CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

9. Cell Division—Proliferation and Reproduction

Unregulated Cell Division Can Result in Cancer

New findings reveal how cells can become tumors.

Cancer occurs when there is a problem with controlling how cells divide and replace themselves. A tumor forms when cells divide in an unregulated manner. As a tumor grows, some of its cells may change and move out of the tumor, enter the circulatory system, and establish new tumors in other places.

Scientists are starting to understand how cell growth is regulated. The picture that is emerging from this research is that many proteins are involved in cell growth regulation. When certain changes occur in the proteins (i.e., histones) that regulate the cell’s growth, the cell might divide when it should not. Sometimes these mutations are inherited. Individuals with these mutations are more likely than others to develop cancer. Sometimes these mutations occur because of exposure to something in the environment.

• How does a mutagen cause cancer?

• How do chemotherapy and radiation treatments stop cancer?

• If components of smoke from coal-fired power plants cause cancer, should laws be passed to regulate such emssions?

ü  Background Check

Concepts you should already know to get the most out of this chapter:

• The organization of the cell and its nucleus (chapter 4)

• The function of enzymes in the cell (chapter 5)

• The genetic information of eukaryotic cells is found in DNA that is packaged into chromosomes (chapter 8)

9.1. Cell Division: An Overview

Two fundamental characteristics of life are the ability to grow and the ability to reproduce. Both of these characteristics depend on the process of cell division. Cell division is the process by which a single cell generates new daughter cells. Cell division serves many purposes. For single-celled organisms, it is a method of increasing their numbers. For multicellular organisms, it is a process that leads to growth, the replacement of lost cells, the healing of injuries, and the formation of reproductive cells.

There are three general types of cell division, each involving a parent cell. The first type of cell division is binary fission (figure 9.1). Binary fission is a method of cell division used by prokaryotic cells. During binary fission, the prokaryotic cell’s single loop of DNA replicates, and becomes attached to the plasma membrane inside the cell. As a membrane forms inside the cell, the two DNA loops become separated into two daughter cells. This process ensures that each of the daughter cells receives the same information that was possessed by the parent cell. Some bacteria, such as E. coli, are able to undergo cell division as frequently as every 20 minutes. The second type of cell division, mitosis, is a method of eukaryotic cell division; like binary fission, it also results in daughter cells that are genetically identical to the parent cell. Eukaryotic cells have several chromosomes that are replicated and divided by complex processes between two daughter cells. The third type of cell division is meiosis, a method of eukaryotic cell division that results in daughter cells that have half the genetic information of the parent cell. These daughter cells contain half the genetic information of the parent cell, are not genetically identical to the parent cell from which they were produced, and can be used in sexual reproduction.

FIGURE 9.1. Binary Fission

This asexual form of reproduction occurs in bacteria. Each daughter cell that results has a copy of the loop of DNA found in the parent cell.

Asexual Reproduction

For single-celled organisms, binary fission and mitosis are methods of asexual reproduction. Asexual reproduction binary fission and mitosis requires only one parent that divides and results in two organisms that are genetically identical to the parent. Prokaryotes typically undergo binary fission whereas single-celled and multicellular eukaryotes undergo mitosis.

In multicellular organisms, mitosis produces new cells that

• cause growth by increasing the number of cells,

• replace lost cells, and

• repair injuries.

In each case, the daughter cells require the same genetic information that was present in the parent cell. Because they have the same DNA as the parent cell, the daughter cells are able to participate in the same metabolic activities as the parent cell.

Sexual Reproduction

Sexual reproduction requires two parents to donate genetic information when creating offspring. The result of sexual reproduction is a genetically unique individual.

Meiosis is the process that produces the cells needed for sexual reproduction. Meiosis is different from mitosis; in meiosis, reproductive cells receive half of the parent cell’s genetic information. The full complement of genetic information is restored after the reproductive cells (sperm and egg) join.

Understanding the purposes of cell division is an important part of understanding how cell division ensures that the daughter cells inherit the correct genetic information.

9.1. CONCEPT REVIEW

1. What are the three general types of cellular reproduction?

2. What is the purpose of binary fission and mitosis in comparison to meiosis?