Cell and Molecular Biology-Chapter 13

Meiosis is the nuclear division that produces the specialized reproductive cells in animals called gametes. To understand meiosis you must understand certain things about the chromosomal organization of a cell. All somatic cells in a diploid organism contain two of each kind of chromosome. Members of each pair of the same kind of chromosomes are referred to as homologous chromosomes. Homologous chromosomes are the same length and have their centromeres located the same distance along their length. They also contain the same set of genes and each gene occurs at a particular location (locus) along the DNA (Fig 13.3). A gene is a length of DNA that when translated generates a functional polypeptide (or sometimes is transcribed into a functional RNA). Typically a gene controls a specific character associated with the organism (e.g., hair color). Diploid cells contain two copies of each gene, but these copies need not be identical (e.g., one may be for blond hair color and the other for brown) but they could be identical. Each copy is referred to as an allele. The egg of a female contains one of each kind of chromosome (i.e., it is haploid) and is fertilized by a sperm that also is haploid and contains one of each kind of chromosome. Therefore, the fertilized egg (the zygote) becomes diploid and through mitotic divisions and cytokinesis gives rise to all future somatic cells in the multicellular animal. (Fig. 15.4, 13.5)

There is one exception to diploid cells having two of each kind of chromosome and this involves one pair of chromosomes called the sex chromosomes. In mammals, females contain two relatively long sex chromosomes (X chromosomes) that are like other homologous chromosomes. Mammalian males contain one X chromosome but another small sex chromosome (Y chromosome) that, despite missing many genes found on the X chromosome, behaves during meiosis as if it is homologous to the X chromosome. All other chromosomes are referred to as autosomal chromosomes (autosomes).

There are three basic life cycles and all eukaryotic species employ one of these life cycles. In animals, meiosis immediately precedes the formation of gametes. In the other life cycles, gametes are produced by the mitotic division of haploid cells. Note that there is an alternation of generations in the life cycle of plants (Fig. 13.6).

Meiosis represents a series of two consecutive nuclear divisions. (Fig. 13.7, 13.8) The first of these divisions reduces the number of chromosomes in the original nucleus in half. If the original nucleus is diploid, each of the nuclei resulting from the first division will have one chromosome from each homologous pair of chromosomes. One of the two chromosomes making up each homologous pair of chromosomes arose from the paternal ancestor of that cell (the other chromosome coming from the maternal ancestor). This forms the cellular basis for Mendel's Law of Segregation that will be discussed in the next chapter. It is totally random within which of the two nuclei any paternally (or maternally) descended chromosome becomes incorporated. Because of this, each of the two nuclei arising from the first division has one of each kind of chromosome and these are a random mix of paternally and maternally descended chromosomes. This forms the cellular basis for Mendel's Law of Independent Assortment that will be discussed in the next chapter. Because the first meiotic division reduces the number of chromosomes in half, it is often referred to as the reduction division. (Fig. 13.9)

The second division in meiosis produces two nuclei from each of the two nuclei produced from the reduction division. But, each nucleus produced from the second division has the same number of chromosomes as the nuclei did following the reduction division. Therefore this second division is mitotic-like and is referred to as the equational division. Like mitosis, each meiotic division is broken down into stages. Those stages have a number appended to their name that is I for stages of the reduction division and II for stages of the equational division. Prophase II, Metaphase II, Anaphase II, and Telophase II are quite similar to their counterparts in mitosis. However. Prophase I, Metaphase I, and Anaphase I are quite different. (Fig. 13.9)

In Prophase I, homologous chromosomes pair up with each other in a precise alignment.This pairing is called synapsis. Each chromosome consists of two identical sister chromatids. During Prophase I, chromatids randomly break and rejoin along their lengths. Sometimes simultaneous breaks occur in chromatids from homologous chromosomes and rather the chromatids rejoining and forming their previous configuration, segments of nonsister chromatids from homologous chromosomes join forming uniquely new chromatids. This process is referred to as crossing over. After crossing over occurs Xs appear to exist that unite homologous chromosomes during Prophase I. These are called chiasmata. (Fig. 13.8, 13.9)

The paired homologous chromosomes migrate to the equatorial plane under the control of microtubules bound to kinetochores. The homologous chromosomes arrange themselves such that one member of each pair line up opposite each other, one on each side of the equatorial plane. It is random which side of the equatorial plane the maternally and paternally descended chromosomes for any homologous pairs are positioned. This stage is Metaphase I.

In Anaphase I all chromosomes on the same side of the equatorial plane migrate to a centrosome on that side, the other chromosomes migrate to the other centrosome.

After both meiotic divisions have occurred the result (assuming cytokinesis) is 4 cells each with a haploid set of chromosomes. In animals, these cells go through certain developmental modifications and become sperm or eggs. When a sperm fertilize an egg the zygote formed is once again diploid.

Meiosis leads to genetic variability. If we momentarily disregard crossing over, we recognize that each gamete typically contains a random collection of some maternally and paternally descended chromosomes. In addition, some of these chromosomes are new combinations of some of the original maternally and paternally decended chromosomes that arose due to crossing over in Prophase I. This shuffling of the genetic messages is called genetic recombination. (Fig. 13.11, 13.12)