Chapter 6.5: Recombination at the DNA Level

Question 1

What is a recombianation event?

Answer 1

o Use the term recombination event to describe the molecular process initiated by Spo11, whether or not it results in crossing-over

Question 2

Describe step 6 of recombination: Resolution of the Holliday Junction

Answer 2

• The two interlocked nonsister chromatids must disengage. Separation is achieved by breakage of two DNA strands at each Holliday junction by an enzyme called resolvase; the strands are subsequently rejoined by DNA ligase. Crossing-over results because each of the four strands is cut once and rejoined.

Question 3

What are the steps of recombination? (6)

Answer 3

Step 1: Double-strand break formation Step 2: Resection Step 3: First strand invasion Crossover Pathway Step 4: Formation of a Double Holliday Junction Step 5: Branch Migration Step 6: Resolution of the Holliday Junction Noncrossover Pathway: Step 4’: Strand Displacement and Annealing Step 5’: DNA Synthesis and Ligation

Question 4

Define recombination

Answer 4

the generation of allele combinations through genetic exchange between homologous chromosomes

Question 5

What are the products of crossing over?

Answer 5

recombinants

Question 6

Describe step 4 of recombination: Formation of a double holliday junction

Answer 6

• New DNA added to the invading 3’ tail enlarges the D-loop until the single-stranded bases on the displaced strand can form complementary base pairs with the 3’ tail on the nonsister chromatid. New DNA added to this latter tail re-creates the DNA duplex on the bottom chromatid. At each side of the original break, the 3’ end of the newly synthesized DNA is connected to a 5’ end left after resection, and DNA ligase forms phosphodiester bonds to rejoin DNA strands without the loss or gain of nucleotides. The resulting X-shaped structures are called Holliday junctions

Question 7

Describe step 3 of recombination: First strand invasion

Answer 7

 The protein Dmc1 collaborates with other proteins to help one of the tails to invade and open up the other chromatids double helix. Dmc1 then moves along the double helix, prying it open. The invading strand scans the base sequence it passes in the momentarily unwound stretches of DNA duplex. As soon as it finds a complementary sequence of sufficient length, the invading strand becomes immobilized by dozens of hydrogen bonds and forms a stable heteroduplex. The strand displaced by the invading tail forms a D-loop (for displacement loop), which is stabilized by binding of replication protein A (RPA)

Question 8

What are chromosomes that carry a mix of alleles derived from different homologs?

Answer 8

recombinants

Question 9

Describe step 5 of recombination: Branch migration

Answer 9

• The two invading strands tend to “zip up” by base pairing with the complementary strands of the parental double helixes they invade. The DNA double helixes unwind in front of this double zippering action and two newly created heteroduplex molecules rewind behind it. Branch migration thus lengthens the heteroduplex region of both DNA molecules.

Question 10

What are the six properties of recombination?

Answer 10

 Homologs physically break, exchange parts, and rejoin  Crossing-over occurs between nonsister chromatids after DNA replication  Breakage and repair generate reciprocal products of recombination  Recombination events can occur anywhere along the DNA molecule  Precision in the exchange—no gain or loss of nucleotide pairs—prevents mutations from occurring during the process  Gene conversion—the physical change of one allele in a heterozygote into the other—sometimes occurs as a result of recombination

Question 11

Describe step 5’ of recombination: DNA synthesis and ligation

Answer 11

• The remaining gap in the double-stranded DNA sequence is filled by DNA synthesis and DNA ligase forms phosphodiester bonds to rejoin the DNA strands. The result is no crossover, but a heteroduplex region remains nonetheless.

Question 12

Define recombinants

Answer 12

chromosomes that carry a mix of alleles derived from different homologs

Question 13

Describe step 2 of recombination: Resection

Answer 13

 An exonuclease (an enzyme that removes nucleotides from an end of a DNA molecule) degrades the 5’ ends on each side of the break to produce two 3’ single-stranded tails

Question 14

Describe step 4’ of recombination: Strand displacement and annealing

Answer 14

• The invading strand was extended by DNA synthesis using the nonsister chromatid as a template. An anticrossover helicase enzyme has disentangled the invading strand and the nonsister chromatid

Question 15

What is the generation of allele combinations through genetic exchange between homologous chromosomes?

Answer 15

recombination

Question 16

Describe step 1 of recombination: Double-strand break formation

Answer 16

 During meiotic prophase, Spo11 protein makes a double-strand break on one of the chromatids by cleaving the phosphodiester bonds between adjacent nucleotides on both strands of DNA

Question 17

Define gene conversion

Answer 17

the physical change of one allele in a heterozygote into the other

Question 18

What is the physical change of one allele in a heterozygote into the other?

Answer 18

gene conversion