Timing of Cell Division

One of the most important questions in biology is what signals cells when to divide and when to stop. The activation and inhibition of cell division are subjects of intense research for obvious reasons such as management of cancer and tissue repair. Cells divide when (1) they grow large enough to have enough cytoplasm to distribute to their two daughter cells; (2) they have replicated their DNA, so they can give each daughter cell a duplicate set of genes; (3) they receive an adequate supply of nutrients; (4) they are stimulated by growth factors, chemical signals secreted by blood platelets, kidney cells, and other sources; or (5) neighboring cells die, opening up space in a tissue to be occupied by new cells. Cells stop dividing when nutrients or growth factors are withdrawn or when they snugly contact neighboring cells. The cessation of cell division in response to contact with other cells is called contact inhibition.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Describe the genetic roles of DNA helicase and DNA polymerase. Contrast the function of DNA polymerase with that of RNA polymerase.
  2. Explain why DNA replication is called semiconservative.
  3. Define mutation. Explain why some mutations are harmless and others can be lethal.
  4. List the stages of the cell cycle and summarize what occurs in each one.
  5. Describe the structure of a chromosome at metaphase.
Figure 4.14 Chromosome Structure. (a) A metaphase chromosome. (b) Transmission electron micrograph. (c) Scanning electron micrograph.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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