Meiosis occurs in eukaryotic organisms that reproduce sexually. This includes plants and animals. Meiosis is a two-part cell division process that produces sex cells with one half the number of chromosomes as the parent cell.
InterphaseEd Reschke/Getty Images
There are two stages or phases of meiosis: meiosis I and meiosis II. Before a dividing cell enters meiosis, it undergoes a period of growth called interphase. At the end of the meiotic process, four daughter cells are produced.
- G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.
- S phase: The period during which DNA is synthesized. In most cells, there is a narrow window of time during which DNA is synthesized. Note that the S represents synthesis.
- G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesizes proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.
- In the latter part of interphase, the cell still has nucleoli present.
- The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.
- In animal cells, two pairs of centrioles formed from the replication of one pair are located outside of the nucleus.
At the end of interphase, the cell enters the next phase of meiosis: Prophase I.
Prophase IEd Reschke/Photolibrary/Getty Images
In prophase I of meiosis, the following events occur:
- Chromosomes condense and attach to the nuclear envelope.
- Synapsis occurs (a pair of homologous chromosomes lines up closely together) and a tetrad is formed. Each tetrad is composed of four chromatids.
- Genetic recombination via crossing over may occur.
- Chromosomes thicken and detach from the nuclear envelope.
- Similar to mitosis, the centrioles migrate away from one another and both the nuclear envelope and nucleoli break down.
- Likewise, the chromosomes begin their migration to the metaphase plate.
At the end of prophase I of meiosis, the cell enters into metaphase I.
Metaphase IEd Reschke/Photolibrary/Getty Images
In metaphase I of meiosis, the following events occur:
- Tetrads align at the metaphase plate.
- Note that the centromeres of homologous chromosomes are oriented toward the opposite cell poles.
At the end of metaphase I of meiosis, the cell enters into anaphase I.
Anaphase IEd Reschke/Photolibrary/Getty Images
In anaphase I of meiosis, the following events occur:
- Chromosomes move to the opposite cell poles. Similar to mitosis, microtubules such as the kinetochore fibers interact to pull the chromosomes to the cell poles.
- Unlike in mitosis, sister chromatids remain together after the homologous chromosomes move to opposite poles.
At the end of anaphase I of meiosis, the cell enters into telophase I.
Telophase IEd Reschke/Photolibrary/Getty Images
In telophase I of meiosis, the following events occur:
- The spindle fibers continue to move the homologous chromosomes to the poles.
- Once movement is complete, each pole has a haploid number of chromosomes.
- In most cases, cytokinesis (the division of the cytoplasm) occurs at the same time as telophase I.
- At the end of telophase I and cytokinesis, two daughter cells are produced, each with one-half the number of chromosomes of the original parent cell.
- Depending on the kind of cell, various processes occur in preparation for meiosis II. There is, however, a constant: The genetic material does not replicate again.
At the end of telophase I of meiosis, the cell enters into prophase II.
Prophase IIEd Reschke/Photolibrary/Getty Images
In prophase II of meiosis, the following events occur:
- The nuclear membrane and nuclei break up while the spindle network appears.
- Chromosomes do not replicate any further in this phase of meiosis.
- The chromosomes begin migrating to the metaphase II plate (at the cell's equator).
At the end of prophase II of meiosis, the cell enters into metaphase II.
Metaphase IIEd Reschke/Photolibrary/Getty Images
In metaphase II of meiosis, the following events occur:
- The chromosomes line up at the metaphase II plate at the cell's center.
- The kinetochore fibers of the sister chromatids point toward opposite poles.
At the end of metaphase II of meiosis, the cell enters into anaphase II.
Anaphase IIEd Reschke/Photolibrary/Getty Images
In anaphase II of meiosis, the following events occur:
- Sister chromatids separate and begin moving to opposite ends (poles) of the cell. Spindle fibers not connected to chromatids lengthen and elongate the cell.
- Once the paired sister chromatids separate from one another, each is considered a full chromosome. They are referred to as daughter chromosomes.
- In preparation for the next stage of meiosis, the two cell poles also move further apart during the course of anaphase II. At the end of anaphase II, each pole contains a complete compilation of chromosomes.
Following anaphase II of meiosis, the cell enters into telophase II.
Telophase IIEd Reschke/Photolibrary/Getty Images
In telophase II of meiosis, the following events occur:
- Distinct nuclei form at the opposite poles.
- Cytokinesis (division of the cytoplasm and the formation of two distinct cells) occurs.
- At the end of meiosis II, four daughter cells are produced. Each cell has one-half the number of chromosomes as the original parent cell.
Stages of Meiosis: Daughter CellsEd Reschke/Photolibrary/Getty Images
The final result of meiosis is the production of four daughter cells. These cells have one half the number of chromosomes as the original cell. Only sex cells are produced by meiosis. Other cell types are produced by mitosis. When sex cells unite during fertilization, these haploid cells become a diploid cell. Diploid cells have the full complement of homologous chromosomes.