Worked Problems

1. The following diagram (below0 represents the template strands of a replication bubble in a DNA molecule. Draw in the newly synthesized strands and label the leading and lagging strands.

Origin

Unwinding

Origin

Unwinding

To determine the leading and lagging strands, first note which end of each template strands is 5' and which end is 3'. With a pencil, draw in the strands being synthesized on these templates, and label their 5' and 3' ends, recalling that the newly synthesized strands must be antiparallel to the templates.

Origin

Unwinding

Unwinding

Origin

Unwinding

Next, determine the direction of replication for each new strand, which must be 5':3'. You might draw arrows on the new strands to indicate the direction of replication. After you have established the direction of replication for each strand, look at each fork and determine whether the direction of replication for a strand is the same as the direction of unwinding. The strand on which replication is in the same direction as unwinding is the leading strand. The strand on which replication is in the direction opposite that of unwinding is the lagging strand. Make sure that you have one leading strand and one lagging strand for each fork.

Origin

Unwinding

Unwinding

Origin

Unwinding that would appear if replication were (b) conservative; (c) dispersive.

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