Question

2. Haploid yeast cells that preferentially repair double-strand breaks by homologous recombination are especially sensitive to agents that cause double-strand breaks in DNA. If the breaks occur in the G1 phase of the cell cycle (before DNA replication), most yeast cells die; however, if the breaks occur in the G2 phase (after DNA replication), a much higher fraction of cells survive.

(a) What are the key proteins required for homologous recombination in yeast (a eukaryote)?

(b) Yeast can also carry out repairs by non-homologous end-joining (NHEJ). How does this form of repair different from repair of double-strand DNA breaks by homologous recombination?

(c) Explain the observations described above.

Answer 1

Answer a:)

Two main proteins Rad51 and Dmc1 are important in the homologous recombination in yeasts. Rad 51 and DMc1 work as pairing and shuffling the homologous chromosomes.

Answer b:)

In the non-homologous end-joining (NHEJ) method, it does not require a homologous sequence for the DNA repairing. This method occurs mainly in G1 phase.

Answer c:)

If DNA breaks occur in G1 phase, there is a maximum probability that the DNA will get mutated and damaged, which is the form of genotoxic stress. This causes the arrest of the cell cycle and leads to the death of the cell. DNA breaks in the G2 phase allow the DNA to get repair through the homologous recombination that is the best method to correctly repair the DNA. That is why DNA breaks in G1 phase do not provide more production of cells than DNA breaks in the G2 phase.