Question

Q1. What is the difference in mating-specific gene expression between a MATa haploid and a diploid yeast strain? What causes this difference?

Q2.  The O region of the phage lambda genome has three sub-sequences, OR1-OR3. The three sequences are:

TATCACCGCAAGGGATA OR3

TAACACCGTGCGTGTTG OR2

TACCTCTGGCGGTGATA OR1

a. From highest to lowest affinity, list the binding sites for Cro and CI.

b. The sequences are very similar. What about them determines the binding preferences of the two proteins?

c. How would deleting OR1 (but not PR) affect CI regulation of cro and cI transcription?

Answer 1

1. . Cells are MATa if they have a single gene a1, In MATacells, genes encoding general mating functions, such as components of a signalling pathway mentioned below, are expressed independently of the mating-type proteins; they are expressed constitutively. In haploid MATa cells, genes required for expression of MATa-cell-specific mating functions are also constitutively expressed because the a1 protein has no role in haploid cells. The different gene expression patterns of haploids and diploids are again due to the MAT locus. Haploid cells only contain one copy of each of the 16 chromosomes and thus can only possess one allele of MAT (either MATaor MATα), which determines their mating type. Diploid cells result from the mating of a cell and an α cell and thus possess 32 chromosomes (in 16 pairs), including one chromosome bearing the allele and another chromosome bearing the MATα allele. The combination of the information encoded by the MATaallele (the a1 gene) and the MATα allele (the α1 and α2 genes) triggers the diploid transcriptional program. Similarly, the presence of only a single allele of MAT, whether it is Mata or MATα, triggers the haploid transcriptional program

In the yeast, Saccharomyces cerevisiae is a simple single-celled eukaryote with both a diploid and haploid mode of existence. The mating of yeast only occurs between haploids, which can be either the a or α (alpha) mating type and thus display simple sexual differentiation.[1] Mating type is determined by a single locus, MAT, which in turn governs the sexual behaviour of both haploid and diploid cells. Through a form of genetic recombination, haploid yeast can switch mating type as often as every cell cycle.