21. Which one of the following statements about DNA replication is INCORRECT?

(1) During DNA replication,

When adjacent bidirectional forks converge, the lagging strandwill meet the leading strand of the same template strand

(2) Mispaired nucleotide at the 3′-OH end of the primer strand triggers the 3-5′ exonucleolyticproof reading activity

(3) In a replication bubble moving bidirectionally, the same parental DNA strand cannot serve as a template for both the lagging and leading strand synthesis
(4) DNA replication involves a RNA-DNA chimeric molecule

Analysis of Each Statement

(1) During DNA replication, when adjacent bidirectional forks converge, the lagging strand will meet the leading strand of the same template strand.

• This is correct. As two replication forks converge, the lagging strand synthesized from one fork will meet the leading strand synthesized from the opposite fork on the same template strand. This is supported by models of replication termination where the two forks meet and their respective leading and lagging strands are completed and joined27.

(2) Mispaired nucleotide at the 3′-OH end of the primer strand triggers the 3-5′ exonucleolytic proofreading activity.

• This is correct. DNA polymerases possess 3’→5′ exonuclease activity that removes mispaired nucleotides from the 3′-OH end, serving as a proofreading mechanism to maintain fidelity during replication.

(3) In a replication bubble moving bidirectionally, the same parental DNA strand cannot serve as a template for both the lagging and leading strand synthesis.

• This is incorrect. In reality, each parental DNA strand serves as a template for both leading and lagging strand synthesis at different replication forks. For example, at one fork, one parental strand is the template for the leading strand, while at the other fork, the same parental strand is the template for the lagging strand. Thus, the same parental DNA strand can indeed serve as a template for both leading and lagging strand synthesis, depending on the direction of fork movement138.

(4) DNA replication involves an RNA-DNA chimeric molecule.

• This is correct. During lagging strand synthesis, Okazaki fragments are initiated by short RNA primers, resulting in RNA-DNA chimeric molecules until the RNA is replaced by DNA.

Correct Answer

(3) In a replication bubble moving bidirectionally, the same parental DNA strand cannot serve as a template for both the lagging and leading strand synthesis.

Conclusion

The incorrect statement is option (3). In a bidirectional replication bubble, each parental DNA strand can act as a template for both leading and lagging strand synthesis at different forks, depending on the direction of synthesis relative to the replication fork.

Keywords: DNA replication, leading strand, lagging strand, replication fork, bidirectional replication, proofreading, RNA-DNA chimeric molecule, Okazaki fragments, molecular biology