Unit 6: The Cell Cycle

Objectives:

• List the phases and describe the processes that are undergone in each

phase of the cell cycle.

• Compare/contrast binary fission, conjugation, and mitosis.

• Differentiate between chromatin, chromosomes, and chromatids.

• Explain what signals cells to divide.

• Explain the effects of telomeres and telomerase on cell division.

• Explain what can sometimes cause cancer and how to avoid the problem.

Vocabulary:

Mitosis * G Phase * Interphase * S Phase * Prophase * Somatic * Metaphase * Anaphase * Telophase * Centromere * Centriole * Spindle Fibers * Cytokinesis * Chromatid * Chromosome * Histone Chromatin * Telomere * Telomerase * Apoptosis * Contact Inhibition * Cancer * Oncogenes * Binary Fission * Conjugation * Pili * Synthesize * Karyokinesis * Benign Tumors * Malignant

The nutrition of a cell and its ability to eliminate wastes is very much

dependent on the ratio between its cell membrane and internal area.

Since all nutrients must enter a cell through its membrane, and all

wastes leave through the membrane, a cell’s growth and survival is

dependent upon adequate movement of these substances into and out

of a cell.

When a cell grows too large, the ratio of cell membrane to internal

area decreases. Therefore, it becomes difficult to transport enough

materials across the membrane to support the cell’s life. To avoid

death, the cell will begin the process of cell division (The smaller the

cell, the greater its surface {cell membrane} area to internal area ratio.).

In more primitive single cells, like the prokaryotic cells of bacteria and

the simple eukaryotic cells of some protists, binary fission is possible. Binary

fission is a form of asexual cell reproduction where a cell simply copies its

genetic information and other cellular materials before splitting itself in two. The

new cells are genetically identical. This is a simple way for a single-celled

organism to divide in a stable environment.

Some of the same single celled organisms that undergo binary fission can

also undergo conjugation. Conjugation is the exchange of genetic information

from one organism to the next. (Some call it sex without reproduction.) This

may only involve only a small portion of the genetic material being copied and

transferred across a straw-like bridge often called a pili. For example, a

bacterium resistant to penicillin can transfer just that part of its genome to non-

resistant bacteria. Unfortunately, this can create enormous colonies of anti-

biotic resistant bacteria. Conjugation is beneficial when exposed to a hostile

environment. However, no eggs or sperm are produced and the cell may or

may not even divide after the genetic exchange.

Mitosis in multi-cellular organisms is very similar to binary fission. Like

binary fission, mitosis produces two identical daughter cells. However, in most

cases, each daughter cell is specialized to perform the function of the original

cell. Ex: skin cells produce more skin cells There is no exchange of genetic

information.

The Cell Cycle

1) Interphase used to be thought of as a resting stage because it was difficult to see anything going on under a microscope. We now know that this is the phase where the genetic material is copied and organelles double in number. During this phase, the genetic material remains loose and “stringy” in appearance. This is called chromatin and it is in this state that DNA can be copied.

Interphase is divided into the:

a) G1, or “gap”, phase which is when the cell will receive the signal to either divide, take time to repair DNA, continue functioning normally but not divide, or die. (Programmed cell death is called apoptosis.) This phase can go very quickly or last for years before a signal changes the normal, non-dividing functional state. If the cell is preparing to divide, it will make (synthesize) proteins, lipids, and carbs that will be needed for the two daughter cells to form correctly. Cells in the intestines, bone marrow, bladder, etc. may go through this phase very quickly while other cells take years. This is unusual in that all of the other phases take relatively the same amount of time to complete in all cells.

b) S (synthetic) phase activity includes copying the DNA by assembling nucleotides into the new DNA strands. In addition, proteins included in the chromosomes and many enzymes are synthesized.

c) G2 phase involves more protein synthesis, cell membrane material is made and stored for use in the daughter cells, and DNA begins to tighten around the histone protein that forms the center part of chromosomes. Interphase is followed by the M phase, or mitosis. Mitosis is the division of somatic (non-sex cells, not sperm or eggs) and can be divided into the following phases:a) Prophase - Chromatin wraps tightly around the histone protein forming “sister” chromatids (identical copies of the original

chromosome) held together by centromeres. In most animal cells, centrioles will form now. Centrioles will later reach out protein microtubules called spindle fibers. These will later reach out to the centromeres to help separate the sister chromatids to opposite sides of the cell.

b) Metaphase - The nuclear membrane begins to break up. The spindle fibers reaching out from the centriole guide the chromosomes to the equator (middle) of the cell where they are nicely aligned.c) Anaphase - The sister chromatids’ centromeres are tugged apart by the spindle fibers, separating the sister chromatids to opposite sides of the cell.d) Telophase - The separated sisters are now individual chromosomes that are surrounded during telophase by new nuclear membranes. This separation of the nuclear material is sometimes called karyokinesis.

After mitosis, the full division of the cells occurs, i.e. the cytoplasm is divided in two, forming two completely separate cells. This is called cytokinesis. In animal cells, the cell membrane pinches inward to separate the cells. In plant cells, a new cell wall is built between the new nuclear areas. In some slime molds, etc. karyokinesis is not followed by cytokinesis. This creates a a multi-nucleated cell.

The cell cycle has several checkpoints to keep cells healthy and

controlled in number. “Proofreading” enzymes check for misread DNA,

clip out the wrongly substituted base, and glue in the correct base -

when all is performing well. And to prevent over multiplication of an

original cell, chromosomes have end-caps called telomeres. These

are repetitive sequences of the 6 base DNA code TTAGGG. Each time

a cell divides, it normally loses part of the telomere end-cap. This acts

as a sort of “biological clock”.

Eventually, the end-cap is so short, the cell will no longer divide and

natural cell death (apoptosis) occurs. However, some cells that need to

continually divide, like sperm cells, can keep their telomeres long. A

special reverse transcriptase enzyme called telomerase reads the RNA

sequence AAUCCC to continually add more to the telomeres.

In addition, most cells exhibit contact inhibition. For example, when

a cut destroys some skin cells, new skin cells fill the area but sense

when to stop dividing when they touch other healthy skin cells.

Unfortunately, if something interferes with this process, tumors can

develop. Tumors that do not spread to other areas are called benign.

When cell growth is unchecked, tumors form. Benign tumors usually are not life threatening unless they grow very large and put pressure on other organs, stopping them from functioning correctly. However, benign tumors sometimes convert to malignant tumors. This is when some of the unchecked, dividing cells break off and are transported to other parts of the body where they start reproducing out of control. The cause of these cancers is not always known. Viruses and environmental chemicals that enter the nuclear area of normal cells can disrupt and damage the DNA. This can cause loss of reproductive control. Maintaining a healthy liver and kidneys can help remove environmental toxins from the body before they do damage. In Europe, milk thistle (silymarin) has been used to help the liver regenerate and repair itself so that it can detoxify harmful chemicals effectively. The herb called gravel root is often used for kidney health. Healthy cell membranes (omega-3 fatty acids from salmon, flaxseed, etc, vitamin E, and more) can help resist viral entry into the cell. And, certain herbs and pharmaceutical drugs block the specific receptor sites in cell membranes where certain viruses need to dock. (Exs. Melissa officianalis, aka Lemon Balm, blocks the entry point for herpes viruses that cause cold sores, shingles, mononucleosis, etc. Elderberry, Sambucus nigra, blocks the respiratory flu virus.) And, vitamin & mineral deficiencies leave us at higher risk for cancer.

Oncogenes can also cause cancer. These are genes that are

normally supposed to stimulate cell reproduction for growth and repair

but are now out of control so they don’t turn off when they should. Or,

tumor suppressor genes which normally stop the cell from dividing can

be turned off when they should be on. A non-functioning p53 gene is

often the cause here, but there are other genes that also can be to

blame.

Remember, certain nutrients reduce the risk of getting cancer in the

first place. Zinc is necessary for normal cell apoptosis. Selenium

seems to prevent many cancers. In fact, zinc and selenium seem to be

very effective at reducing the risk of prostate cancer. Calcium appears

to reduce the risk of colon cancer. And, remember, cancer cells have

24 times more glucose receptors than normal cells! Sugars/carbohydrates feed

cancer! Vitamin C can be absorbed by cancer cells to help kill them if sugar is

kept very low and vitamin C is kept very high in the diet. Some nutritionally

oriented medical practitioners will use vitamin C intravenously to get it high

enough to help without causing diarrhea or stomach upset.