Transcript Unit 6: The Cell Cycle

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 nonresistant bacteria. Unfortunately, this can create enormous colonies of antibiotic 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.