6

6.1 Prokaryotes Have a Simple Cell Cycle

6.2 Eukaryotes Have a Complex Cell Cycle

6.3 Chromosomes

6.4 Cell Division

6.5 Controlling the Cell Cycle

6.9 Discovery of Meiosis

6.10 The Sexual Life Cycle

6.11 The Stages of Meiosis

6.12 Unique Features of Meiosis

6.13 The Evolutionary Consequences of Sex

6.6 What Is Cancer?

6.7 Cancer and Control of the Cell Cycle

6.8 Curing Cancer

Animations
(Animation Requirements)

• DNA Packaging
• Mitosis

Art Labeling Activities

• Plant Cell Mitosis

Explorations

Mitosis: Cell Cycle Regulation
In this exercise, you can explore the ways a cell decides if and when it will divide, a decision that, if made inappropriately, may lead to cancer.

Exploring Meiosis
In this exercise, you can explore how the failure of chromosomes to separate during meiosis can result in Down syndrome.

Curing Cancer. In 2000, over half a million Americans died of cancer, a growth disorder of cells caused by damage to genes regulating the cell division cycle. In some cases cancerous growth is initiated by the inappropriate activation of proteins that regulate the cell cycle. In others, it results from inactivation of proteins that normally suppress cell division.

1. Can scientists starve cancer tumors?
2. Understanding the processes that lead to cancer.
3. Why is deadly lymphoma becoming more common?
4. High-fiber diets don't protect against colon cancer.
5. Curing colon cancer by blocking cell receptors.
6. The cancer war is yielding promising results.
7. Oncogene blockers — the newest weapons against cancer.

Progress in Curing Cancer
Cancer is a cell growth defect, a breakdown in the cell's control of its proliferation. Progress in curing cancer is focused on the elements of this breakdown. Two sorts of damage to genes must occur to produce cancer. As a crude analogy, imagine a car parked on the side of a road. To get it moving you must step on the accelerator and release the brake. Accelerators: Chemical signals called growth factors pass a "divide" signal from the cell surface to the nucleus, where the divide instructions are to be carried out. Any change that facilitates any stage of this information pathway will promote cancer. Releasing the Brakes: Cells have three safeguards against uncontrolled cell division, all of which must be removed before cancer can occur. First, cells have proteins called tumor suppressors that normally block cell division which are temporarily inactivated during normal cell division. In cancer cells they are permanently inactivated. Second, cells have error-correcting proteins such as p53 that detect damage to genes that might initiate cancer. In most cancers this error-detection has been disabled. Third, cancer cells rebuild the tips of their chromosomes, removing the normal limit on the cell's life span.

Can Cancer Tumors Be Starved to Death?
One of the most exciting recent developments in the war against cancer is the report that it might be possible to starve cancer tumors to death. In order for a tumor to grow, it needs an ample supply of blood. A tumor increases its blood supply by secreting substances into the surrounding tissues that encourage angiogenesis, the formation of small blood vessels. As a solid tumor grows and outstrips its blood supply, its interior becomes hypoxic (oxygen depleted). In response to hypoxia, it appears that genes are turned on that promote survival under low oxygen pressure, including ones that promote angiogenesis. How does hypoxia within a tumor promote angiogenesis?

Randall Johnson of the University of California, San Diego, is studying one possible way. A tumor responding to hypoxia induces a gene-specific transcription factor (a protein that activates the transcription of a particular gene) that promotes angiogenesis. Called HIF-1, for hypoxia inducible factor-1, this transcription factor appears to play a critical role in inducing the transcription of genes necessary for blood vessel formation.