Question

DNA replication requires many enzymes to accomplish the process successfully. This results in a replication process that is known to be bi-directional, semi-conservative and semi-discontinuous. You discover a new prokaryotic organism that lacks the gene coding for an DNA polymerase I enzyme, and therefore this enzyme is not made in this species. Additionally, the DNA polymerase III enzyme from this organism lacks any 3’ to 5’ nuclease activity but has 5’ to 3’ nuclease activity. Based on what you know about DNA replication and the functions of DNA polymerase I and DNA polymerase III, discuss the effect of these changes on DNA replication in this organism. Please explain your answer in full.

Answer 1

Answer:

DNA polymerase I is the enzyme which has 5' to 3' polymerase activity which it uses to synthesise DNA by using DNA as template.

This enzyme also has 3' to 5' exonuclease activity which it uses for proofreading if the wrong nucleotide is added. It moves back one nucleotide and by 3' exonuclease activity it removes the wrong nucleotide and replaces it with correct nucleotide.

The DNA polymerase I also has 5' to 3' exonuclease activity. This activity of DNA Polymerase I is used to remove the RNA Primers. The DNA Polymerase I removes the Ribonucleotides from the 5' end of the RNA Primer and replaces it with the deoxyribonucleotide through 5' to 3' Polymerase activity. In this way it replaces the RNA primer with DNA segment leaves only a nick which is filled by the DNA ligase.

If the cell does not have the gene for the DNA Polymerase I , then it will not be able to remove the RNA primers and thus Okazaki Fragments will not be ligatated.

The DNA Polymerase III is the main enzyme responsible for DNA replication in bacteria and has the 3' to 5' exonuclease activity which it uses for the proofreading when a wrong nucleotide is added. The enzyme moves one nucleotide backwards and removes the wrong nucleotide by it's 3' to 5' exonuclease activity and then replaces the wrong nucleotide with the correct nucleotide by 5' to 3' polymerase activity.

This DNA Polymerase lacks any 5' to 3' exonuclease activity, thus it cannot be used to remove the RNA primers which are removed by DNA Polymerase I.

If the DNA Polymerase III lacks the 3' to 5' exonuclease activity it means, proofreading cannot occur by this enzyme. Thus replication will result in errors and hence mutations will occur as this is the main enzyme responsible for prokaryotic replication.

However if in a cell the DNA Polymerase III attains the the 5' to 3' exonuclease activity it can remove the RNA primers and thus Okazaki fragments will be ligatated by Ligase.

Thus, in this mutated cell there will be much more errors during replication because of the loss of the 3' to 5' exonuclease activity of DNA Polymerase III and the role of DNA Polymerase I will be fulfilled by the DNA Polymerase III by having 5' to 3' exonuclease activity.