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A tetrad is the association of a pair of homologous chromosomes (4 sister chromatids) physically held together by at least one DNA crossover. This physical attachment allows for alignment and segregation of the homologous chromosomes in the first meiotic division. In most organisms, each replicated chromosome (composed of two identical sisters ...
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion. [1] Gene conversion can be either allelic, meaning that one allele of the same gene replaces another allele, or ectopic, meaning that one paralogous DNA sequence converts another.
Just as tandem repeats are further subcategorized based on the length of the repeating sequence, there are many different types of retrotransposons. Long interspersed nuclear elements are typically 3–7 kilobases in length. [23] Short interspersed nuclear elements are typically 100-300 base pairs and no longer than 600 base pairs. [23]
In genetics, a chiasma (pl.: chiasmata) is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis. [1]
In crossing-over, a Spo11 enzyme makes staggered nicks in a pair of sister chromatid strands (in a tetrad organization of prophase). Subsequent enzymes trim back the 5' ends of the strand and a protein complex binds to the 3' single-stranded ends.
Based on the definition of homology specified above this terminology is incorrect since sequence similarity is the observation, homology is the conclusion. [3] Sequences are either homologous or not. [3] This involves that the term "percent homology" is a misnomer. [4]
After strand invasion, a DNA polymerase extends the end of the invading 3' strand by synthesizing new DNA. This changes the D-loop to a cross-shaped structure known as a Holliday junction. Following this, more DNA synthesis occurs on the invading strand (i.e., one of the original 3' overhangs), effectively restoring the strand on the homologous ...
In 1998 it was determined that homologous pairing in Drosophila occurs through independent initiations (as opposed to a directed, 'processive zippering' motion). [4] [8]The first RNAi screen (based on DNA FISH [9]) was carried out to identify genes regulating D. melanogaster somatic pairing in 2012, [10] described at the time as providing "an extensive “parts list” of mostly novel factors".