Question

Molecular mechanism of crossing over including model of recombination

Answer 1

Answer:

This model of recombination was proposed by Holliday in 1964. According to this model, each chromatid is regarded as a single DNA molecule, while a pair of non-sister chromatids is viewed as a pair of DNA molecules.

Answer 2

Answer:

The molecular mechanism or crossing over is explained by the example of homologous chromosomes and the model of recombination is explained by Holiday Model given by Holiday in 1964.

Explanation:

• Crossing over- when 2 sister chromatids of homologous chromosomes exchange genetic material with each other in diploid eukaryotic species, this happens during the prophase I of meiosis I when homologous chromosomes align with each other and exchange the genetic material to form recombinants.

• Recombination- It is the process in which new combinations of alleles are produced when pieces of DNA are broken and recombined.
• Molecular mechanism of Crossing over

• In one of the DNA molecules when the double strands break naturally through agents like carcinogen or radiation or by the action of specific proteins.
• Enzymes exonucleases act on this break and remove the short regions of nucleotides from 5' → 3' orientation from both the strands.
• These 2 loose single strands combine and get coated with proteins known as recombinases that catalyze recombination. Therefore single strands that invade the suitable sequence are catalyzed by these enzymes for base pairing.
• The single-stranded region uses the sequence on the homologous chromosome because non-sister chromatids come closer during prophase-I.
• The Ist invading strand act as a primer & synthesizes a double-stranded region for itself using 1 strand of its sister chromatids as a template.
• Because of this base pairing with the 2nd single strand region, which was initially brought by the exonucleases, occurs, and the complementary strand gets displaced. This leads to an exchange of genetic material between two strands, hence crossing over takes place.
• This cross-like structure forms when 2 strands are being exchanged is called Holiday Junction
• When cross in holiday junction migrates in the lateral direction, a process called Branch migration occurs in which a DNA strand in 1 helix is swapped for a DNA strand in the other helix.
• Heteroduplex is produced by the swapping of DNA strands because the DNA sequences in the homologous chromosomes are similar( may not be identical). Heteroduplex is the region of double-stranded DNA that consist of base mismatches (not perfectly complimentary).
• At last, resolution occurs when 2 DNA strands break and rejoin to form 2 separate chromosomes.
• This is how the Holiday model is associated with general properties of recombinant chromosomes and hence with the Recombination process.