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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.
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.
A key event is the initiation of synapsis between homologous chromosomes, which carry the same genetic information but may have different allelic variations. The homologous chromosomes begin pairing and association along their lengths, facilitated by lateral (axial) elements of the synaptonemal complex protein structure that forms between the ...
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]
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 ...
Next, the 3' end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single-stranded DNA generated from the other end of the initial double-stranded break. The structure that results is a cross-strand exchange, also known as a Holliday junction.
[citation needed] While in this formation, homologous sites on two chromatids can closely pair with one another, and may exchange genetic information. [ 6 ] Because there is a small probability of recombination at any location along a chromosome, the frequency of recombination between two locations depends on the distance separating them.