Meiosis

When a cell is going to divide, either by mitosis or meiosis, the DNA is replicated (forming chromatids) and the chromosomes become shorter and thicker, as previously described. At this point the cell has forty-six chromosomes, each of which consists of two duplicate chromatids.

The short, thick chromosomes seen at the end of the G2 phase can be matched as pairs, the members of each pair appearing to be structurally identical. These matched chromosomes are called homologous chromosomes. One member of each homologous pair is derived from a chromosome

Mitosis And Meiosis Immunofluorescence

■ Figure 3.31 Chromosomes and spindle fibers. The duplicate chromatids are clearly seen in (a), though the spindle fibers are just barely visible. In a technique callled immunofluorescence, the spindle fibers shine in (b) due to a reaction with microtubules, the major constituent of the spindles.

76 Chapter Three

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■ Figure 3.32 A karyotype, in which chromosomes are arranged in homologous pairs. A false-color light micrograph of chromosomes from a male arranged in numbered homologous pairs, from the largest to the smallest.

Table 3.3

Stages of Meiosis

Stage

Events

First Meiotic Division

Prophase I

Chromosomes appear double-stranded.

Each strand, called a chromatid, contains

duplicate DNA joined together by a structure

known as a centromere.

Homologous chromosomes pair up side by side.

Metaphase I

Homologous chromosome pairs line up at

equator.

Spindle apparatus is complete.

Anaphase I

Homologous chromosomes separate; the two

members of a homologous pair move to

opposite poles.

Telophase I

Cytoplasm divides to produce two haploid cells.

Second Meiotic Division

Prophase II

Chromosomes appear, each containing two

chromatids.

Metaphase II

Chromosomes line up single file along equator

as spindle formation is completed.

Anaphase II

Centromeres split and chromatids move to

opposite poles.

Telophase II

Cytoplasm divides to produce two haploid cells

from each of the haploid cells formed at

telophase I.

inherited from the father, and the other member is a copy of one of the chromosomes inherited from the mother. Homologous chromosomes do not have identical DNA base sequences; one member of the pair may code for blue eyes, for example, and the other for brown eyes. There are twenty-two homologous pairs of autosomal chromosomes and one pair of sex chromosomes, described as X and Y. Females have two X chromosomes, whereas males have one X and one Y chromosome (fig. 3.32).

Meiosis, which has two divisional sequences, is a special type of cell division that occurs only in the gonads (testes and ovaries), where it is used only in the production of gametes— sperm cells and ova. (Gamete production is described in detail in chapter 20.) In the first division of meiosis, the homologous chromosomes line up side by side, rather than single file, along the equator of the cell. The spindle fibers then pull one member of a homologous pair to one pole of the cell, and the other member of the pair to the other pole. Each of the two daughter cells thus acquires only one chromosome from each of the twenty-three homologous pairs contained in the parent. The daughter cells, in other words, contain twenty-three rather than forty-six chromosomes. For this reason, meiosis (from the Greek meion = less) is also known as reduction division.

At the end of this cell division, each daughter cell contains twenty-three chromosomes—but each of these consists of two chromatids. (Since the two chromatids per chromosome are identical, this does not make forty-six chromosomes; there are still only twenty-three different chromosomes per cell at this point.) The chromatids are separated by a second meiotic division. Each of the daughter cells from the first cell division itself divides, with the duplicate chromatids going to each of two new daughter cells. A grand total of four daughter cells can thus be produced from the meiotic cell division of one parent cell. This occurs in the testes, where one parent cell produces four sperm cells. In the ovaries, one parent cell also produces four daughter cells, but three of these die and only one progresses to become a mature egg cell (as will be described in chapter 20).

The stages of meiosis are subdivided according to whether they occur in the first or the second meiotic cell division. These stages are designated as prophase I, metaphase I, anaphase I, telophase I; and then prophase II, metaphase II, anaphase II, and telophase II (table 3.3 and fig. 3.33).

The reduction of the chromosome number from forty-six to twenty-three is obviously necessary for sexual reproduction, where the sex cells join and add their content of chromosomes together to produce a new individual. The significance of meio-sis, however, goes beyond the reduction of chromosome number. At metaphase I, the pairs of homologous chromosomes can line up with either member facing a given pole of the cell. (Recall that each member of a homologous pair came from a different parent.) Maternal and paternal members of homologous pairs are thus randomly shuffled. Hence, when the first meiotic

Fox: Human Physiology, I 3. Cell Structure and I Text I I © The McGraw-Hill

Eighth Edition Genetic Control Companies, 2003

Prophase I

Metaphase I

- Tetrad ph if

Anaphase I

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