Question

1. A) List five enzymes (must end in –ase) found in the replisome and in one/two sentences explain their activity (role).

B) Describe the 3’5’ exonuclease activity of DNA polymerase III. Why is this a useful activity (how does it help replication)?

C) Look for the codons coding for Proline in the genetic code. One of them is CCC. What would the codon code for if there is a mutation in the wobble position to an A, G, T/U? What would the codon code for?

Answer 1

A). The replisome is consists of different proteins including RFC, PCNA, helicase, gyrase or topoisomerase, DNA polymerase III, RNAse H, SSB or RPA, primase, and ligase.

Functions:

Helicase - Helicase unwind the double-stranded DNA.

Gyrase – DNA gyrase is a topoisomerase that eradicates the twist that is produced when the two strands of the double helix are unwinded.

Primase - DNA primase is involved in DNA replication and is a type of RNA polymerase. Primase catalyzes the production of a short RNA segment known as a primer which is complementary to an ssDNA template.

DNA polymerase – It is an enzyme that manufactures DNA molecules from deoxy-ribonucleotides. These enzymes are necessary for replication of DNA and generally work in pairs to produce two identical strands of DNA from a single original molecule of DNA.

RNAse - RNase HI is used to demolish the RNA template after the synthesis of cDNA by reverse transcription.

Ligase - a ligase enzyme facilitates the combination of DNA strands cooperatively by catalyzing the construction of a phosphodiester bond.

B). Being the main holoenzyme occupied in the replication activity, the DNA Pol III also has proofreading capability that corrects the replication errors by way of exonuclease activity reading in 3'→5' and synthesizing in 5'→3' direction. Pol III is a part of the replisome, situated at the replication fork. Once an incorrect base pair is acknowledged, the DNA polymerase shifts backward by one DNA base pair. After that the 3'–5' exonuclease enzyme activity of permit the incorrect base pair to be removed. This whole process is called proofreading.

C). The genetic code for proline is CCA, CCC, CCU, and CCG. The third codon is the wobble position (A, C, U, G).

Mutation in the third wobble will lead to the alteration of the protein function called a point mutation or some alteration lead to a missense mutation in which the codon codes for the same protein after mutation.

In the case of proline, the mutation in the wobble position is a type of missense mutation the codons will codes for proline only even after the change in the third codon.

For example – CCC → CCA → CCU→ CCG all codes for proline.