15. General (homologous) recombination. Molecular mechanisms of crossing over.

General Recombination and Crossing Over

• Crossing over refers to the exchange of genetic material between homologous chromosomes, a process known as homologous recombination.
• This process plays a vital role in creating genetic diversity during sexual reproduction.
• In eukaryotes, it primarily takes place during meiotic prophase I but can also occur during mitotic (somatic) cell divisions as a mechanism for DNA repair.

Molecular Mechanism of Crossing Over (during Eukaryotic Meiosis)

1. 1. Initiation with DNA Breaks:

• The process begins with the creation of a double-strand break (DSB) in one of the chromatids.
• This typically occurs during the leptotene stage of prophase I, when chromosomes begin to condense and form looped structures supported by protein scaffolding.

1. 2. Formation of Single-Stranded DNA:

• Enzymes, such as SPO11, create the breaks.
• The ends of the broken DNA strands are resected, producing single-stranded segments that are necessary for the homology search.

1. 3. Strand Invasion and D Loop Formation:

• Special proteins called recombinases, including RAD51 and DMC1, bind to the single-stranded DNA.
• These proteins promote strand invasion into the homologous chromosome, forming a displacement loop (D loop), where the invading strand pairs with the complementary DNA of the homolog.

1. 4. DNA Synthesis and Resolution:

• DNA polymerases extend the invading strand by copying information from the homologous chromosome.
• RAD51 is then removed, and crossover intermediates are further stabilized.

1. 5. Role of the Synaptonemal Complex (SC):

• The synaptonemal complex forms between homologous chromosomes during zygotene and is fully formed by pachytene.
• The SC consists of:
• Lateral elements, partially composed of the protein SYCP3.
• Central elements, including SYCP1.
• Transverse filaments, which connect the homologs.
• The SC aligns chromosomes along their length and facilitates recombination.

1. 6. Recombination Nodules and Resolution:

• Recombination nodules appear as foci along the synaptonemal complex and mark the sites of crossover.
• These nodules help stabilize crossover structures and recruit proteins like MSH4, MSH5, MLH1, and MLH3, which are involved in mismatch repair and resolving recombination intermediates.
• Not all DSBs result in crossover events. Some are resolved without exchange of flanking regions.

1. 7. Formation of Heteroduplex Regions:

• A region called a heteroduplex forms between homologous chromosomes, which can span thousands of base pairs.
• This region may contain mismatched base pairs if the homologs have sequence differences.

1. 8. Chiasma Formation and Separation:

• As meiosis progresses to diplotene and diakinesis, the synaptonemal complex dissolves.
• Crossovers appear as chiasmata, which physically hold the homologous chromosomes together.
• In anaphase I, cohesins are removed from the chromosome arms (but retained near centromeres), allowing homologs to separate.