Meiosis

Meiosis I

During prophase I, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins.
As the nuclear envelope begins to degrade, the proteins associated with homologous chromosomes brings the pairs close to each other.
The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations and then spreads to cover the entire length of the chromosomes.
The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other.
The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover.
As prophase I progresses, the synaptonemal complex begins to degrade and the chromosomes begin to condense. When the synaptonemal complex disappears, the homologous chromosomes remain attached to each other at the centromere and at the chiasmata. The chiasmata remain until anaphase I.

During prometaphase I, of the spindle fiber microtubules are attached to the kinetochore proteins at the centromeres.
- Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle.
Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the center of the cell and attach to one of the two homologous chromosomes. The microtubules attach at each chromosomes' kinetochores.
A kinetochore microtubule is a spindle fiber that has attached to a kinetochore.
At the end of prometaphase I, each homologous chromosome is attached to microtubules from both poles, with one homologous chromosome facing each pole.

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles.
- This is important for determining the genes carried by each gamete, sine each one will only receive one of the two homologous chromosomes.
Since each homologous chromosome has slight differences in their genetic makeup, each gamete will have a unique genetic makeup. This randomness helps create genetic variation in the offspring.

During the anaphase I, the microtubules pull the linked chromosomes apart and the sister chromatids remain tightly bound together at the centromere.
The chiasmata are broken during this process.

During telophase I, the separated chromosomes arrive at opposite poles and the rest of the telophase process may or may not occur, depending on the species.
In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In others, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei.
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.
- Only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids.

During prophase II, 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 interkinesis, a brief interphase period, move toward opposite poles, and new spindles are formed.

During prometaphase II, 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.

During metaphase II, the sister chromatids are maximally condensed and aligned at the center of the cell.

During anaphase II, the sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles.
Non-kinetochore microtubules elongate the cell into an oval shape.

During telophase II, the chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes and cytokinesis separates the two cells into four unique haploid cells.
At this point, the newly formed nuclei are both haploid.
The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombining of maternal and paternal segments of chromosomes that occurs during crossover.