in this ppt, I cover up the mechanism of class switching in yeast to determine there mating type which helpful to understand the role of genetics in mating

1. Mating type in Saccharomyces cerevisiae
subject: Genetics
PRESENTED BY
PARTH SHARMA
MSC 1ST SEMESTER
DEPARTMENT OF BIOTECHNOLOGY

2. Introduction
 Saccharomyces cerevisiae is a simple single-celled eukaryote with both a diploid and
haploid mode of existence.
 Mating of yeast only occurs between haploids, which can be either the a or (alpha)
mating type.
 Haploid cells are capable of mating with other haploid cells of the opposite mating type.
 an a cell can only mate with an cell, and vice versa to produce a stable diploid cell.

3. life cycle of Saccharomyces cerevisiae
stressful condition
Meosis
2 a cells 2 cells
gene activation gene activation
Produce a factor and Ste2 receptor produce factor and Ste3 receptor
cells respond to a factor a cells respond to factors
Diploid cell

5. The MAT locus
• Characterization a and cells are caused by the presence of one of two alleles of a mating-type locus
called MAT: MATa or MAT located on chromosome III.
• MATa allele of MAT encodes a gene called a1, which in haploids direct the transcription of the a-specific
transcriptional program (such as expressing STE2 and repressing STE3) that defines an a cell.
• MAT allele of MAT encodes the α1 and α2 genes, which in haploids direct the transcription of the -
specific transcriptional program (such as expressing STE3, repressing STE2, producing prepro-α-factor)
which causes the cell to be an cell.

6. Mating type switching
• Haploid yeast switch mating type by replacing the information present at
the MAT locus. For example, an a cell will switch to an cell by replacing
the MATa allele with the MAT allele.
• The replacement MAT is possible because yeast cells carry an additional
silenced copy of both the MATa and MAT alleles
• MAT a : HMR (homothallic mating right)
• MAT : HML (homothallic mating left).
• The process of mating type switching is a gene conversion event initiated
by the HO gene.
• HO gene is a tightly regulated haploid-specific gene that is only activated
in haploid cells during the G1 phase of the cell cycle.

7. Mechanics of the mating type switch
HO gene
Codes DNA endonuclease
Cleaves DNA at the MAT locus
Degrade the cut site
Gap genrated
DNA is repaired by copying in the genetic information
present at either HML or HMR

8. THANK YOU