Which of the following statements about DNA synthesis at the replication fork of a replicating DNA molecule is FALSE? Nucleotides are added at the 3′ ends of all the new strands in a replicating DNA molecule. The sequence of each newly synthesized single strand is identical to that of the parental single strand that served as its template. DNA replication requires RNA primers. Prokaryotes do not have telomerase DNA helicase and DNA ligases are required for DNA replication
The false statement is: “Nucleotides are added at the 3′ ends of all the new strands in a replicating DNA molecule.”
Explanation:
DNA synthesis occurs in a 5′ to 3′ direction, meaning that new nucleotides are always added to the 3′ end of a growing strand. This is true for both the leading strand and the lagging strand, though the mechanisms differ for each strand.
- Leading Strand:
The leading strand is synthesized continuously in the 5′ to 3′ direction. DNA polymerase adds nucleotides to the 3′ end of the newly forming strand, using the 5′ to 3′ direction of the template strand. - Lagging Strand:
The lagging strand is synthesized in the opposite direction (3′ to 5′ direction relative to the replication fork), but still, new nucleotides are added to the 3′ end of the growing strand. However, since this strand is synthesized in short segments called Okazaki fragments, the synthesis occurs in the opposite direction relative to the replication fork. These fragments are later joined together by DNA ligase.
Other Statements:
- Sequence of newly synthesized strand identical to the parental strand:
The newly synthesized strand is complementary to the parental strand, not identical. In the case of A pairing with T, and C pairing with G, the sequence on the new strand will be the reverse complement of the template strand. - RNA primers:
RNA primers are essential for DNA replication. They provide a starting point for DNA polymerase to add nucleotides. Without them, DNA polymerase cannot initiate the synthesis of a new strand. - Prokaryotes and Telomerase:
Telomerase is not required in prokaryotes because they generally have circular chromosomes that do not shorten during replication. Telomerase is crucial for eukaryotic chromosomes to prevent the shortening of telomeres during replication. - DNA helicase and DNA ligase:
Both DNA helicase and DNA ligase are essential for replication. DNA helicase unwinds the double helix ahead of the replication fork, and DNA ligase joins Okazaki fragments on the lagging strand.
In summary, the key error in the false statement is the direction in which nucleotides are added. It is 5′ to 3′ for both strands, even though their synthesis occurs differently on the leading and lagging strands.
