Meiosis is a form of cell division that usually occurs only once in the lifetime of a eukaryote, and is vital to the sexual reproduction of eukaryotic organisms. Meiosis forms gametes, or sex cells, by rearranging and mixing genetic material, which ensures genetically-distinct progeny (children) and sufficient variety in the gene pool.
Because meiosis begins with one diploid parent cell and ends with four haploid daughter cells, two division stages are needed: these divisions are called meiosis I and meiosis II. Genetic reassortment occurs during meiosis I. The first meiotic stage is also an example of reductional division, wherein a change in ploidy takes place as a diploid parent cell forms haploid daughter cells. Meiosis II, being an equational division, does not feature a change in ploidy; it instead produces haploid daughter cells from haploid parent cells. Meiosis I, also produces cells in which the chromosomes are still whole and are composed of two chromatids; on the other hand, the separation of sister chromatids occurs in meiosis II.
Meiosis II is generally regarded as being very similar to mitosis, except for the presence of two parent cells, instead of only one. In both meiosis I and II, cytokinesis occurs, and there are two daughter cells per parent cell.
|Meiosis I||Meiosis II|
|Type of Division||Heterotypic or reduction division (diploid cell to haploid cells)||Homotypic or equational division (haploid cells to haploid cells)|
|Relationship to Mitosis||Does not resemble mitosis.||Closely resembles the mitotic process.|
|Duration||Slow. 85%-95% of total time for both divisions is spent in prophase I.||All phases proceed very quickly.|
|Replication||Replication of DNA prior to meiosis.||No replication|
|Interphase||Features G1 phase, S phase, and G2 phase; genetic replication occurs.||In some species, starts with interkinesis, which lacks an S phase and does not result in chromosome duplication, instead of interphase.|
|Prophase||Prophase I: Chromosomes form a tetrad, joined at the chiasmata. Genetic recombination occurs (crossing-over). Has five substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.||Prophase II: Sister chromatids joined at the centromere. Genetic recombination does not occur.|
|Metaphase||Metaphase I: Homologous chromosomes align side-by-side, in random order (either paternal or maternal chromosome align to each side). Spindle fibers attach to chromosomes.||Metaphase II: Chromosomes align single file. Spindle fibers attach to kinetochores, to which they attach when separating sister chromatids.|
|Anaphase||Anaphase I: Homologous pairs separate, with two chromatids going to each of the two poles.||Anaphase II: Chromosomes divide at the centromeres, with one chromatid going to each of the two poles.|
|Telophase||Telophase I: Separates into two daughter cells, both haploid.||Telophase II: Separates into four daughter cells, all haploid.|
In premeiotic interphase, chromosomes are duplicated and other proteins are produced that are needed for meiosis. This is the phase in which all the "building blocks" for meiosis are prepared. The stages are the G1 phase (the first "gap" phase), the S phase, and the G2 phase (the second "gap" phase).
In the G phases, G stands for "gap." During the G1 phase, the cell produces the proteins necessary for replicating DNA.
In this phase, the chromosomes are replicated. Each chromosome, instead of consisting of only one chromatid, now has a pair of sister chromatids, which doubles the amount of DNA in the cell while retaining the original number of chromosomes (2n, or diploid). It is important to note that both a lone chromatid and a pair of sister chromatids are considered one chromosome; thus, the doubling of chromatids does not affect the number of chromosomes, or the ploidy.
This stage is the final preparation for meiosis. The cell produces more proteins, such as microtubules.
Process of Meiosis I
Meiosis I and II, as well as mitosis, have the same five five stages: prophase, prometaphase, metaphase, anaphase, and telophase. With the stages in meiosis I, the primary difference lies in prophase I, which is much longer than either its meiosis II or mitosis counterparts, and is in fact the stage a cell is in for 85%-95% of the time spent in meiosis. This is due to being the stage in which crossing-over, the defining event of meiosis I, occurs.
During prophase I, the chromatin (or loose threads of genetic material) coil and condense such that they are viewable under a microscope. Homologous chromosomes then start to move closer together. A homologous pair is two chromosomes, one maternal chromosome and one paternal, that have the same genes in the same locations. This side-by-side pairing is called synapsis. It is when chromosomes are in synapsis that crossing-over — an exchange of genetic material at points called chiasmata (singular: chiasma) — occurs. After the crossover, the homologous pairs are linked only at the chiasmata in an arrangement called a tetrad.
Prophase I is further divided into five substages:
- Leptotene: Chromatin condense into visible strands.
- Zygotene: Homologous pairs of chromosomes line up.
- Pachytene: Crossing-over occurs; homologous chromosomes exchange segments at homologous areas. This is genetic recombination.
- Diplotene: Homologous chromosomes move a little apart. Chromosomes also uncoil slightly to allow the transcription of DNA.
- Diakinesis: Chromosomes condense more, and the sites of crossover entangle and overlap, rendering the chiasmata visible. The nucleoli disappear, the nuclear membrane disintegrates. The meiotic spindle, which is responsible for dividing a single parental cell into two daughter cells, forms.
In metaphase I, homologous pairs line up side-by-side on the metaphase plate, or the equator, an imaginary line between the two poles of the cell. This is different from the way chromosomes line up single file in mitosis and metaphase II (in meiosis II). The pairs line up in random order in metaphase I, which means that each parental homolog (maternal or paternal) can line up to either pole of the cell. This causes chromosomal differences in the daughter cells of meiosis I.
The two chromosomes of each homologous pair separate due to the action of the meiotic spindle: one homolog goes to one pole, while the other goes to the opposite pole of the cell. Since the meiotic spindle is attached to the chromosomes and not to the kinetochores (the protein structures to which the spindle attaches when pulling sister chromatids apart), the centromeres do not split, and sister chromatids are not yet separated, which is the opposite of the case in anaphase II (meiosis II).
Telophase I begins when the chromosomes arrive at their respective poles. They then decondense, and the nuclear membranes form around them again. Cytokinesis, or when the cell physically divides, occurs then, resulting in two haploid daughter cells.
Process of Meiosis II
Meiosis II is very similar to mitosis. Aside from the four phases being analogous to those in mitosis, the ploidy also remains unchanged throughout the process and stays haploid.
Usually, meiosis II directly follows the cytokinesis in meiosis I; however, in some species interkinesis occurs, which is similar to interphase but lacks the S phase (growth phase) and thus no chromosome replication occurs
Prophase II is much shorter than prophase I (Meiosis I), primarily because no further genetic reassortment, or crossover, takes place. While the chromosomes uncoiled and decondensed in telophase I, in prophase II they condense again. The nuclear membrane disintegrates, and the spindle fibers reform. Sister chromatids grow kinetochores.
The formation of the spindle fibers is completed. The sister chromatids condense completely and align, single file (as opposed to metaphase I and similar to mitosis) on the metaphase plate in preparation for division. The kinetochores of the sister chromatids face their respective poles, and are attached to the spindle fibers from each pole of the cell.
The chromosomes split at the centromeres, and the chromatids move to opposite poles. These chromatids are now called chromosomes, despite only being one chromatid and not two. The term “chromatid” only refers to each molecule in the pair of DNA molecules in a duplicated chromosome, or a chromosome after it has produced another copy of itself that remains attached to the original copy through the centromere. As soon as the sister chromatids detach from each other, the chromosomes return to their “unduplicated” state and become chromosomes by themselves.
The chromosomes reach their respective poles and decondense. The nuclear membrane forms again, then cytokinesis separates each of the two cells into two further cells, totaling four haploid daughter cells. These cells are genetically unique and are rearrangements of the genetic material from the maternal and paternal homologs due to crossover. Each cell contains 23 chromosomes that are each composed of one chromatid.
In males, all four daughter cells become sperm cells (spermatogenesis), while in females, three of the daughter cells become polar bodies and disintegrate, with the one remaining cell becoming the egg cell (oogenesis).
Meiosis I contributes significantly to genetic diversity, which is vital to the adaptation and evolution of a species. The first event in meiosis I that contributes is crossing over, which allows genes from either parent to exchange, changing the genetic information in the chromosomes involved. This leads to new gene combinations and traits in offspring. The second event is the random distribution of chromosomes in metaphase I. The genetic shuffling makes it just as likely for a certain chromosome to end up in either of the daughter cells.
In meiosis II, sister chromatids are separated and randomly distributed among the daughter cells, which means that each resulting gamete has a unique set of genetic material.