CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

9. Cell Division—Proliferation and Reproduction

9.7. Cell Division and Sexual Reproduction

Meiosis is a form of cell division involved in sexual reproduction. Meiosis has a different function than mitosis, the cell division that we have just been discussing. Mitosis is responsible for growth and repair of tissues. Meiosis is responsible for the production of eggs and sperm. The cells of sexually reproducing organisms have two sets of chromosomes and thus have two sets of genetic information. One set was received from the mother’s egg, the other from the father’s sperm. It is necessary for organisms that reproduce sexually to form sex cells having only one set of chromosomes.

If sex cells contained two sets of chromosomes, the zygote resulting from their union would have four sets of chromosomes with twice the total genetic information of the parents. With each new generation, the number of chromosomes would continue to increase. Thus, eggs and sperm must be formed by a method that reduces the amount of genetic information by half.

Scientists have terms to distinguish when a cell has either one or two copies of genetic information. Haploid cells carry only one complete set of their genetic information. Diploid cells carry two complete sets of their genetic information. Meiosis is the cell division process that generates haploid reproductive cells from diploid cells. In many sexually reproducing organisms, such as humans, meiosis takes place in the cells of organs that are devoted to reproduction—the gonads. The gonads in females are known as ovaries; in males, testes. Ovarian and testicular cells that divide by meiosis produce reproductive cells called gametes. Gamete is a general term for reproductive cells like eggs and sperm. These gametes are also referred to as germ cells. Algae and plants also possess organs for sexual reproduction. Some of these are very simple. In algae such as Spirogyra, individual cells become specialized for gamete production. In plants, the structures are very complex. In flowering plants, the pistil produces eggs, or ova, and the anther produces pollen, which contains sperm (figure 9.19).

FIGURE 9.19. Haploid and Diploid Cells

Both plants and animals produce cells with a haploid number of chromosomes. The male anther in plants and the testes in animals produce haploid male cells, sperm. In both plants and animals, the ovaries produce haploid female cells, eggs.

In sexually reproducing organisms, the life cycle involves both mitosis and meiosis. In figure 9.20, the haploid number of chromosomes is noted as n. The zygote and all the resulting cells that give rise to the adult fruit fly are diploid. The diploid number of chromosomes is noted as 2n—mathematically, n + n = 2n. The gametes are produced by meiosis in female and male adult fruit flies. Notice that the male and female gamete each contain 4 chromosomes. Collectively, these 4 chromosomes represent one complete set of all the genetic information that is necessary for a fruit fly.

FIGURE 9.20. The Life Cycle of a Fruit Fly

(a) The diploid cells of this adult fruit fly have 8 chromosomes in their nuclei. (b) In preparation for sexual reproduction, the number of chromosomes must be reduced by half, so that the gametes will be haploid and have 4 chromosomes, (c) When the egg and sperm unite during fertilization, the original diploid number of 8 chromosomes will be restored. (d) The offspring will grow and produce new cells by mitosis.

Fertilization is the joining of the genetic material from two haploid cells. During fertilization, each gamete contributes one set of genetic information (one set of chromosomes) toward forming a new organism. Recall that the zygote is the diploid cell that results from the egg and sperm combining their genetic information. The zygote contains two sets of genetic information on 8 chromosomes (4 from the egg and 4 from the sperm—two sets of chromosomes). The zygote divides by mitosis and the cells grow to become an adult fruit fly, which will then produce either eggs or sperm by meiosis in its gonads. The characteristics of the fruit fly will depend on the combination of genetic information it inherits from both parents on its 8 chromosomes.

Diploid cells have two sets of chromosomes—one set from each parent. Because chromosomes contain DNA, each chromosome has many genes along its length. Each chromosome in a diploid cell can be paired to another chromosome on the basis of the genes on those chromosomes. Homologous chromosomes have the same order of genes along their DNA (figure 9.21). Because of the similarity of genetic information in homologous chromosomes are the same size and their centromeres are found in the same locations. Each parent contributes one member of each of the pairs of the homologous chromosomes. Non-homologous chromosomes have different genes on their DNA. The fruit fly has four pairs of homologous chromosomes—or 8 total chromosomes. Different species of organisms vary in the number of chromosomes they contain (table 9.2).

FIGURE 9.21. A Pair of Homologous Chromosomes

A pair of chromosomes are said to be homologous if they have genes for the same traits. Notice that the genes may not be identical, but the genes code for the same type of information. Homologous chromosomes are of the same length, have the same types of genes in the same sequence, and have their centromeres in the same location—one chromosome came from the male parent and the other from the female parent.

TABLE 9.2. Chromosome Numbers

Before we move on and describe meiosis in detail, consider the different purposes of mitosis and meiosis: Mitosis results in cells that have the same number of chromosomes as the parent cell, whereas meiosis results in cells that have half the chromosomes as the parent cell. An important question to ask is, “how are the processes of mitosis and meiosis different, so that gametes receive only half of the parent cell’s chromosomes?”

9.7. CONCEPT REVIEW

18. How do haploid cells differ from diploid cells?

19. Why is meiosis necessary in organisms that reproduce sexually?

20. Define the terms zygote, fertilization, and, homologous chromosomes.

21. Diagram fertilization as it would occur between a sperm and an egg with the haploid number of 3.