Lecture 2▯
In this lecture we are going to consider experiments on yeast, a very us▯eful organism for
genetic study. Yeast is more properly known as Saccharomyces cerevisiae, which is the
single-celled microbe used to make bread and beer. Yeast can exist as haploids of either
mating type α▯(MATα) or mating tya(MAT a. Haploid cells of different mating type
when mixed together will mate to make a diploid cell.
Haploids and diploids are isomorphic – meaning that a given mutation ▯will cause essentially
the same change in haploid and diploid cells. This allows us to look at the effect of having
two different alleles in the same (diploid) cell.
All yeast needs to grow are salts, minerals, and glucose (minimal mediu▯m). From these
compounds, yeast cells can synthesize all of the molecules such as amino▯ acids and
nucleotides that are needed to construct a cell. The synthesis of complicated molecules
requires many enzymatic steps. When combined, these enzymatic reactions constitute a
biochemicalpathwayy
Consider the pathway for the synthesis of the amino acid histidine.
A →▯ B →▯ C →▯ D → histidine→ Protein
Enzyme: 1 2 3 4
Each intermediate compound in the pathway is converted to the next by an▯ enzyme. For
example, if there is a mutation in the gene for enzyme 3 then intermedia▯te C can not be
converted to D and the cell can not make histidine. Such a mutant will only grow if
histidine is provided in the growth medium. This type of mutation is known as aauuxotrophic mutattonoand is very useful for genetic
analysis.
growth on minimal growth on minimal + histidine
His+ (wild type) + +
–
His – +
Phennotype:All traits of an organism (with an emphasis on trait under investigati▯on)
Hoomozyygote:iploid with two like alleles of same gene
Heeterozygote:: diploid with two different alleles of same gene
Reccessve Allele: trait not expressed in heterozygote
genotype phenotype Mate to : diploid genotype diploid phenotype
– – – – – –
MAT Has3 His MATα His3 His3 /His3 His
– – + – + +
MAT Has3 His MATα His3 His3 /His3 His
– – + –
Based on the His phenotype of the His3 /His3 heterozygote, we would say that His3 is
recessive to wild type.
Let’s consider a different kind of mutation giving resistance to copp▯er that occurs in a
gene known as CUP1.
genotype phenotype Mate to : diploid genotype diploid phenotype
r + r +
MAT Cap1 copper resistant MATα Cup1 Cup1 /Cup1 copper resistant
Doominant Allee:etrait is expressed in heterozygote
Cup1 is dominant to wild type (Cup1 ).
The terms dominant and recessive are simply shorthand expressions for th▯e results of
particular experiments. If someone says a particular allele is dominant that means that
at some point they constructed a heterozygous diploid and found that the▯ trait was
expressed in that diploid. Note: Sometimes an allele will have more than one phenotype and may be recessi▯ve for
one and dominant for another. In such cases, the phenotype must be specified when one
is making statements about whether the allele is dominant or recessive. ▯ Consider for
example, the allele for sickle cell hemoglobin in humans designated Hb . Heterozygous
s a s
individuals (Hb /Hb ) are more resistant to malaria, thus Hb is dominant for the trait of
malaria resistance. On the other hand, Hb /Hb heterozygotes do not the debilitating
s s s
sickle cell disease, but Hb /Hb homozygous individuals do. Therefore, Hb is recessive
for the trait of sickle cell disease.
Once we find out whether an allele is dominant or recessive, we can alre▯ady infer
important information about the nature of the allele. The following conclusions will
usually be true.
Reccessve alleles usually cause the loss of something that is made in wild type
Doominantalleles usually cause increased activity or new activity
It turns out that the Cup allele actually carries more copies of the gene for a copper
binding protein and therefore increases the activity of the gene.
Last lecture we defined the gene structurally as the DNA needed to encode
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