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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
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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.
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In organisms that reproduce sexually, meiosis is a two stage cell
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division process.
The two stages of meiosis are meiosis I and meiosis II.
After meiosis has completed, four distinct daughter cells are
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produced.
The daughter cells that result from meiosis each have one half
of the number of chromosomes of the parent cell.
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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.
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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.
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Meiosis ensures that the correct number of chromosomes per cell is preserved during
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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.
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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. Nondisjunction of the sex chromosomes is typically not as severe as non-disjunction of
the autosomes.
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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
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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.
Next > Stages of Meiosis
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