Meiosis

Stages of Meiosis

Meiosis is more complex than mitosis, requiring two rounds of cell division.

The main events that occur during meiosis are similar to those of mitosis, but completing the process of meiosis requires more steps than mitosis. Mitosis involves one division of the nucleus, while meiosis involves two divisions. Mitosis can thus take place in cells that are either haploid or diploid. Haploid means having a single set of chromosomes, which is half the total of the parent. The haploid condition is designated by 1n or n. Diploid means having a double set of chromosomes in homologous pairs. The diploid condition is designated by 2n. Meiosis takes place in diploid cells. Meiosis begins with a diploid cell and results in four haploid cells after division.

Like mitosis, meiosis is divided into different stages. The actual processes are continuous, and these steps are human inventions to aid in understanding them. Breaking the processes into distinct steps is useful for analyzing what is happening along the way. The steps of meiosis are divided into two groups, meiosis I and meiosis II.

Interphase is a period of growth and duplication of chromosomes. It is not technically a step in meiosis. Instead, it precedes meiosis and ensures the cell is ready to divide. Each cell contains two homologs of each chromosome, one from each parent. A homolog is a member of a pair of homologous chromosomes, with similarity between organisms, anatomical structures, or DNA, based on common ancestry. Each homolog is part of a pair of homologous chromosomes, which are identical pairs of chromosomes that are formed during synapsis of meiosis I in diploid organisms. Homologous chromosomes contain the same genes as in the other homolog but may have different alleles. When these chromosomes are duplicated, the pairs of identical chromosome copies are called sister chromatids. A sister chromatid is an identical half of a replicated chromosome that is still attached at the centromere. Sister chromatids remain held together at the centromere, a region of chromosomes where the chromatids are joined. Since all of the genetic material is being replicated, sex cells spend most of their time in this stage. Interphase also occurs in mitosis.

Meiosis I consists of prophase I, metaphase I, anaphase I, and telophase I. Prophase I is also a long stage of the meiosis process. The replicated chromosomes condense, appearing to become more compact, and pair with their homologs in a process called synapsis, which is the alignment of pairs of replicated homologous chromosomes during prophase I of meiosis. This association of two pairs of sister chromatids forms a structure called a bivalent, or tetrad. A bivalent is a structure consisting of two sets of sister chromatids (from replicated homologous chromosomes) connected by the synaptonemal, ladder-like complex prior to the first meiotic division. Each set of homologous chromosomes forms a bivalent. Once a bivalent is formed, regions of the chromatids within a bivalent may trade places. This exchange of DNA between homologous chromosomes during meiosis is called crossing-over, which is the exchange of DNA between homologous chromosomes during meiosis. Crossing-over occurs between non-sister chromatids on homologous chromosomes during meiosis and serves to increase genetic diversity of the offspring. Further events of prophase I are the breakdown of the nuclear envelope and the formation of spindle fibers. A spindle fiber is a microtubule that attaches to chromosomes and pulls them to opposite poles of a cell during cell division. Prophase I is unique to meiosis. In mitosis the equivalent phase is prometaphase.

In metaphase I the bivalents attach at their centromeres to the spindle fibers and then line up along the equator, or central plane, of the cell. Metaphase I is unique to meiosis.

In anaphase I the spindle fibers pull one chromosome from each bivalent toward each end, or pole, of the cell. Each chromosome still has two chromatids. The separation of the homologous chromosomes is called disjunction. Anaphase I is unique to meiosis. Sister chromatids remain together after meiotic anaphase I. They separate after mitotic anaphase (and after meiotic anaphase II).

In telophase I the chromosomes gather into two nuclei, the regions containing DNA. Each pole now has a chromosome set. A nuclear envelope re-forms, and cytokinesis, or the pinching off of the cytoplasm to form two new cells, takes place. Telophase I is unique to meiosis. Each daughter cell contains duplicated chromosomes composed of sister chromatids. The sister chromatids at this point are not identical as a result of crossing-over.

There is only a brief interphase between telophase I and prophase II. It is used mostly as a resting stage for the cells. During the resting stage, no chromosome duplication happens. This step is unique to meiosis.

Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II. In prophase II, a spindle apparatus containing the spindle fibers forms, the chromosomes condense again, and the nuclear membrane that surrounds them dissolves. This step is analogous to mitotic prophase. The difference is that in mitosis the dividing cell contains both a paternal and a maternal homolog of each chromosome (both duplicated), while in meiosis the dividing cell contains only one or the other.

In metaphase II paired chromatids line up on the equator of the cell. This step is analogous to mitotic metaphase.

In anaphase II the centromeres divide; at this point the sister chromatids begin to separate. Once separated, the now individual chromatids are called chromosomes. Separated chromosomes move to opposite ends of the cell, and this separation of the sister chromatids is called disjunction. This step is analogous to mitotic anaphase.

In telophase II the chromosomes come together into nuclei. Cytokinesis occurs, resulting in four haploid daughter cells in the male. Each of these cells is different genetically from the original because of the crossing-over that occurred in prophase I. In female animals there is an uneven distribution of the genetic material. This results in a single viable haploid (n) cell and three smaller structures called polar bodies. A polar body is a nonviable cell produced at the end of meiosis in females. The function of polar bodies is to sequester an appropriate amount of genetic material away from the viable haploid cell. Polar bodies are usually reabsorbed by the female's body, although females of some species retain them.

This step is analogous to mitotic telophase. However, the end product of mitosis is a diploid cell, while the end product of meiosis is a haploid cell with only one copy of each chromosome.
The stages of meiosis II are similar to the corresponding stages in mitosis. The stages of meiosis I, which leads to the first division, are unique to the production of gametes.