Meiosis Study Guide

Meiosis
In meiosis, pairs of homologous chromosomes (orange) are pulled to opposite ends of the cell by spindles (blue). This results in two cells with half the usual number of chromosomes. Meiosis occurs only in the sex cells.

TIM VERNON / SCIENCE PHOTO LIBRARY / Getty Images

Overview of Meiosis

Meiosis is a two-part cell division process in organisms that sexually reproduce. Meiosis produces gametes with one half the number of chromosomes as the parent cell. In some respects, meiosis is very similar to the process of mitosis, yet it is also fundamentally different from mitosis.

The two stages of meiosis are meiosis I and meiosis II. At the end of the meiotic process, four daughter cells are produced. Each of the resulting daughter cells has one half of the number of chromosomes as the parent cell. Before a dividing cell enters meiosis, it undergoes a period of growth called interphase.

During interphase the cell increases in mass, synthesizes DNA and protein, and duplicates its chromosomes in preparation for cell division.

Key Takeaways

  • In organisms that reproduce sexually, meiosis is a two stage cell division process.
  • The two stages of meiosis are meiosis I and meiosis II.
  • After meiosis has completed, four distinct daughter cells are produced.
  • The daughter cells that result from meiosis each have one half of the number of chromosomes of the parent cell.

Meiosis I

Meiosis I encompasses four stages:

  • Prophase I - chromosomes condense and attach to the nuclear envelope and begin migrating toward the metaphase plate. This is the stage where genetic recombination may occur (via crossing over).
  • Metaphase I - chromosomes align at the metaphase plate. For homologous chromosomes, the centromeres are positioned toward opposite poles of the cell.
  • Anaphase I - homologous chromosomes separate and move toward opposite cell poles. The sister chromatids remain attached after this move to opposite poles.
  • Telophase I - cytoplasm divides producing two cells with a haploid number of chromosomes. Sister chromatids remain together. While different cell types may prepare differently for meiosis II, there is one variable that does not change: the genetic material does not undergo replication in meiosis II.

Meiosis II

Meiosis II encompasses four stages:

  • Prophase II - chromosomes begin migrating to the metaphase II plate. These chromosomes do not replicate again.
  • Metaphase II - chromosomes align at the metaphase II plate while the kinetochore fibers of the chromatids are oriented toward opposite poles.
  • Anaphase II - sister chromatids separate and begin moving to opposite ends of the cell. The two cell poles also grow further apart in preparation for telophase II.
  • Telophase II - new nuclei form around daughter chromosomes and the cytoplasm divides and forms two cells in a process known as cytokinesis.

At the end of meiosis II, four daughter cells are produced. Each of these resulting daughter cells is haploid.

Meiosis ensures that the correct number of chromosomes per cell is preserved during sexual reproduction. In sexual reproduction, haploid gametes unite to form a diploid cell called a zygote. In humans, male and female sex cells contain 23 chromosomes and all other cells contain 46 chromosomes. After fertilization, the zygote contains two sets of chromosomes for a total of 46. Meiosis also ensures that genetic variation occurs through genetic recombination that happens between homologous chromosomes during meiosis.

Meiosis Problems

While the meiotic process generally ensures that the correct number of chromosomes is preserved in sexual reproduction, sometimes errors may occur. In humans, these errors can lead to problems that may ultimately result in a miscarriage. Errors in meiosis can also lead to genetic disorders.

One such error is chromosomal non-disjunction. With this error, the chromosomes do not separate as they should during the meiotic process. The gametes that are produced do not have the correct number of chromosomes. In humans, for example, a gamete may have an extra chromosome or be missing a chromosome. In such cases, a pregnancy that resulted from such gametes could end in a miscarriage. Non-disjunction of the sex chromosomes is typically not as severe as non-disjunction of the autosomes.

Stages, Diagrams, and Quiz

  • Overview
  • Stages of Meiosis - Get a thorough overview of the stages of both meiosis I and meiosis II.
  • Meiosis Diagrams - see diagrams and pictures of each of the stages of meiosis I and II.
  • Glossary of Terms - the cell biology glossary contains important biological terms related to the meiotic process.
  • Quiz - Take the Meiosis Quiz to find out if you have mastered the intricacies of meiosis I and meiosis II.