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Chromatid pairs are normally genetically identical, and said to be homozygous. However, if mutations occur, they will present slight differences, in which case they are heterozygous . The pairing of chromatids should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.
A sister chromatid refers to the identical copies formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad.
The list of organisms by chromosome count describes ploidy or numbers of chromosomes in the cells of various plants, animals, protists, and other living organisms.This number, along with the visual appearance of the chromosome, is known as the karyotype, [1] [2] [3] and can be found by looking at the chromosomes through a microscope.
Crossing over occurs between prophase I and metaphase I and is the process where two homologous non-sister chromatids pair up with each other and exchange different segments of genetic material to form two recombinant chromosome sister chromatids. It can also happen during mitotic division, [1] which may result in loss of heterozygosity.
The search for the homologous target, helped by numerous proteins collectively referred as the synaptonemal complex, cause the two homologs to pair, between the leptotene and the pachytene phases of meiosis I. [4] Resolution of the DNA recombination intermediate into a crossover exchanges DNA segments between the two homologous chromosomes at a ...
The introduction of DSBs in DNA often employs the topoisomerase-like protein SPO11. [3] CO recombination may also be initiated by external sources of DNA damage such as X-irradiation, [4] or internal sources. [5] [6] There is evidence that CO recombination facilitates meiotic chromosome segregation. [2]
[3] [4] When each tetrad , which is composed of two pairs of sister chromatids , begins to split, the only points of contact are at the chiasmata. The chiasmata become visible during the diplotene stage of prophase I of meiosis , but the actual "crossing-overs" of genetic material are thought to occur during the previous pachytene stage.
3.1.1.6 Meiotic spindle formation. ... homologous chromosomes pair with each other and undergo ... (n chromosomes, 23 in humans) which each contain chromatid pairs ...