Question

part c

the following picture shows the two holliday junctions (labeled junction x and junction y) that are formed during the double holliday junction model of homologous repair. after the resolution step, you do not want crossing over to have happened. in what ways can resolution occur in order to not observe crossing over?

select all that apply.

resolution of x at 1 and y at 2
resolution of x at 2 and y at 1
resolution of x at 2 and y at 2
resolution of x at 1 and y at 1
resolution of y at 1 and y at 2
resolution of x at 1 and x at 2

Explanation:

Understand Holliday junction cleavage planes

Using the Double Holliday Junction Model and Homologous Recombination Repair knowledge points:
Each Holliday junction (Junction X and Junction Y) can be resolved in one of two planes:

• Plane 1 (vertical cleavage): Cleaves the crossing strands (non-parental/recombinant strands).
• Plane 2 (horizontal cleavage): Cleaves the non-crossing strands (parental/non-recombinant strands).

Determine outcomes of double junction resolution

Using the Double Holliday Junction Model knowledge point:
When resolving two adjacent Holliday junctions, the overall genetic outcome (crossover vs. non-crossover) depends on the combination of cleavage planes used at each junction:

• Crossover outcome: Occurs when the two junctions are resolved in different planes (one in Plane 1 and the other in Plane 2).
• Resolution of X at 1 and Y at 2
• Resolution of X at 2 and Y at 1
• Non-crossover outcome: Occurs when both junctions are resolved in the same plane (both in Plane 1, or both in Plane 2).
• Resolution of X at 1 and Y at 1
• Resolution of X at 2 and Y at 2

Evaluate the given options

Using the Double Holliday Junction Model knowledge point:
We want to identify the ways resolution can occur to not observe crossing over (non-crossover outcomes):

• Resolution of X at 1 and Y at 2: Different planes \(

ightarrow\) Crossover.

• Resolution of X at 2 and Y at 1: Different planes \(

ightarrow\) Crossover.

• Resolution of X at 2 and Y at 2: Same plane \(

ightarrow\) Non-crossover (Correct).

• Resolution of X at 1 and Y at 1: Same plane \(

ightarrow\) Non-crossover (Correct).

• Resolution of Y at 1 and Y at 2: Invalid option (describes resolving a single junction in two ways).
• Resolution of X at 1 and X at 2: Invalid option (describes resolving a single junction in two ways).

Answer:

• Resolution of X at 1 and Y at 2
• Resolution of X at 2 and Y at 1
• Resolution of X at 2 and Y at 2 (Correct answer)
• Resolution of X at 1 and Y at 1 (Correct answer)
• Resolution of Y at 1 and Y at 2
• Resolution of X at 1 and X at 2