Which of the following does not occur during the normal course of DNA replication in a typical human cell? A. Proofreading of newly synthesized DNA by DNA polymerase B. Copying of non-coding repeat sequences at the ends of chromosomes C. Ligation of completed Okazaki fragments to the neighboring primers D. Initiation at multiple replication origins E. Association of single-strand binding protein with DNA
The correct answer and explanation is:
Correct Answer: C. Ligation of completed Okazaki fragments to the neighboring primers
Explanation (≈300 words):
DNA replication is a vital process during cell division in human cells. It ensures each daughter cell receives an exact copy of the DNA. Let’s evaluate each option to understand why C is the correct answer (i.e., the one that does not occur):
A. Proofreading of newly synthesized DNA by DNA polymerase –
✔️ Occurs.
DNA polymerase has 3’ to 5’ exonuclease activity that allows it to remove incorrectly paired bases and replace them with the correct ones, ensuring high fidelity in replication.
B. Copying of non-coding repeat sequences at the ends of chromosomes –
✔️ Occurs.
These are the telomeres, and while normal DNA polymerase cannot replicate them completely, the enzyme telomerase extends them. This is especially important in germline and stem cells, though less active in most somatic cells.
C. Ligation of completed Okazaki fragments to the neighboring primers –
❌ Does NOT occur.
This is the correct answer. DNA ligase does not ligate Okazaki fragments to RNA primers. Instead, RNA primers are first removed by DNA polymerase I (in prokaryotes) or an enzyme complex involving RNase H and DNA polymerase δ/ε (in eukaryotes). Then, the resulting gaps are filled with DNA, and finally, DNA ligase seals the nicks between DNA fragments—not between DNA and RNA.
D. Initiation at multiple replication origins –
✔️ Occurs.
Human DNA is massive, so replication starts at multiple origins of replication to ensure the genome is copied efficiently and in a timely manner.
E. Association of single-strand binding protein with DNA –
✔️ Occurs.
SSB proteins (or RPA in humans) bind to the single-stranded DNA during replication to prevent re-annealing and protect the strands from degradation.
Summary:
Option C is correct because DNA ligase only connects DNA to DNA—not DNA to RNA primers, which must be removed first.