6.5 Recombination at the DNA level

Question 1

What is recombination

Answer 1

It is when new combinations of alleles are created

Question 2

What two events created by both meiosis I and II cause for recombination

Answer 2

independent assortment and crossing over

Question 3

Explain what independent assortment is

Answer 3

It is when each pair of homologous chromosomes segregate freely from the other.. This creates new allele combinations for unlinked genes

Question 4

Explain what is crossing over

Answer 4

This is when two homologous chromosomes exchange portions of DNA. This creates new allele combinations for linked genes and ensures proper chromosome segregation during meiosis.

Question 5

What protein initiates recombination by breaking double strand breaks

Answer 5

Spo11

Question 6

Give the 6 steps of the crossing over pathway of recombination

Answer 6

1. Double strand break formation (Spo11 breaks phosphodiester bonds of both strands of one chromatid)
2. Resection (Then 5’ ends of broken strand are degraded by an exonuclease to create 3’ single-stranded tails)
3. Strand invasion(Dmc1 helps one single-strand tail invade a non-sister chromatid and froms a stable heteroduplex. Displacement loop from invaded chromatid is stabalised by single-stranded binding protein)
4. Formation of double Holliday junction (D-loop and other 3’ tail form a second heteroduplex.
   DNA synthesis replaces the DNA degraded during formation of the 3’ tails.
   DNA ligase reseals the DNA backbones.)
5. Branch migration (Heteroduplex region of both DNA molecules is lengthened)
6. Resolution of the Holliday junction (Holliday junctions are resolved by resolvase and ligase.)

Question 7

Give the 5 steps of the non-crossing over pathway of recombination

Answer 7

Step 1:Double-strand break formation
Step 2:Resection
Step 3:Strand invasion
Step 4:Elongation of invading strand – but anti-crossover helicase disentangles the invading strand from the nonsister chromatid preventing Holliday junction formation
Step 5:DNA synthesis of remaining gaps occur – DNA ligase links strands – forms a heteroduplex (but not Holliday juctions)

Question 8

Why do recombination events not always result in crossing over

Answer 8

Because this depends on how the Holliday junction is resolved.