1. Day 9 September 25th Chapter 6
Dr Amy B Hollingsworth
The University of Akron
2. On My To-Do List
1. Grade Homework from Sept 19th (me)
2. Post Homework opportunity on Springboard
(me) – will be due Sunday at 11:30
3. Post last lecture recording (I was in hospital)
4. Answer student emails
5. Input Interim Reports
3. Chapter 6: Chromosomes
and Cell Division
Insert new photo (Jackson 5)
Continuity and variety
Lectures by Mark Manteuffel, St. Louis Community College
5. 6.1 Immortal cells can spell
trouble: cell division in
sickness and health.
6. Telomeres
The telomere is like a
protective cap at the
end of the DNA.
Every time a cell
divides, the telomere
gets a bit shorter.
Insert new fig 6-1
15. Complementarity
The characteristic that in the double-stranded DNA
molecule the base on one strand always has the
same pairing-partner (called the complementary
base) on the other strand
Every “A” (adenine) pairs with “T” (thymine) and
vice-versa.
Every “G” (guanine) pairs with “C” (cytosine) and
vice-versa.
18. Mutation
A variety of errors can occur during replication.
Several DNA repair processes occur after
replication.
If an error remains, however, the sequences in a
replicated DNA molecule (including the genes) can
be different from those in the parent molecule.
19. 6.6 Most cells are not immortal:
Mitosis generates replacements.
What is dust?
Why is it your fault?
20. Mitosis has just one purpose:
To enable cells to generate new, genetically
identical cells.
There are two different reasons for this need:
1. Growth
2. Replacement
21.
22. Apoptosis
The pre-planned process of cell suicide
Certain cells are targeted for apoptosis.
23. Mitosis
The number of (somatic) cells that must
be replaced by mitosis every day is huge.
The rate at which mitosis occurs varies
dramatically.
36. Cancer
Unrestrained cell growth and division…
…can lead to tumors…
…the second leading cause of death in the
United States! (20%, leading is heart
disease)
42. Why is the treatment for cancer often
considered as bad as the disease?
43. Cancer is unrestrained cell growth and division.
Cancer can lead to large masses of cells called
malignant tumors that can cause serious health
problems.
Treatment focuses on killing or slowing the
division of the cells using chemotherapy and/or
radiation.
Editor's Notes
Look around your dorm room. Dust is everywhere. What is it? It is primarily dead skin cells. In fact, you and your roommates slough off millions of dead skin cells each day. Yet your skin is not disappearing. How can that be? Obviously, your body is replacing the sloughed off cells. The cells have simply worn out, so your body creates replacements, preferably identical copies of the old cells. How does it do this? Mitosis.
1. Growth. During growth and development, organisms get bigger and must add new cells. In fact, if you want to see cell division in action, one sure-fire place to look is at the tip of a plant root because that is one of the fastest growing parts of a plant, at about half an inch per day (Figure 6-7 Part 1 Reasons for mitosis).
Some other cells that must be replaced actually die on purpose, in the pre-planned process of cell suicide called apoptosis. This seemingly counterproductive strategy is employed in parts of the body where the cells are likely to accumulate significant genetic damage over time and are therefore at high risk of becoming cancer cells (a process described later in this chapter). Cells targeted for apoptosis include many of the cells lining the digestive tract as well as those in the liver, two locations where cells are almost constantly in contact with harmful substances.
Every day, a huge number of cells in an individual must be replaced by mitosis. In humans this number is in the billions. Nearly all of the somatic cells of the body—that is, everything other than sperm- and egg-producing cells—undergo mitosis with a few notable exceptions. Brain cells and heart muscle cells, in particular, do not appear to divide, or, if they do divide, they do so at very, very slow rates. (It is not known why this is so.)
The rate at which mitosis occurs varies dramatically. The most rapid cell division occurs in the blood and among the cells lining various tissues and organs. The average red blood cell, for example, is in circulation only for about six weeks and then must be replaced, and the cells lining the intestines are replaced about every three weeks. Hair follicles, too, are among the most rapidly dividing cells.
For mitosis to begin, the parent cell replicates its DNA, creating a duplicate copy of each chromosome. Once this task is completed, the remainder of mitosis can take place, in which the chromosomes are separated into identical sets in two separate nuclei, and then the cell can divide into two duplicate cells, the daughter cells.
Mitosis occurs in just four steps. It cannot begin, however, until after an important event occurs during the previous portion of the cell cycle, interphase. During the synthesis portion of interphase, all of the chromosomes replicate. Mitosis then begins with 1) the condensing of the chromosomes, which during interphase are all stretched out and stringy. 2) Next, all of the duplicated and condensed pairs of chromosomes move to the center of the cell. 3) Each chromosome is pulled apart from its duplicate. 4) And finally, new cell membranes form around each complete set of chromosomes and the cytoplasm duplicates as well. Where once there was one cell, now there are two (Figure 6-10 A simplified introduction to mitosis).
Let’s look at the process in a bit more detail, keeping in mind that the ultimate consequence of the process is to produce two cells with identical chromosomes.
Interphase: in preparation for mitosis, the chromosomes replicate—Processes essential to cell division take place even before the mitotic phase of the cell cycle begins. During the DNA synthesis part of interphase, every chromosome creates an exact duplicate of itself by replicating. Prior to replication, each chromosome was just a long linear strand of genetic material. Following replication each chromosome is a pair of identical long linear strands, held together at the center, a position called the centromere. (Figure 6-11 Part 1 Mitosis: cell duplication, step by step).
Mitosis
1) The long, linear chromosomes that have replicated condense—Looking at a cell through a microscope, you won’t generally see any chromosomes. Mitosis officially begins when the chromosomes in the cell’s nucleus become more and more tightly coiled. As they condense, they become thick enough that they can be seen through a light microscope.
At this point, each chromosome looks like the letter X. This appearance is misleading; Chromosomes are not actually X-shaped. They are linear. Each X consists of two identical linear DNA molecules—a chromosome and its identical, replicated copy—joined at the centromere.
Each of the identical DNA molecules is called a chromatid; together, the two are called sister chromatids.
The reason for the X shape in most photos of chromosomes is that the only time the chromosome is coiled tightly and thus thick enough to be seen (and photographed) is during cell division.
How does cancer actually cause death? Somewhat surprisingly, it’s not because of some chemical or genetic property of the cancer cells. It’s simpler than that. As a tumor gets larger, it takes up more and more space, pressing against neighboring cells and tissue. Eventually, it may block them from carrying out their normal function and even kill them. This dysfunction or cell death can have disastrous consequences when the affected normal tissue controls processes critical to life, such as the regulation of breathing, heart functioning, or detoxification processes in the liver.
Figure 6-14 Cancer’s effects.
In order to treat cancer, the rapidly dividing cells must be removed surgically, killed, or at least slowed down. Currently this is done in two ways: chemotherapy and radiation. In chemotherapy, drugs are administered that interfere with cell division. The drugs are called anti-mitotic agents. Chemotherapy can slow the growth of tumors, but because the drugs interfere with cell division throughout the body they have terrible side effects. In particular, chemotherapy drugs disrupt normal systems that rely on the rapid and constant production of new cells. Chemotherapy almost always causes extreme fatigue and shortness of breath, for instance, as it reduces the rate at which red blood cells are produced, limiting the amount of oxygen that can be transported throughout the body. By interfering with bone marrow stem cell division, too, chemotherapy also reduces the production of platelets and white blood cells and thus increases bruising, causes bleeding, and increases an individual’s susceptibility to infection.
Radiation also works by disrupting cell division. Unlike chemotherapy drugs which circulate throughout the entire body, however, radiation therapy directs high-energy radiation at the part of the body where a tumor is located. Like chemotherapy, the radiation process is not perfect and nearby tissue is often harmed as well. The significant negative effects of chemotherapy and radiation treatment on normal tissue and the suffering this leads to has caused patients to comment that the treatment for cancer is often worse than the disease itself.
Is there hope for a complete cure soon? Not yet. Nonetheless, many potential therapies for cancer treatment and prevention are on the horizon. Extensive research is being conducted into the mechanisms by which damage to DNA controlling the cell cycle occurs, for example, and how it might be prevented or reversed.
In addition, there is a rapidly growing field of research into the many naturally occurring, cancer-inhibiting chemicals that have evolved in many plant species, such as resveratrol, which is found in grapes and peanuts, and that may be less harsh than current chemotherapy drugs. Advances are also being made in the area of environmental and behavioral modifications that reduce cancer risk, too, from changes in industrial processes to simple dietary or lifestyle changes. Advances in early screening, particularly in identifying cancer prior to metastasis, continue to have important benefits as well.
