Cell Replication / Cell Division

Copyright 1998-1999 by Randall Oelerich. All Rights Reserved. No part of this document may be reproduced except for phyiscal hard-copy printouts for personal study or classroom teaching handouts. That being said, please do add links to this webpage on the internet. Detailed information is contained withing this document, comparable to that in a college textbook. It is hoped that this will serve as an electronic reference for virtual students and the public seeking knowledge on this topic

All life seeks to propagate itself, to continue, to produce progeny for the future. This occurs of course with entire organisms, such as when we as humans mate and have children. Propagation of life also occurs at the cellular level, as unicellular life forms, and individual cells of multicelled organisms, replicate. Indeed, without cell replication, you would not exist, since all cells (and you are composed of cells) come from preexisting cells. This tenet was proposed by the German pathologist Rudolf Virchow in 1858, and is the basis of what is known as cell doctrine.

[Key Point: The cell doctrine states that all cells arise from preexisting cells.]

Unicellular life forms replicate for continuance of the species (a bacterium, for example), while sometimes providing biomass for organisms higher up on the food chain. Cells of multicellular life forms replicate during the creation and maturation of the organism from an embryo or larvae, for the growth of and repair of tissues, and for the creation of sex cells (gametes) such as sperm and egg cells.

The manner in which cells replicate, however, differs with various classes of life forms, as well with the end purpose of the cell replication. For example, some unicellular life forms replicate by simple splitting of the cell into two cells, a process known as binary fission. Sometimes bacteria meet and form an intercellular bridge and exchange genes, a process called conjugation.

Cell that comprise tissues in multicelled organisms typically replicate by an organized duplication and spatial separation of the cellular genetic material, a process called mitosis. Similar to mitosis, but with differing significantly in that the cellular progeny have their complement of genetic material reduced to half that of the parent cell, meiosis is the mode of cell replication for the formation of sperm and egg cells in plants, animals, and many other multicelled life forms.

No matter what specific method is used for replication of a cell, the process that a cells undergoes in order to replicate is known as the cell cycle.

[Key Point: Binary fission, budding, mitosis, and meiosis are four methods of cell replication, but all involve what is called the cell cycle- the sequence of events involved in cell replication. Conjugation is often studied along with cell replication, but occurs only in bacteria and does really involve cell replication, but rather sharing of genetic material.]

[Figure: Illustration of Cell Cycle- 3D translucent cells in phases of cell cycle, showing cutaways of cell interiors (nuclei, chromosomes, etc.]

Generally there is an increase, usually a doubling, in cell biomass before the cell cycle ends with cell division into two cells. The cell cycle can be defined as the orderly duplication of intracellular components, including the cell genome (DNA), followed by division of the cell into two cells.

Unicellular organisms simply undergo one cell cycle so as to produce a new organism. Multicellular organisms, however, undergo many, indeed often times trillions, of cell cycles in order to produce a new organism from a fertilized egg.

How frequently are cells replicating, that is, moving through cell cycles? Microbes are generally very active- constantly moving through the cell cycle as long as nutrients are plentiful. In fact, if nutrients were unlimited and waste products could be removed from the vicinity of bacteria, a period of 24 hours would produce a pile of bacteria four feet high on the entire surface of our planet! Yet in multicelled organisms such as adult humans, cells vary considerably in their cell cycle activity.

Generally, the more specialized the cell type the less the cell replicates. Cells of a developing embryo are undifferentiated, and replicate at very fast rates. In contrast, nerve cells as well as muscle cells rarely, if ever, replicate in a adult animal. Cells lining your surfaces, that is epithelial cells, are constantly replicating. For example, the cells that line the human gut generally undergoing a cell cycle about every twenty-four hours. So your entire gut lining is replaced every day.

[Key Point: Cells replicate at different rates. The more specialized a cell, the less frequently it replicates, and vice versa.]

[SIDE BAR: Think you don't need any fat in your diet? Consider all the cells that are constantly replaced in your body, especially those shed each day from the lining of your gut and of your skin. The new cells that are formed to replace all those lost cells synthesize lots of new cell membrane components and cell organelles such as golgi, endoplasmic reticulum, vacuoles, lysosomes, nuclear membrane, and mitochondria- cell components that have as one of their principle membranous biochemical substances phospholipids. ]

In this chapter you will explore each of the above described cell replication processes in more detail. We will begin with the simpler methods of cell replication (fission, budding) and progress to the more complex (mitosis, meiosis).

Binary fission is the normal method of replication among bacteria. In this method of cell replication, the bacterial cells replicate intracellular ribosomes, enzymes, and other cell components in the cytosol, and duplicate their chromosome (dsDNA). Having replicated genome and components in the cytosol, new plasma membrane and cell wall material is laid down along the cell midline between the two daughter chromosomes. The cell then splits down the midline into two cells.

[SIDEBAR: Since bacteria only have a single, circular double-stranded DNA molecule, and hence a single chromosome, it is a simple matter for a bacterium to quickly replicate their genome. They do this quickly, in a matter of minutes. Consider that during the replication of a bacterial chromosome, the chromosome, that is the double-stranded circular DNA molecule, rotates at over 1000 r.p.m.! ]

The process of binary fission is complete, and one cell has become two cells in an extremely short period of time. How short? Most bacteria replicate (given adequate nutrients) every five to twenty minutes. Because of the extremely rapid rate of replication of bacteria, these microbes can be grown cheaply and efficiently, and may one day prove to be the answer to global population food shortages. Surgeons and other health care workers must be extremely cautious of microbial contamination of patients.

[Key Point: Binary fission is simple, quick, and only occurs in prokaryotic cells such as bacteria. Binary fission is how bacteria normally replicate.]

[Exercise: Suppose you and your team member were assisting with abdominal surgery, and one of you contaminates the patient's abdomen with a single bacterial cell. How serious is this? How many bacteria would arise from this single cell overnight? Working together with several classmates, calculate the number of bacteria formed from one starting cell, after a total time period of 12 hours, given a doubling time of 5 minutes. Do the same for doubling times of 10, 20, and 30 minutes. Graph your results on a piece of scientific graphing paper (Semi-log paper if available, otherwise graph Log10[number of cells] instead of the number of cells. Your instructor will help you with this.). Compare results with other teams, and show your results to your instructor.]

Conjugation is another means of bacterial "replication" although the cells do not really replicate, as for example with binary fission. But conjugation is important for propagation of bacteria. In conjugation, two bacterial cells meet, form a bridge, and one of the cells generally donates some replicated DNA. This allows for sharing of genes among bacteria, even among different genera. Conjugation is one of the ways in that bacteria share genes coding for resistance to drugs, causing great concern among the medical community.

[Key Point: Conjugation is not really a method of cell replication, but is often studied as part of microbial cell growth and replication. ]

[Thought Question: Suppose you work in a biomedical research and development laboratory. What use might intentional promotion of conjugation between two or more species of bacteria serve?]

The cell cycle in eukaryotic cells is much more complex than that in prokaryotic cells such as the bacteria. This is a result of a number of factors unique to eukaryotic cells: they are much larger than prokaryotic cells, they have a more complex armada of intracellular components (golgi bodies, endoplasmic reticulum, etc.), they have a nucleus and nucleolus, they have multiple chromosomes containing the cellular genome, and they have additional genetic material inside mitochondria (and inside chloroplasts if plant cells). In order for a eukaryotic cell to successfully complete a cell cycle, it will need to conduct a complex cell symphony of a sort, coordinating the replication and sorting of many different cell organelles and chromosomes.

Eukaryotic cells undergo a cell cycle that can be divided into the following stages as seen in figure ___ : Interphase (subdivided into G1(Gap1), S(Synthesis), and G2(Gap2) phases), mitosis (subdivided into prophase, metaphase, anaphase, and telophase), and cytokinesis (cell splitting and the distribution of cellular components). Mitosis and cytokinesis constitute the M phase of the cell cycle.

[Key Point: The cell cycle of eukaryotic cells includes the non-dividing interphase and the dividing M phase. Interphase includes two gap phases (G1,G2) with a DNA synthesis S phase occurring in-between G1 and G2.]

Mitosis is the common method of cell replication for tissue growth and regeneration among all multi-cellular organisms. Typically it may last only an hour, with most of a cell's activity spent in interphase, the time between M phases (mitotic cell divisions).

Mitosis is also the method of cell replication used for growing an adult multicelled organism from a primitive fertilized egg, or zygote, as cells of an embryo or larvae divide to increase the cell numbers of the developing organism, the new cells formed then differentiating into specialized structures that help define the adult organism.

[Thought Question: Which is better as a means of cell replication- binary fission, budding, or mitosis?]

[SIDEBAR: Meiosis is a mode of cell replication that occurs only in the gonads of eukaryotes, where germ cells (sperm and egg cells, that is sex cells) are created. Meiosis is a reduction division, where a cell's content of genetic material is reduced to form daughter cells having 1/2 the amount of DNA (and genes) found in regular body cells. Following meiosis, sperm and egg cells potentially combine during fertilization to form a fertilized egg called a zygote. The zygote now has the full complement of genetic material (1/2 + 1/2=1). The zygote undergoes repeated mitotic cell divisions to ultimately form the adult organism of the species that originally donated egg or sperm, thus completing the life cycle, and the propagation of, the species.]

[Key Point: Mitosis is cell replication that occurs in eukaryotic tissue cells. Meiois is cell replication that occurs in testes or ovaries and leads to sperm and egg cells.]

During mitosis, replication of cell genetic and cytoplasmic material occurs, followed by a highly organized splitting of cell contents, that is cytokinesis. The two cells formed following mitosis, called daughter cells, are genetically identical, and since a cell approximately doubles its volume and mass prior to division, each daughter cell has approximately the cell mass of the original interphase parent cell.

For human convenience and for communication among scientists, mitosis is artificially divided into discrete stages or phases known as prophase, metaphase, anaphase, and telophase. Each of these phases can also be divided into early, middle, and late stages or phases. Remember though, that mitosis is a continuous process, but with visibly unique phases.

Interphase is technically not part of mitosis. It is the non-dividing stage of a cell's life cycle. A cell in interphase, however, is certainly not resting. During interphase a cell is active performing its normal cell functions, duplicating cell components, and genome replication, in preparation for cell replication. During interphase, the cellular genome, that is the double-stranded DNA (dsDNA) molecules, exists in an uncoiled, unraveled state, and is not easily visible using ordinary dyes and light microscopes. The uncoiled genome is said to exist as chromatin.

Interphase occurs as gap (G) and synthesis (S) phases. At the start of interphase, the cell is in Gap1 (G1) of interphase, and is active, making cell components that will be contributed to the cell progeny, that is the daughter cells that will result when the cell cycle is completed.

Following G1(Gap1), the S(synthesis) phase of interphase occurs, during which synthesis of new DNA occurs, as well as synthesis of many cell proteins- most notably proteins playing an important role in cell replication, such as spindle fiber proteins that will help separate the chromosomes (structures visible by light microscopy using dyes, consisting of dsDNA molecules supercoiled around proteins during cell splitting.

It is during the S phase that chromosomal DNA is replicated (forming two sister chromatids) so that later during mitosis when the cell divides the DNA to each newly forming daughter cell, each new cell will receive a chromatid (daughter chromosome) and hence exactly the same amount of DNA, and hence genes, as the parent cell originally possessed.

[SIDEBAR: Detecting what phase of the cell cycle a cell or culture of cells is in can be important in cell research. Cells in the M phase of the cell cycle can be identified easily by microscopy, noting the presence of chromosomes. Interphase G1, S, and G2 phases of the cell cycle need to be identified by non-microscopic methods. Cells in the S phase of the cell cycle can be detected by noting the cellular uptake of radioactive thymidine, a building block of cellular DNA.

Cell DNA content (quantified by incubating cells with a fluorescent dye that binds to DNA, and then quantifying the intensity of the fluorescence of cells using a flow cytometer) also gives evidence as to the stage of the cell cycle that a cell is at. Animal cells in G1 have the normal diploid complement of DNA (two of each chromosome) and are said to be 2n (n refers to haploid, that is the amount of DNA found in gametes such as sperm and egg cells).

Cells in the S phase replicate their DNA so their DNA content increases from 2n to (2nx2=)4n. The cell remains 4n (G2, phases) until the M phase subsequently completes (karyokinesis and cytokinesis) and the cell again becomes (4n/2cells=) 2n (diploid).]

Interphase completes itself with the second gap phase, Gap2(G2), during which additional cell proteins are made, cell membrane material is created and stored in preparation for laying down new cell membrane down the center of the cell when the cell will split into two cells, and cellular DNA condenses by supercoiling to form chromosomes. The condensation of the DNA by means of supercoiling around histone proteins is very important, allowing for efficient organization of the chromosomes during mitosis. During the DNA condensation, chromatin condenses by about a factor of one thousand. This highly compact form of DNA is such that genes are unable to be decoded (transcribed).

[Key Point: Histone proteins allow coiling of eukaryotic cell DNA into chromatin and chromosomes.]

[Thought Question: What would be the consequence to a cell that had a genetic defect so that histone proteins were not properly synthesized?]

Prophase is the first stage of mitosis. By ordinary light microscopy of stained cell preparation, this stage of mitosis is evident when chromosomes are seen as dense, highly coiled structures randomly arranged in the nuclear region. The supercoiling of the dsDNA molecules that leads to chromosomes involves the DNA interaction with unique cell proteins called histone proteins. At both sides of the chromosomes, mitotic spindles are seen to form. These are arrays of microtubules.

[SIDEBAR: Histone proteins are one of the few types of cellular proteins with a net positive electrical charge at physiological pH - important when studying cells by polyacrylamide gel electrophoresis, also known as PAGE or SDS-PAGE. PAGE involves loading mixtures of cell proteins onto the top of a thin vertical gel, with a positively charged electrode wire placed at the bottom of a tank containing electrolyte (electrical conducting) solution. The electrode attracts negatively charged proteins (most cell proteins have a negative charge at normal cell pH), so the protein mixture is separated by size and charge as the proteins migrate downward through the PAGE gel. Positively charged proteins migrate vertically upward and are normally lost into the tank electrolyte solution. If it is desired to study histone proteins, the electrical current in the electrode wires must be reversed.]

In animal cells, microtubules of the spinal apparatus emanate from structures called centrioles (originally called centrosomes before electron microscopes revealed their fine structure), and will function to separate the DNA molecules (which will be termed chromatids when in the supercoiled state and part of a chromosome) of the chromosomes. Centrioles are in fact short arrays of microtubules- nine rods, each itself made of three microtubules. As prophase begins, the centriole replicates, with a second centriole forming adjacent, and at ninety-degrees orientation, to the original centriole.

The centrioles move from their original position near the nucleus to opposite poles of the cell, driven by centrosome-associated motor proteins that hydrolyze ATP for this energy-requiring process. When the microtubules elongate (by means of alpha and beta tubulin protein subunit additions), the spindle apparatus forms and is visible. The microtubules grow in length until they encounter a centromere along a chromosome, at which point the microtubules cease elongation. The two sets of microtubules from each pole of the cell together constitute the mitotic spindle, or spindle apparatus. The two centrioles, or centrosomes, that give rise to the mitotic spindle are called spindle poles.

In plant cells, a similar process occurs, that is spindle apparatus formation and microtubule elongation, but without centrioles. Microtubule attachment to centromeres prepares a cell for subsequent separation of replicated genetic material (chromosomes, chromatids) to each pole of the cell- a process that will occur as mitosis proceeds beyond prophase.

During late prophase, or prometaphase (the transition from prophase to metaphase), the nuclear membrane and also the nucleolus dissolve and are no longer visible. This occurs as intermediate filaments of the nuclear lamina are phosphorylated and subsequently disassemble, leading to nuclear membrane breakdown into small vesicles. This is an important event, as you will see, so that the chromosomes are freely available to interact with the mitotic spindle microtubules in the cytosol that will help align and then separate the chromosomes.

[SIDEBAR: Protein complexes form on each chromosome to function as attachment points for the microtubules. These protein complexes are known as kinetochores. Each chromatid of a chromosome has a kinetochore protein complex created on each chromatid's centromere, so that each chromosome at prophase consists of two kinetochores- one on each sister chromatid.

The kinetochores of each sister chromatid face in opposite directions- critical because the microtubules from each cell pole of the mitotic spindle will reach out and grasp kinetochores. Since they face in opposite directions, a sister chromatid will be grasped by mitotic spindle fiber microtubules from each pole of the cell, assuring that equal amounts of genetic material in the form of chromatids, or daughter chromosomes (when the chromatids separate), separate to each newly forming daughter cell.]

[Key Point: Microtubules of the spindle apparatus emanating from the centrioles at each pole of the cell attach to kinetochore proteins at the centromeres of each chromatid.]

Metaphase follows prophase, when the spindle fibers attach to the region of each chromatid called a centromere. Tension is applied to each chromatid from mitotic spindle fibers on each pole of the cell, and the chromosomes align along the equatorial plane. Chromosomes are pulled toward the centromeres by shortening of the spindle fibers, but this is countered by an opposite force, known as polar wind, that repels chromosomes from the centrosomes. A metaphase plate is thus formed, with chromosomes aligned down the middle of the cells.

This phase of mitosis is easily recognizable by microscopy. A stained cell preparation viewed using an ordinary light microscope reveals the chromosomes in alignment in this strict fashion along the center of the cell.

Metaphase prepares the cell for separation of chromatids (replicated DNA) to each pole of the cell so that each new cell will receive a complement, that is an amount, of DNA equal to what the parent cell had. In this way, progeny cells are genetically identical to the parent cell.

[SIDEBAR: Colchicine is a chemical that interferes with microtubule function, preventing tubulin addition to the mitotic spindle. Colchicine prevents separation of chromatids at metaphase, since tubulin addition is needed for spindle pole migration (as metaphase proceeds to anaphase). Cells exposed to colchicine never progress beyond metaphase. They are ' arrested' in metaphase.

The use of colchicine is common in laboratories that study the karyotype of a cell, that is the chromosomal compositions of a cell, such as in done following amniocentesis to check for chromosomal diseases such as Down's syndrome and many other conditions where a chromosome may be missing in its entirety, may be missing a part, or where there may extra chromosomes.

Amniocentesis involves the aspiration of some fetal cells from the amniotic sac in the uterus, culturing of the cells, then exposure of the cells to colchicine. The cells become arrested in metaphase in synchrony, and are then lysed, with the chromosomes being splayed on a microscope slide. The chromosomes are photographed with the use of a microscope and camera, and are then studied. Each of the chromosomes has a characteristic 'X' appearance, caused by the fact that each chromosome has had its DNA replicated and so consists of two chromatids joined together at the crossing point of the 'X', that is the centromere region.]

During anaphase, the stage of mitosis that continues after mitosis, the mitotic spindle apparatus microtubules function to separate the sister chromatids of each chromosome, each chromatid now moving to one side of the cell (at a rate of about 1um/minute). The sister chromatids are separated as enzymes break protein connections that have been bonding sister chromatids together on each chromosome. Each newly separated chromatid can now also be called a daughter chromosome, since each chromatid is by itself a visible entity, a colored body (Greek chroma=color[ed] + soma=body).

[Key Point: A chromosome is a visible supercoiled complex of DNA and histone proteins. During mitosis, chromosomes contain two chromatids until anaphase when the sister chromatids separate, at which point they are known as daughter chromosomes.]

[SIDEBAR: Anaphase can be divided into two processes- anaphase A and anaphase B. During anaphase A, microtubules of the mitotic spindle that are attached to the chromatids (at their kinetochores) shorten by depolymerization of the tubulin subunits of the microtubules. This results in movement of the daughter chromosomes (sister chromatids) towards the centrosomes (centrioles). During anaphase B, microtubules that are not attached to chromosomes elongate by means of tubulin subunit addition (polymerization) and the effects of motor proteins, serving to push the centrosomes further away from the metaphase plate, and thus helping to separate the chromosomes to each pole of the cell.]

In animal cells, microfilaments form beneath the cell membrane in the center region of the cell, in effect pinching the cell into two cells while new cell membrane material is being formed (remember the plasmalemma vesicles formed during interphase in preparation for this?). This process is called cell furrowing. An acute microscopist can see the cleavage furrow that forms as a result. Animal cells also form asters that will help direct cleavage furrow formation.

Asters are star-shaped microtubule outgrowths of the spindle fiber apparatus. Normally two asters are formed during mitosis. If asters are not formed, a cell undergoing mitosis will not divide into two cells, but will form one larger cell having two nuclei, increased cell mass, and increased amounts of cell organelles. If repeated, lack of aster formation during cell cycles can lead to a cell having many nuclei and increased cell size- such a cell is called a syncytium.

[SIDEBAR- Syncytial cell formation is normal in certain tissues and cells. Animal embryos, in their early stages, form syncytial cell masses that invade the uterus to feed on maternal nutrients much like an amoeboid protozoan. Voluntary skeletal muscle cells (containing up to a hundred nuclei) are formed as syncytia. Certain immune system white blood cells called giant cells formed during certain disease processes (tuberculosis, temporal artery inflammation).

Some fungi form as a syncytium and are termed slime molds (one such slime mold, a single syncytial cell, is many acres in size, covering an entire forest floor in the subsoil!). For an interesting twist on this topic, the reader might wish to read the fictional Sci-Fi Horror genre of novel title FireFly by Pierce Anthony.]

Recall that plant cells have, in addition to a cell membrane, a cell wall composed of cellulose starch. When plant cells divide during mitosis they also need to lay down new cell wall material in addition to cell membrane. This is done during cell furrowing as necessary cell wall components are synthesized and laid down on top of the new cell membrane being laid down for the daughter cells.

The final stage of mitosis, telophase, occurs following anaphase. Daughter chromosomes have migrated to opposite ends, or poles, of the cell, and the cell prepares to split into two daughter cells. No longer needed, the mitotic spindle fibers dissolve and disappear. Karyokinesis, that is the replication and separation of the cellular genome (DNA, genes) to each end of the cell, is complete.

The nucleus of each newly cell needs to be reassembled. This occurs as nuclear membrane vesicles congregate around each set of daughter chromosomes. The vesicles fuse, and a new nuclear membrane is formed around each set of chromosomes. Nuclear pores reform. Intermediate filaments that were phosphorylated during dissolution of the nuclear membrane in prophase now are phosphorylated and reform, integrating into the nuclear envelope to form the nuclear lamina. Nuclear proteins are transported through nuclear pores into the nucleoplasm and the nucleus expands. The dense chromosomes uncoil to reform chromatin so that genes can again become active. Telophase, and mitosis, is complete except for the completion of cytokinesis.

Cytokinesis, that is the replication and separation of other cellular components (golgi, ribosomes, and so on), having begun in early anaphase, is all that needs to be completed in preparation for physical splitting of the cell into two daughter cells. Recall that the synthesis of these cell components occurred primarily during the S phase of interphase. The cell separates these cell components during, or shortly after karyokinesis, so that each newly forming daughter cell will have its own complement of not only genome, but also of intracellular machinery that will allow each daughter cell to function and thrive.

Finally, during late telophase, karyokinesis and cytokinesis being completed, the cell physically separates into two daughter cells. Each daughter cell (barring any errors during mitosis) is genetically identical, and has about the same biomass, shape, and appearance, and are identical to the parent cell they came from, except for being somewhat less in size (this will correct as the daughter cells grow).

[Key Point: Karyokinesis is the distribution of genetic material to each newly forming cell during mitosis, whereas cytokinesis is the distribution of non-genetic cellular material during mitosis.]

[SIDEBAR: During anaphase, and in animal cells, a contractile ring composed of actin and myosin (the same contractile proteins in muscle cells that cause muscle cell shortening) forms on the inside of the cell membrane, encircling the metaphase plate, that is the center region of the dividing cell. During telophase, this contractile ring helps squeeze the cell into the two daughter cells.

Plant cells do not form a contractile ring (not surprising since plants do not have muscle cells!), but in place of this plant cells simply form a new cell wall down the middle of the cell (surrounded by cell membrane), the process being guided by a structure called a phragmoplast (formed from microtubule fragments). Cell wall building blocks- glycoproteins and polysaccharides - are transported to the forming cell wall by vesicles that then fuse to the growing cell wall. Cellulose fibers are integrated into the newly forming cell wall, and eventually the plant cell splits into two daughter cells.]

[Key Point: Animal cells use a contractile ring for cell furrowing. Plant cells use a phragmoplast.]

[Exercise: Given drawing tools and artistic paper by your instructor, create an illustration depicting the phases of mitosis, illustrating key structures at each phase. Strive for realism. You may choose either a plant cell or an animal cell.]

A natural question might be, "What signals a cell to replicate, and how often?" There is no simple answer to this question. Multiple factors regulate cell replication, including intracellular and extracellular control factors.

Whatever the mechanisms, cells have a genetic internal 'clock' of a sort that determines how many times a cell divides over its lifetime (a number known as the Hayflick limit), and also how often the cell divides (specific to tissue cell types).

The more specialized a cell type is, the less often it replicates, and vice versa. For example, nerve cells are highly specialized for electrical signally, memory, and other complex functions; however nerve cells do not replicate (in adults). Muscle cells also rarely if ever replicate in a adult organism. Less specialized cells such as those that protect body surfaces (epithelial cells) replicate perhaps every twenty-four hours. Blood cells formed in the bone marrow also replicate very rapidly.

[SIDE_BAR: When cancer cells are treated through the use of chemotherapy and nuclear medicine (radiation) therapy, the rapidly dividing cancer cells are greatly affected by these therapies. This is because dividing cells are very active metabolically and their DNA (genes) are more sensitive to chemicals or radiation that might damage the DNA as it is being replicated.

The problem is, non-cancerous cells that also replicate rapidly like cancer cells are also sensitive to chemotherapy and radiation. Cells such as epithelial cells and bone marrow blood cells. This explains why chemotherapy and radiation therapy causes patients to suffer from abnormal blood counts and to have problems holding down food (vomiting a lot). Their bone marrow precursor blood cells, and also their epithelial cells lining their gut, are severely affected by the cancer treatment (designed to affect cancer cells that are also in high states of metabolism and that also have their genes exposed frequently as their chromosomes are frequently in existence.]

[SIDEBAR: The ability to identify cells that are undergoing mitosis is very important in the field of pathology, where tissue biopsies are often checked for cancer or pre-cancerous conditions. Looking at a biopsy, the pathologists looks for the ratio of dividing cells to non-dividing cells. This ratio is called the mitotic index.

Different tissues have different mitotic index values. For example, the mitotic index of adult brain or muscle tissue should be zero, whereas the mitotic index value of cervical tissue (part of the female uterus) might be 1:10. Suppose a biopsy specimen of muscle tissue revealed by light microscopy that about 1 in 10 muscle cells were undergoing mitosis (see photograph). Would you suspect a neoplastic condition (cancer or pre-cancer condition)?]

Normal cells of tissues and organs exhibit a common growth control phenomenon known as contact inhibition. When normal cells replicate, they cease dividing when they encounter neighboring cells. Contact inhibition of cells in tissues generally leads to monolayer tissue growths in culture.

[SIDE_BAR: Cancer cells do not exhibit contact inhibition and continue to replicate in spite of contacting neighboring cells. It is this feature of cancer cells that makes them so damaging to an organism. Without contact inhibition, cancer cells grow, and grow, and grow, damaging the organism by a multitude of mechanisms. Cancer cells use precious nutrients, but do not perform useful, normal physiological function for the organism. Cancer cells sometimes grow into nearby organs, disrupting normal anatomy and physiological function, or perhaps impinging on nerve tracts and causing subsequent sensory or motor deficits.]

[Key Point: Cancer cells do not stop growing when they contact neighboring cells. They do not exhibit contact inhibition. Normal cells do, and form tissue culture monolayers as a result.]

[SIDEBAR: Uncontrolled replication of cells leads to tumors, that is cell overgrowths,, whether benign or cancerous. Benign tumors are simply excessive cell growths that will not cause any significant harm. Malignant tumors, that is cancers, are cell growths where the cells are replicating without any inhibition of cell growth, and they will cause death to the organism if allowed to continue growing. Here are the naming conventions used for the more common tumors.

[Carcinomas: cancers of epithelial tissues (cells lining the surfaces of an organism). Sarcomas: cancers of connective tissues. Leukemias: cancers of white blood cells. Lymphomas: tumors of the lymph nodes. Osteomas: tumors of bone. Osteosarcomas: sarcomas of bone tissue. Neuromas: benign tumors of nerve tissue. Leiomyomas: benign tumors of smooth muscle tissue. Rhabdomyomas: benign tumors of voluntary (skeletal) muscle. Chrondromas: benign tumors of cartilage. Chrondrosarcomas: malignant tumors of cartilage. Adenomas: benign tumors of glandular tissue. Adenocarcinomas: malignant tumors of glandular tissue.]

Agents that can trigger cells to become tumorous include: environmental carcinogens in food, water, or air; cancer-causing genes called oncogenes that are transmitted by certain viruses; and inherent oncogenes, triggered by repeated trauma to a cell.

[Karl Loren Note: Notice that this author completely omits the action of free radicals -- the most basic cause of cancer.]

In addition to contact inhibition regulation of cell replication, cell growth, cell replication, and cell activity are governed by intracellular and extracellular signals.

Intracellular signals include kinases and cyclins- two molecules that interact by means of a negative feedback loop to cause an internally regulated- and phasic- cell replication cycle, as well as regulatory check points that control the progression of the cell cycle from one phase to another.

[SIDEBAR: Eukaryotic cells have a cell division cycle (cdc) kinase (an enzymatic protein that modifies other cell proteins through the addition of a phosphate chemical group, PO4) called cdc2 kinase. When combined with a cell protein called cyclin (produced during interphase), a molecular complex is formed (cdc2*cyclin) that then converts to a key intracellular protein called maturation promoting factor (MPF).

Cellular mitosis is promoted by MPF. At the same time, the MPF protein triggers enzymes that degrade cyclin (this process is delayed, and lags behind MPF promotion of mitosis). As cyclin is destroyed, MPF production (and promotion of cell mitosis) diminishes.

With diminishing MPF, cyclin-degrading enzymes lessen in activity. The cell resumes cyclin production during interphase, formation of the cdc2*cyclin complex, and formation of active MPF. Mitosis promotion resumes.]

[Key Point: Cytoplasmic cdc2 kinase protein combines with cyclin protein to form MPF that then stimulates mitosis to occur, while at the same time degrading cyclin.]

[SIDEBAR: Internally, cells have built-in checkpoints that oversee the readiness of a cell to progress to the next phase in the cell cycle.

A G1 checkpoint looks for damaged DNA, and if found, produces a protein (p53) that inhibits progression of the cell cycle to the S phase. The G1 checkpoint prevents the synthesis, that is the replication of damaged and mutated DNA that would otherwise be passed on to daughter cells.

A G2 checkpoint looks for DNA that has not yet replicated, or that is damaged, and if found, a signal stops the cell cycle from progressing until all DNA has been replicated or repaired if damaged. The G2 checkpoint does not allow a cell to proceed to M phase (mitosis) until the S phase (DNA synthesis) has completed. This prevents disastrous cell replication that would otherwise result in daughter cells with unequal amounts of, or mutated DNA, and hence defective daughter cells.

A metaphase checkpoint also occurs in cells during mitosis (M phase), during which the cell checks for the proper number of chromosomes by noting their alignment on the mitotic spindle apparatus. Continuation of M phase from metaphase to anaphase will not occur until this checkpoint succeeds.]

[Key Point: Cellular checkpoints are internal controls that prevent haphazard cell cycles.]

Extracellular Signals include hormones, neurotransmitters, and cytokines. All these signaling molecules originate outside the cell, and sometimes are very similar in structure and action, but differ in the mode of delivery- hormones are delivered by means of blood, neurotransmitters are released from nerve cells to interact with adjacent cells, and cytokines are released from many cell types, to interact with other nearby cells.

[SIDEBAR: Sometimes the definition of a hormone, neurotransmitter, and cytokine gets blurred- for example many hormones originate from part of the brain called the posterior pituitary gland. These hormones are released into the blood, and travel by blood to target cells, but the cells that make the hormones are nerve cells.]

[SIDEBAR: Adrenaline is a molecule that helps the body during a stressful situation. Adrenaline is secreted by the adrenal gland in times of stress, diffusing into the blood vessels coursing through the adrenal gland, and then dispersing throughout the body rapidly by means of the blood to find its target cells. Adrenaline is also a neurotransmitter, released from many nerve endings to diffuse a very short distance (across what it called a synaptic space) to interact with another nerve cell membrane, a muscle cell membrane, or a gland cell membrane.]

Hormones are molecular messengers transported by blood (or in the case of animals without blood, such as insects, hemolymph or other circulatory body fluids) to cells. Some hormones (peptide hormones) interact with cell membrane proteins that then interact with intracellular enzymes to regulate cell activities. For these types of hormones, the hormone itself is the first messenger, signaling a cell membrane protein. Part of the cell membrane protein receptor for the hormone contains enzymatic activity on the inside of the cell that acts on ATP to convert it to cyclic adenosine monophosphate (cAMP). The cAMP formed as a result of the peptide hormone interaction with the membrane protein is known as the second messenger, a fundamental concept in cellular biology and physiology, since cAMP is a common signaling mechanism in many cell types. Once formed cAMP leads to changes in other intracellular enzymes that in turn affect cellular metabolism and function. Can you see why cAMP is called the second messenger?

[Key Point: cAMP is a second messenger that causes internal cell events to occur. The first messenger that stimulates cAMP formation may be a peptide hormone, a neurotransmitter, or some other molecule.]

[SIDEBAR: Oxytocin is a peptide hormone, comprised of only a few amino acids. Oxytocin is produced by the pituitary gland of the brain, and when released into the blood of animals oxytocin attaches to muscle cells of the female uterus and causes muscle contraction. When a female is ready to give birth to a fetus, the brain releases significant amounts of oxytocin that then trigger labor, that is uterine contractions. A synthetic form of oxytocin, Pitocin, is used in hospitals to induce labor in certain instances.]

Other hormones (steroid hormones such as androgens and estrogens, as well as thyroid hormone) enter cells, are ushered to the cell nucleus by an intracellular transport protein. Once in the nucleus, the hormone binds to genes on cellular DNA, affecting cell activity at the genetic level. Sometimes the hormone enhances gene activity, sometimes the hormone decreases gene activity. Either way, cellular transcription of gene products is affected, in turn affecting the cell function.

Neurotransmitters are molecules released from nerve cells to diffuse across a microscopic space to interact with a target cell - another nerve, muscle, or gland cell. Sometimes the rate and quantity of nerve cell release of neurotransmitters onto the target cell can affect its growth and replication. For example, increased firing of nerve cells (and release of neurotransmitters) onto developing muscle cells can affect whether the muscle cells develop into muscle cells adapted for strength or endurance. During early childhood development, the interaction of nerve cells with other nerve cells greatly affects the development of brain cells, and hence brain function.

Cytokines are molecules released from many different cell types that then interact with and affect a variety of nearby cells. Lymphokines are cytokine molecules synthesized and released from white blood cells called lymphocytes- immune system cells important in fighting off virus infections, producing antibodies, and regulating immune system function. Some lymphokines stimulate mitosis of other white blood cells, whereas other lymphokines inhibit the function of certain white blood cells.

[SIDEBAR: The human immunodeficiency virus (HIV) that is associated with acquired immunodeficiency syndrome (AIDS) primarily attacks a type of lymphocyte called a helper T cell (Th). Helper T cells manufacture and secrete a cytokine (lymphokine) stimulates another type of lymphocyte called a killer T cell (Tk), a cell type that attacks and destroys cells infected with viruses (acting to diminish spread of a virus through the body), that attacks cancer cells, and that helps fight infections with fungi and tuberculosis (TB). Can you see why the HIV virus can lead to the danger of an infected person contracting a fungal infection, or TB, or succumbing to a cancer that is developing in the body?]

[Group Exercise: In a small group, come up with a couple of hypothetical methods that for treating AIDS that would manipulate helper and killer T cells, so as to get at the base problem caused by the HIV virus.]

Growth factors are another class of cytokine. They do what their name implies- stimulate cellular mitosis, and hence tissue growth. Various growth factors are secreted by cells of different tissues. Skin cells secrete what is called Epidermal Growth Factor (EGF). Fibroblast Growth Factor (FGF) promotes mitosis of connective tissue cells called fibrocytes (cells that make collagen and other connective tissue proteins), and also of endothelial cells (cells that create the lining of blood vessels). Megakaryocytes, gigantic cells in the bone marrow that produce blood platelets (cell fragments that help plug damaged sites in blood vessel walls), produce platelet-derived growth factor (PDGF). PDGF activates fibroblasts to undergo mitosis in the area of a wound, where platelets will be found as a result of blood vessel trauma.

Though occurring naturally in the body, growth factors are being used therapeutically for a variety of medical purpose, including acceleration of the healing process in damaged eye surfaces (the cornea), promotion of skin regeneration when skin grafts are applied to denuded skin following severe burns, and stimulation of wound closure following surgical closure of a sutured site.

[SIDEBAR: Cancer cells secrete a growth factor, Angiogenesis Growth Factor, that stimulates new blood vessel growth into the vicinity of the cancer, a process called angiogenesis. This allows the cancer to increase its ability to harvest blood-delivered nutrients from the body. One common method of detecting cancer is to do a type of radiological scan of the body that looks for increased blood flow in an area of the body (a " hot-spot")- suggesting a tumor or cancer.

Newer anticancer drugs include drugs that prevent angiogenesis, such as angiogenesis inhibiting factor (AIF) that has been found to cure cancers in mice- not just one type of cancer, but many types. Such drugs hold promise for the treatment of cancers.]

The normal process and regulation of mitosis is complex, as you have seen. Intracellular and extracellular regulatory process work in concert to orchestrate the many events needed to bring about a successful cell division. Is it any wonder then, that sometimes a cell loses its proper mitotic regulation, and begins replicating out of control, that is it becomes cancerous? What exactly causes a cell to lose control of its replication, to has become tumorous or cancerous?

Conversion of a normal cell into a cancerous cell, and subsequently a cancerous tissue mass, occurs by several mechanisms, all grouped under the term carcinogens. Carcinogenic agents that can lead to cellular conversion to a cancer include viral mediated acquisition of cancer genes, conversion of normal genes into cancerous genes by viruses, dietary or environmental chemicals, and damaging radiation.

[Karl Note: Notice that this author ascribes causation to a virus. This is false data. Read my article about the virus.]

Sometimes normal cell genes that inhibit uncontrolled mitosis do not function correctly (for example if mutated) or are absent as a result of an inherited genetic disease. At other times, the reverse situation occurs in which a gene or genes that activate cell replication (oncogenes) become overly active, or mutate so as to become constantly active. Abnormal oncogene activation has even been documented to occur by means of viral transmission of oncogenes, or viral infection with concomitant activation of endogenous oncogenes. For example, the Rous sarcoma virus has been shown to cause mammary gland (breast) cancer in mice. Epstein-Barr virus has long been know to cause a cancer called Burkitt's lymphoma.

It may be that viruses may prove to cause a number of cancers that we have yet to understand. And yet viruses may themselves prove to be a major weapon in the fight against cancer, as laboratory genetically engineered ' friendly' viruses are designed that invade cancerous cells and surgically correct or kill the cancer cell at the level of the genome.

Chemical carcinogens function to convert normal cells to cancerous cells by inducing mutations in genes that are critical for regulation of cell replication. A simple screening test for chemicals to check whether they are likely to be carcinogenic to many life forms is the Ames test, developed at Ames University in Iowa, USA. The test involves the exposure of special strains of a bacterium to the suspected carcinogen. If the suspected carcinogen induces nutritional mutations in the bacteria, it is deemed wise not to allow higher life forms to be exposed to the suspected carcinogen.

The use of bacterial (prokaryotic) cells for testing suspected carcinogens is advantageous because the cells replicate so rapidly (every 5-30 minutes). This allows the exposure of the bacterial cell DNA (genes) to the suspected carcinogen during DNA replication. The use of bacteria also allows researchers to see the effects, if any, of the suspected carcinogen on extremely large numbers of cell progeny and cell generations.

[Key Point: Conversion of a normal cell to a cancer cell can occur by random damage to cell growth regulatory genes, activation of naturally occuring oncogenes, infection of a cell with oncogenes with a virus as a vector, or by chemicals or irradiation invoking mutational damage to cell growth regulatory genes in a cell.]

[SIDEBAR: Chemical carcinogens are categorized as direct carcinogens, procarcinogens, and promoter carcinogens. Direct carcinogens cause fibroblasts (connective tissue cells) grown in culture and subjected to the chemical to become cancerous. Examples of direct carcinogens include many toxic and complex organic chemicals used in industry. Procarcinogens are not carcinogenic when applied to cells, but become carcinogenic when metabolized by cells. Their metabolic intermediates are carcinogenic. Examples of procarcinogens include nitrates and nitrites such as are found in pepperoni and other foods, and tar created from inhaled tobacco products. Promoter carcinogens, like procarcinogens, are not directly carcinogenic, but rather amplify the carcinogenic effect of other carcinogens. In a sense, promoters act synergistically with other carcinogens. Examples of promoter carcinogens include many chemicals in tobacco smoke and drinking alcohol (ethanol).]

[Key Point: Carcinogens stimulate conversion of cells to cancer cells. Some substances may act directly as carcinogens, some may act synergistically with other agents to become carcinogenic, and some substances may be metabolized to become carcinogenic.]

Cell replication can not continue forever. At some point the organism containing the cell will die, leading to cell death. With death of the organism, nutrient intake diminishes rapidly, cells are unable to produce ATP energy from nutrients, the cells lose the ability to maintain activity of sodium-potassium-ATPase pumps, and osmotic influx of extracellular water leads to cellular swelling and eventual lysis. Cell death can also occur by trauma, whether accidental or as a result of chronic damage to the cell from normal physiological activities in an organism.

Programmed cell death will occur if a cell is infected with a virus, if DNA damage occurs within the cell genome, or as a normal process during embryonic development as a means of remodeling a developing organism. Normal cells also have some sort of built-in clock that allows them to replicate only a certain number of times, this being dependent on the tissue cell type. Upon reaching it allocated number of replications, a cell enters what is known as replicative senescence. Evidence suggests that replicative senescence occurs as telomeres increase in a cell. Telomeres are strands of DNA that tie the ends of chromosomes.

[Key Point: Cells can die as a result of trauma, and from genetic clocks that keep track of how many times a cell has replicated and tell a cell to die after a certain number of divisions.]

Could cells be made immortal if the mystery of telomeres were solved? Perhaps, since cancer cells themselves are in fact immortal, dividing without limitations. Research has demonstrated that cancer cells exhibit active telomerase enzyme activity. Telomerase repairs telomeres, giving cancer cells immortality. Scientists have studied the addition of telomerase to normal non-cancerous cells, with the result being that the cells acquire a longer life span. Perhaps one day there will a magic pill that will allow a person to never grow old, as our cells are given unlimited potential for regenerating damaged tissues and organs that occur as our lives progress.

[Key Point: Telomeres are thought to promote cell aging, and are repaired by enzymes called telomerases. Immortal cells such as cancer cells have high telomerase activity.]

[Exercise: Discuss in a small group the ethics of developing a drug that could create cellular immortality, and hence would allow humans to stop aging at a predetermined age. What age would you choose to stop aging? Would you take the drug if you could never have the opportunity of aging beyond the age your chose to stay at?]

Cells replicate themselves by a variety of methods depending on the type of cell (prokaryote or eukaryote). Such methods include: binary fission (the normal method in bacteria), budding (common in yeasts), mitosis (in body tissues of eukaryotic plant and animal cells), and meiosis (for producing egg and sperm cells).

Cells go through a cell cycle as the grow and divide. The cell cycle involves a non-dividing phase as well as a dividing phase. The non-dividing phase, known as interphase in eukaryotic cells, involves two gap phases (G1 and G2) and a synthesis (S) phase, in the sequence G1, S, and G2. The gap phases involve cellular metabolism as well as replication of intracellular non-genetic components. The S phase involves synthesis of new genetic material (DNA) in preparation for cell division. The dividing phase of a eukaryotic cells is called the M phase and is known as mitosis.

Karyokinesis and cytokinesis occur when cells divide. Karyokinesis is the replication and splitting of genetic material. Cytokinesis is the replication and splitting of non-genetic cellular materials. These two processes are much more complex in eukaryotic cells than in prokaryotic cells, because eukaryotic cells have multiple chromosomes and many membranous cellular organelles, as well as a nucleus.

Mitosis can be looked at, for human convenience, as four distinct phases or states: prophase, metaphase, anaphase, and telophase. During prophase the nuclear membrane and nucleus disappears, and cellular genetic material (DNA) has become highly condensed as bodies seen by light microscopy that are termed chromosomes. The condensation of DNA is helped by histone proteins. Since the DNA has already been replicated during the S phase, each chromosome actually consists of two dsDNA molecules known as sister chromatids. An array of microtubules emanates from centrosomes, or centrioles, at each pole of the cell. This array is known as the spindle apparatus or mitotic spindle. Microtubules attach to kinetochore proteins at the centromere regions of each chromatid and begin to shuffle chromosomes to the cell equator. When this has occurred, the cell is at metaphase, and chromosomes are aligned to create a metaphase plate. As the microtubules separate sister chromatids from each chromosome, the cell enters anaphase. Sister chromatids, now also called daughter chromosomes, migrate to opposite poles of the cell during anaphase. Telophase occurs as new cell membrane and cell wall (if a plant cell) components begin to be laid down in the middle of the cell creating a cell furrow. Chromosomes uncoil to become chromatin once again. A nuclear membrane forms around each set of chromosomes. Cytokinesis, that is the distribution of cell organelles and molecules to each half of the dividing cell, completes, and the cell splits down the middle into two new daughter cells. Mitosis is complete.

Meiosis is a cell division similar to mitosis, except that a reduction division occurs so that daughter cells (sperm or egg cells) are created that have a haploid (n) complement of DNA or chromosomes, exactly one-half that of the normal diploid (2n) complement of DNA or chromosomes found in typical body tissue cells. Meiosis only occurs in the gonads (testes or ovaries) of plants or animals.

Cell replication is regulated by internal cellular genetic mechanisms, intracellular regulatory proteins (cyclins and kinases), extracellular factors (growth factors, hormones, neurotransmitters), and cell contact with other cells.

Normal cells exhibit contact inhibition, ceasing to replicate when they contact other cells, generally forming a tissue monolayer when grown in culture. When control mechanisms for cell replication fail to work properly, a cell may replicate with loss of contact inhibition and convert to a cancerous cell. Cancer cell do not form tissue culture monolayers, but pile up when grown in culture.

Cancers can be caused by inherent cancer-causing genes in cells that are called oncogenes, by genetic mutations (caused by viruses, carcinogens, irradiation, or spontaneous damage to cell DNA), or by viral transmission and insertion of oncogenes into normal cells.

Cells die either from internal genetic cell clocks that cause cell self-destruction after a certain number of cell divisions, from trauma to cells, or by immune system destruction of cell that are detected to be cancerous or infected with a virus. Regions of DNA called telomeres are thought to cause cell aging. Telomerase enzyme- active in immortal cancer cells - repairs telomeres and may prove to be the solution to creating cells that never age, allowing tissues and organs to remain vital throughout life.

KEY TERMS (Put in column next to the SUMMARY of KEY CONCEPTS with specific terms sorted so as to be next to appropriate concept)

Answer: d. metaphase. Note the alignment of the chromosomes along the cell equator, forming the metaphase plate.

2. What structure forms on the ends of chromosomes that is thought to play a key role in cell aging?

4. What structure in plant cells performs a function similar to the contractile ring of animal cells?

5. What proteins help promote chromatin condensation during the supercoiling of chromosomes?

6. How many times does a normal cell divide before it can no longer divide, hence entering replicative senescence?

Answer: d. depends on cell type. Some cells such as those in tissues lining the body can replicate many thousands of times, whereas other cells such as brain tissue nerve cells do not replicate at all (in adulthood).

7. Look at the microphotograph below of a cell undergoing replication. What type of replication is illustrated?
Cell Replication

Answer: d. budding Note the unequal splitting of the cell, a key point that distinguishes budding from the other listed forms of cell replication.

8. Why is cell replication more complicated in eukaryotic cells in contrast to prokaryotic cells?

a. prokaryotic cells have only a single chromosome, whereas eukaryotic cells have multiple chromosomes.

b. eukaryotic cells have a greater number and variety of cell organelles than do prokaryotic cells.

d. Non-dividing muscle, skin, and nerve cells never have a haploid complement of chromosomes.

Answer: c. (A false statement, since the gamete sex cells such as sperm and egg cells are considered haploid (n). They have half the amount of chromosomes (and genes) as other cells found in tissues.

Answer: a. bacteria Mitosis only occurs in eukaryotic cells, and bacteria are prokaryotic cells.

11. Fill in the blanks for the following: The internal cell growth regulatory protein ___________ combines with cyclin to form ______, a protein complex that then stimulates cellular mitosis as well as ________ degradation. Answer: cdc2, MPF, cyclin

12. What characteristics do cancer cells exhibit that normal cells do not exhibit?

a. cancer cells exhibit contact inhibition and cell culture monolayer formation.

b. cancer cells exhibit telomerase activity to a much greater extent than do normal cells.

1. Colchicine is a poison that prevents microtubules from separating the chromosomes after metaphase. Predict what phase of mitosis cells exposed to colchicine would be arrested in? Metaphase, because the spindle fibers will not separate the chromatids normally to cause anaphase. Colchicine is commonly used when culturing cells to visualize chromosomes, as with karyotyping, the technique of studying chromosomes. Karyotyping is done on fetal cells following amniocentesis to check for possible genetic defects of a fetus.

2. How does budding differ from binary fission? Budding involves the unequal splitting of the cell mass. Budding is common in yeast (1 celled fungi) whereas binary fission is the normal mode of replication in bacteria. How are budding and binary fission similar? Both are asexual modes of cell replication (requiring only 1 cell, not a male and female cell) and both occur only in microorganisms.

3. What is conjugation and what purpose does it serve a species of bacteria? It is the joining of 2 bacterial cells with the subsequent exchange of DNA between the cells, usually 1 of the cells doing the DNA donation (after replication the DNA to be donated). Conjugation increases genetic diversity among bacterial genera. Medically, conjugation is a means for bacteria to exchange genes coding for toxins and antibiotic resistance (much to the chagrin of non-microbes).

4. How does meiosis differ from mitosis? How is it similar? Where does meiosis occur? Mitosis occurs in almost all organs and tissues of an organism, and the process of mitosis occurs when a tissue needs to replicate its cells for organ growth or for tissue repair or normal tissue cell turnover.

5. What would be the consequence to a cell if the gene that codes for production of cellular protein p53 is damaged? Since p53 is produced in response to damaged DNA during the G1 checkpoint to prevent subsequent S phase replication of damaged DNA, mutated cells would be propagated.

6. You suspect a cancer exists in a tissue biopsy from a tumor growing in a patient's brain. What would you look for in the stained tissue preparation from the biopsy that would cause you to suspect brain cancer? Answer: Cells in mitosis, that is cell with visible chromosomes. Nerve cells should not be dividing, so should not exhibit M phase cell cycle characteristics.

7. List three agents that may cause cells to become cancerous. Viruses, repeated trauma (physical, chemical exposure, or radioactive exposure), inherent gene malfunction.

1. Chemical substance X was applied to a cell culture but did not cause the cells to become cancerous. Chemical substance Y was also applied to the same cell culture (with substance X absent) but did not cause the cells to become cancerous. When both substances X and Y were applied to the cell culture the cells became cancerous. What can you deduce about substances X and Y as to whether they are carcinogenic, and as to their category of carcinogen, if any? One of the substances, X or Y, must be a weak carcinogen, and the other substance is a promoter carcinogen that amplifies the carcinogenic ability of the other.

2. Illustrate the phases (stages), in order, for mitosis, labeling each phase (stage). PROPHASE (DNA and associated histone proteins coils into dark staining bodies called chromosomes, easily visible with a light microscope; chromosomes are now actually comprised of 1 original chromosome and 1 replicated chromosome; each half of each chromosome at this stage is a chromatid, consisting of a dsDNA molecule; nuclear membrane dissolves), METAPHASE (chromosomes align in the middle of the cell, that is along an equatorial plane; centrioles extend spindle fibers that attach near the center of each chromatide); ANAPHASE (chromatids are pulled apart by spindle fiber microtubules so that 1/2 of the DNA (one set of chromosomes) goes to one end of the cell and 1/2 of the DNA goes the other end of the cell); TELOPHASE (chromosomes uncoil and become non-visible again, as in interphase, and the nuclear membrane forms again around the DNA; cell membrane splits down the center of the cell to begin forming two cells; cytokinesis.)

3. Working for an industrial biotechnology company, you are asked to grow a large quantity of bacteria to be processed for creation of a new food source to help solve world hunger (The bacteria have been genetically engineered to produce various palatable flavors as part of their cytoplasm). Grown in what is known as a chemostat, the culture will be provided with adequate nutrients and cell waste products will be removed. You begin the culture with one bacterial cell having a biomass of 0.1 nanogram (0.1ng). The cell replicates every 10 minutes. How much bacterial biomass can you create in two days, that is 48 hours, of culturing? {Hint: Determine the total number of doublings that will occur in 48 hours and call this quantity 'n'. Then the total number of cells grown will be 2 raised to the n.} If the cells are 80% water, what amount of dried bacterial food chips will you be able to create form the 48 hour culture?

Answers: At 6 doublings per hour (60 minutes/10 minutes per doubling) there will be 48x6=288 doublings or cell generations total. The total number of cells at 48 hours will be 2 raised to 288, that is 4.9732 followed by 86 zeros (4.9732 EE86)! The viable cell mass would be 4.9732 EE85 nanograms, or 4.9732 EE75 grams, that is 4.9732 EE72 kilograms! Multiplied by .20 (20%), the dried cell biomass would be 0.99464 EE72 kilograms!

4. A muscle biopsy was taken from Mr. S., a 53 year-old man who complained of chronic muscle pain. A cervical biopsy was taken from Ms.Q. who was noted to have irregular menstrual periods. The muscle biopsy is seen below on the left. The cervical biopsy is seen below on the right. Both microphotographs were taken at 400x total magnification. Do either or both of the biopsies appear abnormal? {Hint: Estimate the mitotic index of each tissue.} The muscle biopsy is abnormal, showing cells undergoing mitosis, that is mitotic figures are evident. Muscle tissue in an adult should not show cells dividing. The cervical biopsy is normal in that no cells are seen in mitosis.

5. Two substances, S1 and S2, are show to stimulate cell growth and cell replication in a cell line. Research data is collected for the two substances as seen in the table below, showing cellular ATP levels at various time intervals after S1 and S2 are applied to the cell cultures. Deduce whether S1 or S2 uses the second messenger mechanism.

Cytosol ATP Levels in Cell at Various Times After Dripping S1 or S2 onto Cell Cultures.

S1 uses cAMP (second messenger), evident from the sharp drop in cell ATP levels as ATP is used to create cAMP. S2 simply shows variations in ATP levels as a result of cell metabolism.

E1. Write an argument, approximately 500-1000 words in length, for either binary fission or mitosis as a more efficient mechanism for cell replication. Binary fission or mitosis - your choice. You may be opinionated, but you must back your opinions with factual knowledge from the chapter.

E2. Telomeres have been discovered to play a critical role in cell aging. Hypothesize as to three other factors that occur as part of the cell cycle that might play important roles in cell aging and eventual cell death. Support your hypothesis with factual knowledge gained from the study of this chapter.

Copyright 1998-1999 by Randall Oelerich. All Rights Reserved. No part of this document may be reproduced, except for printout for personal study, without the express written permission of the author. Please DO add links to this webpage on the internet.

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Cell Replication / Cell Division

Cancer & Biopsy

Karl Loren Comment: Some of this is excellent material. Some is false. Read with care. The false data concerns "how" cancer is started. This is a "fair comment" on copyrighted material.

Cell Replication / Cell Division

Mitosis, Meiosis, Budding, Fission. The biology of cell replication (cell division) including implications of cell division to human aging and cancer.