2. this yeast is used for winemaking, baking and brewing
It is one of the most intensively studied eukaryotic model
organisms in molecular and cell biology
There are two forms in which yeast cells can survive and
grow: haploid and diploid
Why it is preferred organism for genetic research?
1. simple life cycle
2. Alternating haploid and diploid phases
3. short generation time, and easy to-identify meiotic products
3. Haploid S. cerevisiae has 16 linear chromosomes.
Each chromosome contains:-
1. One centromere /kinetochore
2. Two termini consisting of longer subtelomeric repeats
3. Followed by short telomere repeats at the very ends
4. Multiple origins of replication spaced approximately 30–40 kb
Chromosomes are packaged into nucleosomes consisting of the
core histones H2A, H2B, H3, and H4.
4.
5. Pulse-field gel electrophoresis, which separates intact yeast
chromosomes, produces molecular “karyotypes” of the yeast
genome.
Chromosome I, about 235 kb in length smallest one.
Chromosome XII is the largest; its size varies between about 2060
and 3060 kb because of a variable number of tandem ribosomal
RNA genes (rDNA).
6. The total genetic map length, a function of the frequency of
meiotic recombination, is about 4400 cM. With an average of
3 kb/cM.
Total genetic map of yeast is extraordinarily long compared to
the genetic map of other fungal organisms.
7. 6000 and 6500 genes
2000 have no known function, known as orphan genes
To predict the number of genes that code for proteins from
the number of open reading frames (ORFs) identified in the
yeast genomic DNA sequence
2,60,000 ORFs were identified assuming a length of 2–99
amino acids.
About 1,14,000 ORFs assuming a length of 15–99 amino acids.
8. Among the protein-coding genes, introns occur in only 4% to 5%
of all yeast genes.
274 tRNA genes.
71 small nucleolar RNAs.
5 small nuclear RNAs that function in intron splicing, a few RNAs
of unknown function, and three RNAs that serve as functional
subunits of the enzymes RNase P, endoribonuclease MRP, and
telomerase.
9. yeast genome is composed of unique, nonreiterated DNA
sequences.
Approximately 75% of the genome is transcribed into RNA.
Reiterated DNA sequences:-
1. 100–200 copies of rDNA
2. tRNA genes
3. subtelomeric repeats
4. Transposable elements
5. 385 solo LTRs
10. Yeast cells can proliferate both as haploids (1n, one
copy of each chromosome) and as diploids (2n, two
copies of each chromosome).
Haploid cells have one of two mating types: a or α
(alpha).
Two haploid cells can mate to form a zygote; since
yeast cannot move, cells must grow towards each
other (shmoos).
In Conditions such as nutrient depletion, Diploid cell
can undergo meiosis to produce four haploid spores:
two a spores and two α spores.
11. 'a' cells produce ‘a-factor’, a mating pheromone which
signals the presence of an a cell to neighbouring α cells. &
alpha cell produces alpha -factor & form conjucation duct.
These phenotypic differences between a and α cells are
due to a different set of genes being actively transcribed
a cells activate genes which produce a-factor and produce
a cell surface receptor (Ste2) which binds to α-factor and
triggers signaling within the cell. a cells also repress the
genes associated with being an α cell Similarly, α cells
result in production of (ste3) receptor.
12. The different sets of transcriptional repression and
activation which characterize a and α cells are caused
by the presence of one of two alleles of a locus called
MAT: MATa or MATα located on chromosome III.
The 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) which defines an a cell.
The 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) which defines an a cell.
13. Like the differences between haploid a and α cells,
different patterns of gene repression and activation
are responsible for the phenotypic differences
between haploid and diploid cells.
In addition to the specific a and α transcriptional
patterns, haploid cells of both mating types share a
haploid transcriptional pattern which activates
haploid-specific genes (such as HO) and represses
diploid-specific genes (such as IME1). Similarly,
diploid cells activate diploid-specific genes and
repress haploid-specific genes.