Question

What DNA products would be generated if one of the proteins including DNA polymerase, DNA ligase, sliding clamp for DNA polymerase, nuclease that removes DNA primers, DNA helicase, and primase, were missing?

Answer 1

1. DNA polymerase: DNA polymerase adds complementary nucleotides to the newly synthesizing DNA strand. They are also involved in DNA repair. If there is no DNA polymerase, then replication cannot take place. Only NA will unwind and RNA primers will bind to the origin of replication. However, no new DNA synthesis will occur.

2. DNA ligase: DNA ligase adds phosphates to the gap in the phosphate-sugar backbone. Thus, it seals gaps in DNA strands. DNA ligase is required to seal the gaps on Okazaki fragments in the lagging strand. Hence, in absence of this enzyme, the DNA fragments will not be ligated and remain as fragments. However, the nucleotides in these fragments are not affected. Leading strand replication is not affected.

3. Sliding Clamp for DNA polymerase: The sliding clamp is a protein fold that functions as a processivity promoting factor. It will bind to DNA polymerase and prevent its dissociation from the template. Lack of sliding clamp will slow down replication, as the DNA polymerase will keep falling off the template. There will be replication but due to constant dissociation of the polymerase, the replication rate is affected.

4. Nuclease that removes the primers: The nuclease cleaves off the RNA primers that have are required for synthesis of lagging strand. If there is no nuclease, the primers will remain attached to the Okazaki fragments. DNA ligase therefore cannot be able to ligate or seal the gaps between the Okazaki fragments. Hence, the newly replicated DNA has DNA as well as RNA.

5. DNA helicase: Replication starts at the origin of replication. Initiation of replication involves formation of Replication fork. The DNA helicase unwinds the DNA from the origin of replication to separate the two strands of DNA. In absence of DNA helicase, the DNA strands will not separate and cause the DNA polymerase to stall replication. Little or no new replicated DNA will be formed.

6. DNA Primase: After the DNA strands have been separated, short sequences of RNA called primers bind to the DNA strand. The Primase enzyme synthesizes these primers. A single primer is required for leading strand (3’ to 5’). However, for the lagging strand (5’ to 3’), many primers have to be synthesized for generation of Okazaki fragments. As primase is absent, the RNA primers are not synthesized. Hence, DNA replication is not initiated. No new DNA stands can be synthesized.