At the sites where exchange happens, chiasmata form. Anaphase I[ edit ] Kinetochore microtubules shorten, pulling homologous chromosomes which consist of a pair of sister chromatids to opposite poles.
In this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. No DNA replication occurs during this stage. This allows the sister chromatids to remain together while homologs are segregated.
The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatidswith one chromosome coming from each parent.
Diplotene[ edit ] During the diplotene stage, also known as diplonema, from Greek words meaning "two threads", : In G1, each of the chromosomes consists of a single linear molecule of DNA.
In mitosis, the force of kinetochore microtubules pulling in opposite directions creates tension. In some plants, fungi, and protists meiosis results in the formation of spores: The physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of DNA within chromosomes.
The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.
There is no current consensus among biologists on the questions of how sex in eukaryotes arose in evolutionwhat basic function sexual reproduction serves, and why it is maintained, given the basic two-fold cost of sex.
This replication does not change the ploidy of the cell since the centromere number remains the same. The chromosomes uncoil back into chromatin.
Nuclear envelopes reform and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
Because the ploidy is reduced from diploid to haploid, meiosis I is referred to as a reductional division. This attachment is referred to as a bipolar attachment.
The cells then proceed to a second division without an intervening round of DNA replication. Leptotene[ edit ] The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".
Growth 2 G2 phase: The process ends with telophase II, which is similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle.
Growth 1 G1 phase: Meiotic prophase corresponds most closely to the G2 phase of the mitotic cell cycle.
Some eukaryotes, like bdelloid rotifersdo not have the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.
Other microtubules will interact with microtubules from the opposite centrosome: Meiosis generates gamete genetic diversity in two ways: The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore.
In human fetal oogenesisall developing oocytes develop to this stage and are arrested in prophase I before birth. As kinetochore microtubules from both centrosomes attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes.
In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. Nonkinetochore microtubules lengthen, pushing the centrosomes farther apart.
This often results in chromosomal crossover. Meiosis uses many of the same mechanisms as mitosisthe type of cell division used by eukaryotes to divide one cell into two identical daughter cells.
There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I.recombination meiosis shuffles the genes between the two chromosomes in each pair (one received from each parent), producing recombinant chromosomes with unique genetic combinations in every gamete: mitosis occurs only if needed to repair DNA damage; usually occurs between identical sister chromatids and does not result in genetic .Download