Why can’t both strands of dna be replicated at the same time?

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

Acertain species of fish can have either long or short fins. the allele for long fins is dominant over the allele for short fins. a heterozygous, long-finned fish is crossed with a homozygous, short-finned fish. of the offspring, will have long fins and be , and will have short fins and be .

Select the correct answer from each drop-down menu. which phase of mitotic division is the highlighted cell undergoing? the cell is undergoing , which is the stage before .