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Lecture

Readings 5 notes


Department
Biological Sciences
Course Code
BIOC51H3
Professor
Maydianne Andrade

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Readings 5
Pgs.143-166
-meoisis reshuffles existing alleles into new combinations
-mutation is the only process that creates new alleles and new genes (source of genetic
variation)
-adenine and guanine are purines (two) while cytosine and thymine are pyrimidines (one)
-a single strand of DNA consists of a sequence of bases attached to sugar-phosphate groups
that form a backbone
-purine-pyrimidine combinations are called complementary base pairs
-because 20 amino acids need to be specified by 64 codons of the genetic code, the genetic
code is redundant in that the same amino acid can be specified by more than one codon
-genes are defined as stretches of DNA that code for RNA or a protein
-alleles are defined as versions of the same gene that differ in their base sequence
-mutations are defined as any type of change in base sequence of DNA
-altering a single point in the base sequence of a gene is called a point mutation
-point mutations are caused by random errors in DNA synthesis or random errors in the
repairing of sites damaged by chemical mutagens or radiation
-if DNA polymerase mistakenly substitutes a purine for another purine, the point mutation is
called transition (most common)
-if a purine is substituted for a pyrimidine or a pyrimidine for a purine, the point mutation is
called transversion
-changes in the first or second position of a codon almost always lead to a change in the amino
acid specified by the resulting mRNA
-point mutations that result in an amino acid change are called replacement substitutions
-point mutations that result in no amino acid change are silent site substitutions
-in studying mutation rates, the observable mutant phenotypes were due to loss-of-function
mutations
-loss-of-function mutations are changes in DNA that inactivate a gene and prevent product
-mutation rate per cell division is equal in most organisms leading to a single common mutation
rate hypothesis
-mutation introduces a great deal of variation into populations in every generation
-mitochondria lack some of the DNA repair enzymes, so they experience more mutations
-in investigating the effect of natural selection on mutation rate, researchers found that
enzymes responsible for copying and repairing DNA are under intense selection
-DNA polymerases vary in their accuracy as well due to mutations either increasing or
decreasing error rate
-point mutation rates also depend on how efficiently mistakes in DNA synthesis are corrected
-the efficiency of DNA mismatch repair is a trait with heritable variation because mutations that
occur on the genes responsible for repairing base-pair mismatches affect mutation rates
-cells with elevated mutation rates have higher fitness than cells with normal rates when grown
in novel environments
-a vast majority of mutations are deleterious in that if they are allowed to accumulate, fitness
declines
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