Cells which are diploid have two sets of chromosomes - for most organisms this means the cells have one set of chromosomes from their mother and one set from their father. The gametes in animals are sperm male and eggs female. The gametes in flowering plants are pollen male and ovules female.
The offspring produced in sexual reproduction are genetically different to each other and to their parents. This process results in variation within a population because it involves the mixing of genetic information. This short video explains the role of meiotic cell division. During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells.
Then in anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell. Finally, during telophase II, the chromosomes are enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of the two cells. At the conclusion of meiosis, there are four haploid daughter cells that go on to develop into either sperm or egg cells.
Further Exploration Concept Links for further exploration cell division replication metaphase anaphase telophase linkage chromosome cytokinesis haploid prometaphase principle of segregation principle of independent assortment spindle fibers gamete DNA chromatin nucleus cytoplasm eukaryote prophase recombination principle of segregation Principles of Inheritance.
Related Concepts You have authorized LearnCasting of your reading list in Scitable. Do you want to LearnCast this session? In the non-lichenized ascomycetes fertilization via an antheridium is not always necessary. Many species of fungi produce vegetative propagules called conidia which may be haploid or not and these may germinate to produce a new mycelium. Various fungi produce tiny conidia, seemingly with a primary role as fertilizing agents much like a human sperm , rather than as the sources of new mycelia, and such conidia are sometimes referred to as microconidia or spermatia.
Note that even a haploid macro conidium or a haploid ascospore could act as a fertilizing agent. In many lichens ascogonia have been seen to form near the thallus surface with trichogynes growing from the ascogonia and protruding beyond the surface. Many lichen species produce microconidia and these have been seen on trichogynes, where it is likely that those conidia are acting as fertilizing agents.
However research is still needed to confirm these suppositions and to reveal the precise processes. As recently as it was noted that:. Migration of conidial nuclei through the trichogyne and pairing with ascogonial nuclei likely occurs, but has never been documented in lichen-forming ascomycetes.
In many species ascogonia were never found with protruding trichogynes; To complete the account of nuclear donation and acceptance I'll note that in various non-lichenized ascomycetes donation-acceptance of nuclei can occur after hyphal fusion and that the basidiomycetes never produce any distinct donating and receiving organs.
Instead receipt or donation of basidiomycete nuclei often occurs after hyphae from two mycelia have come into contact and fused. A haploid conidium or haploid basidiospore could also act as a donor to a haploid mycelium that comes into contact with the conidium or spore, assuming there is mating type compatibility between the mycelium and the conidium or spore.
Dikaryotic hyphae are common in the basidiomycetes and mycelia composed of dikaryotic hyphae are formed when two, compatible haploid mycelia meet. For the non-lichenized basidiomycetes this happens out of sight within some substrate for example soil, wood, dung and the resulting dikaryotic mycelium also remains out of sight. It is potentially long-lived and will produce the visible fruiting bodies such as mushrooms, puffballs, bracket fungi, etc.
In such fruiting bodies the basidia are produced and it is in the rudimentary basidia that karyogamy occurs. As in the rudimentary asci the diploid state in each basidium is very short-lived since the newly formed diploid cell quickly undergoes meiosis to produce new haploid nuclei.
These become the basis for the development of new spores which will mature as the basidium develops. During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set.
There are two possibilities for orientation at the metaphase plate. The possible number of alignments, therefore, equals 2n, where n is the number of chromosomes per set. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition. In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set.
In this example, there are four possible genetic combinations for the gametes. In anaphase I, the microtubules pull the attached chromosomes apart.
The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart. In telophase I, the separated chromosomes arrive at opposite poles.
In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. Then cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow constriction of the actin ring that leads to cytoplasmic division. In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate.
This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells. Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present.
Although there is only one chromosome set, each homolog still consists of two sister chromatids. During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. Meiosis II initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed.
In contrast to meiosis I, meiosis II resembles a normal mitosis. In some species, cells enter a brief interphase, or interkinesis, before entering meiosis II. Interkinesis lacks an S phase, so chromosomes are not duplicated. The two cells produced in meiosis I go through the events of meiosis II together. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.
If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes that were duplicated during interphase I move away from each other toward opposite poles and new spindles are formed.
The nuclear envelopes are completely broken down and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.
The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
Non-kinetochore microtubules elongate the cell. Meiosis I vs. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I.
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