The Meiotic Division of an Animal Cell

Chromosomes begin to condense, and homologs loosely pair along their lengths, aligned gene by gene

crossing over (the exchange of corresponding segments of DNA molecules by nonsister chromatids) is completed while homologs are in synapsis, held tightly together by proteins along their lengths

synapsis ends in mid- (this stage), and the chromosomes in each pair move apart slightly

each homologous pair has one or more chiasmata, points where crossing over has occurred and the homologs are srill associated due to cohesion between sister chromatids (sister chromatid cohesion)

centrosome movement, spindle formation, and nuclear envelope breakdown occur as in mitosis

In late (this phase), microtubules from one pole or the other attach to the two kinetochores, protein structures at the centromeres of the two homologs. The homologous pairs then move toward the metaphase plate

pairs of homologous chromosomes are now arranged on the metaphase plate, with one chromosome in each pair facing each pole

both chromatids of one homolog are attached to kinetochore microtubules from one pole; those of the other homolog are attached to microtubules from the opposite pole

breakdown of proteins responsible for sister chromatid cohesion along chromatid arms allows homologs to separate

the homologs move toward opposite poles, guided by the spindle apparatus

sister chromatid cohesion persists at the centromere, causing chromatids to move as a unit toward the same pole

at the beginning of this phase, each half of the cell has a complete haploid set of replicated chromosomes. Each chromosome is composed of two sister chromatids; one or both chromatids include regions of nonsister chromatid DNA

this (division of the cytoplasm) usually occurs simultaneously with this phsae, forming two haploid daughter cells

in animal cells, a cleavage furrow forms (in plant cells, a cell plate forms.)

in some species, chromosomes decondense and the nuclear envelope re-forms

no replication occurs between meiosis I and meiosis II

a spindle apparatus forms

in late this phase, chromosomes, each still composed of two chromatids associated at the centromere, move toward the metaphase II plate

the chromosomes are positioned on the metaphsae plate as in mitosis

because of crossing over in meiosis I, the two sister chromatids of each chromsome are not genetically identical

the kinetochores of sister chromatids are attached to microtubules, extending from opposite poles

breakdown of proteins holding the sister chromatids together at the centromere allows the chromatids to separate. the chromatids move toward opposite poles as individual chromosomes

nuclei form, the chromosomes begin decondensing and cytokinesis occurs

the meiotic division of one parent cell produces four daughter cells, each with a haploid set of (unreplicated) chromosomes

each of the four daughter cells genetically distinct from the other daughter cells and from the parent cell