EasyBiologyHigh School (11–12)Heredity and Evolution › Molecular Basis of InheritanceConceptualAPA-LevelIB
A replication fork opens towards the right…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A replication fork opens towards the right. The top template strand of the parent DNA runs from its 3′ end on the left to its 5′ end on the right. The bottom template strand runs from its 5′ end on the left to its 3′ end on the right.
- Along which template is the leading strand made?
- Along which template are Okazaki fragments made?
- In which direction does the new strand along the top template grow, and in which direction does each fragment along the bottom template grow?
Need a reminder? The key ideaShow
A DNA polymerase adds nucleotides only to a 3′ end, so a new strand grows 5′ to 3′. The leading strand is made in one piece. The lagging strand is made as Okazaki fragments, which DNA ligase joins. An origin starts two forks, and the copying takes .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. The fork moves to the right. Top template: 3′ on the left to 5′ on the right. Bottom template: 5′ on the left to 3′ on the right.
Plan. A new strand runs opposite to its template and grows 5′ to 3′. Then see whether that is towards the fork or away from it.
Step 1. (a) The new strand along the top template has its 5′ end on the left and grows to the right, which is towards the fork. So the leading strand is made along the top template.
Step 2. (b) The new strand along the bottom template has its 5′ end on the right and grows to the left, which is away from the fork. It can be made only in pieces, so the Okazaki fragments are along the bottom template.
Step 3. (c) The top strand grows to the right. Each fragment grows to the left.
Answer: (a) top; (b) bottom; (c) right, and left.
Check. The two new strands run in opposite directions, as the two parent strands do.
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