Meiosis occurs during the life cycle of sexually reproducing organisms, including animals, plants, and fungi, and is the process by which reproductive cells duplicate their chromosomes and divide to form new cells. The reproductive cells produced during meiosis are called gametes and each contains half the number of chromosomes compared to other cells in the body, or somatic cells. Meiosis is what makes sexual reproduction possible by evening out the number of chromosomes when a male gamete meets up with a female gamete. In the case of human beings, the male gametes are called sperm and they fertilize female gametes which are called eggs. Each sperm cell and each egg cell contain 23 chromosomes; these are called haploid cells and contain exactly half of the 46 chromosomes found in diploid, somatic cells.
Similar to mitosis, meiosis occurs as part of the cell cycle alternating with interphase. It is during interphase that the cell prepares to enter meiosis by duplicating its chromosomes. The chromosomes that duplicate come from a diploid parent cell containing one set of chromosomes from the mother and one from the father. As meiosis begins, the cell contains two identical sets of chromosomes organized into sister chromatids.
Before we dig into the details of Meiosis, here is a short video to give you a general overview:
Similar to mitosis, meiosis occurs as part of the cell cycle alternating with interphase. It is during interphase that the cell prepares to enter meiosis by duplicating its chromosomes. The chromosomes that duplicate come from a diploid parent cell containing one set of chromosomes from the mother and one from the father. As meiosis begins, the cell contains two identical sets of chromosomes organized into sister chromatids.
Before we dig into the details of Meiosis, here is a short video to give you a general overview:
As shown in the video, there are distinct and important processes that occur during meiosis. The entire process can be broken down into two major phases: meiosis I and meiosis II. In general, meiosis I is the first division of the cell and meiosis II refers to the second cell division.
Meiosis I
Meiosis I is critical for the role it plays in genetic variability. There are 4 phases in meiosis I: prophase I, metaphase I, anaphase I, and
telophase I/cytokinesis I. Prophase I and metaphase I are the most critical phases in all of meiosis.
Meiosis I
Meiosis I is critical for the role it plays in genetic variability. There are 4 phases in meiosis I: prophase I, metaphase I, anaphase I, and
telophase I/cytokinesis I. Prophase I and metaphase I are the most critical phases in all of meiosis.
Prophase I
Prophase I is the most critical phase of meiosis because of the genetic exchange that occurs resulting in genetic variation in all of the gametes that are produced from the parents germ cells. During prophase I a crossing over occurs between the attached non-sister chromatids and genetic material is rearranged, this is called recombination. The following video describes the process of crossing over during prophase I:
Prophase I is the most critical phase of meiosis because of the genetic exchange that occurs resulting in genetic variation in all of the gametes that are produced from the parents germ cells. During prophase I a crossing over occurs between the attached non-sister chromatids and genetic material is rearranged, this is called recombination. The following video describes the process of crossing over during prophase I:
Let’s take a moment and consider how crucial prophase I is in creating the infinite number of combinations that result from sexual reproduction. The chromosomes virtually get together and “shuffle” their maternal and paternal genes. This is a major source of genetic variability in children of the same parents.
Metaphase I
The second phase of meiosis I is critical for achieving highly varied genetic combinations. During metaphase I, the tetrads randomly arrange themselves along the equator of the cell, or metaphase plate. This independent assortment of chromosomes allows the two daughter cells to “cut the deck”, producing two cells with different sets of shuffled chromosomes.
Anaphase I
The homologs or non-sister chromatids separate and the sister chromatids move towards opposite poles.
Telophase I and Cytokinesis
Nuclear envelope may form along with a cleavage furrow in animal cells, or cell plate in plant cells.
Meiosis II
Meiosis II is the stage in which the total number of chromosomes are divided among four daughter haploid cells, or gametes. Cell division without duplication of DNA results in four distinct haploid cells that are prepared to join with another haploid cell during sexual reproduction and fertilization.
Metaphase I
The second phase of meiosis I is critical for achieving highly varied genetic combinations. During metaphase I, the tetrads randomly arrange themselves along the equator of the cell, or metaphase plate. This independent assortment of chromosomes allows the two daughter cells to “cut the deck”, producing two cells with different sets of shuffled chromosomes.
Anaphase I
The homologs or non-sister chromatids separate and the sister chromatids move towards opposite poles.
Telophase I and Cytokinesis
Nuclear envelope may form along with a cleavage furrow in animal cells, or cell plate in plant cells.
Meiosis II
Meiosis II is the stage in which the total number of chromosomes are divided among four daughter haploid cells, or gametes. Cell division without duplication of DNA results in four distinct haploid cells that are prepared to join with another haploid cell during sexual reproduction and fertilization.
Prophase II
Spindle forms and chromosomes begin to move towards that metaphase plate.
Metaphase II
The chromosomes line up along the metaphase plate. Recall from meiosis I that due to the crossing over of genetic information, none of the sister chromatids are identical.
Anaphase II
Sister chromatids separate and move towards opposite poles.
Telophase II and Cytokinesis
Nuclear envelope may form along with a cleavage furrow in animal cells, or cell plate in plant cells. This final step results in four genetically unique daughter haploid cells.
Spindle forms and chromosomes begin to move towards that metaphase plate.
Metaphase II
The chromosomes line up along the metaphase plate. Recall from meiosis I that due to the crossing over of genetic information, none of the sister chromatids are identical.
Anaphase II
Sister chromatids separate and move towards opposite poles.
Telophase II and Cytokinesis
Nuclear envelope may form along with a cleavage furrow in animal cells, or cell plate in plant cells. This final step results in four genetically unique daughter haploid cells.