15.1: Introduction

Meiosis

Introduction to Meiosis (aka “Reduction Division”)

Meiosis is a special type of cell division in which the daughter cells produced have half the number of chromosomes (n) as their parent cell. In animals, this division occurs in the reproductive organs (gonads -- testes of males or ovaries of females) of species that reproduce sexually, and results in the formation of gametes (eggs or sperm) that contain half the number of chromosomes as the original cell. Sexual reproduction involves the joining of gametes (fertilization) to form a zygote, which then has two copies of each chromosome (2n). Meiosis is a critical process, as it increases genetic diversity within a species.

Cells that divide by meiosis prepare for cellular division (during interphase) much like every other cell. Meiosis progresses through the same phases as mitosis (prophase, anaphase, metaphase, telophase, and cytokinesis). However, unlike mitosis, meiosis involves two rounds of cellular division (meiosis I and meiosis II). Meiosis involves only one round of DNA replication where each chromosome replicates to form sister chromatids. Therefore, when meiosis is completed, each daughter cell contains only half the number (n) of chromosomes as the original cell.

The stages of meiosis:

Meiosis I

• Prophase l
• Metaphase l
• Anaphase l
• Telophase l
• Cytokinesis l

Meiosis II

• Prophase ll
• Metaphase ll
• Anaphase ll
• Telophase ll
• Cytokinesis ll

Prophase I: During prophase of meiosis I, the chromosomes join in homologous pairs. Homologous chromosomes (aka homologs) are the same length, have their centromere in the same position, and carry genetic information (genes) for the same traits, but not necessarily the same versions (alleles) of the gene.

For example, human chromosome #19 contains a gene for eye color. In one person, one allele might code for blue eyes and the other allele codes for green eyes. Since every human inherits two copies of chromosome 19 (one from the mother’s egg and one from the father’s sperm) a person could have 2 blue alleles, 2 green alleles, or one of each.

Paired homologous chromosomes are called tetrads and are said to be in synapsis. During synapsis, equivalent pieces of homologous chromatids are exchanged between the chromosomes. This is called crossing-over and can occur several times along the length of the chromosomes. As occurs in the mitotic division, prophase of meiosis I also involves the degradation of the nuclear membrane, disassembly of the nucleolus, and formation of spindle fibers.

Metaphase I: Metaphase of meiosis I occurs when the joined homologous chromosome pairs are moved to the center of the cell by spindle fibers. The fibers arrange the pairs so that homologs are on opposite sides of the metaphase plate (aka equatorial plane).

Anaphase I follows, as homologs are pulled apart, toward opposite poles of the cell. [*Note: this is significantly different from the separation of sister chromatids that occurs during mitosis].

Telophase I marks the end of meiosis I, as new nuclei form and cytokinesis separates the cytoplasm forming two daughter cells.

At the end of meiosis I, the two daughter cells have half the number of chromosomes as did their parent cell. Thus, the cells have been reduced from diploid (2n) to haploid (n). [n refers to the number of chromosomes in a set that are characteristic for a species. Humans have one set (n) of 23 unique chromosomes (n = 23). A diploid human cell has 2 sets (2n) of 23 unique chromosomes (2n = 46).

*Meiosis II follows meiosis I, which proceeds very much like mitosis.

During Prophase II, chromosomes containing two sister chromatids are lined up on the equator of each daughter cell by the spindle fibers. This is completed by the end of Metaphase II.

The centromeres separate and sister chromatids are pulled to each pole of the cell during Anaphase ll. Cytokinesis II occurs after Telophase II to complete cell division and ultimately the production of four (4) daughter cells (Figure 11). The cells produced (egg or sperm, in humans) are haploid (n rather than 2n) and will either unite (via fertilization) or die. They do not divide further on their own as meiosis is not a cycle.

References

Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks. 18. CC-BY

https://oer.galileo.usg.edu/biology-textbooks/18