Question

why cant dna polymerase run in reverse and synthesize/elongate in the 3 → 5 direction? why is lagging strand synthesis necessary?
if dna polymerase could synthesize in both the 5 → 3 and 3 → 5 directions, the various dna polymerases at each replication bubble would run into each other and cause incomplete replication.
dna polymerase can synthesize in the 3 → 5 direction, but it would result in a double - stranded dna molecule that has parallel orientation rather than an antiparallel orientation.
elongating dna in the 3 → 5 direction is slower than elongating dna in the 5 → 3 direction. lagging strand synthesis allows dna replication of both strands at an equal rate.
the 5 end of a dna molecule is structurally different from the 3 end and would not fit into the active site of dna polymerase.

Explanation:

DNA polymerase has a specific active site that is complementary to the 3' end structure of the DNA strand. The 5' end has a different structure (such as a phosphate group in a configuration that is not compatible with the enzyme's active - site geometry for elongation). This structural difference means that the 5' end cannot fit into the active site of DNA polymerase for synthesis in the 3'→5' direction.

Answer:

The 5' end of a DNA molecule is structurally different from the 3' end and would not fit into the active site of DNA polymerase.