BIOL 1020 Lecture Notes - Vernon Ingram, Silent Mutation, Sickle-Cell Disease
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Chapter 5: Mutation and Genetic Variation (pages 143 – 163)
The genetic variation that natural selection and other evolutionary forces act on
originates in mutation. Meiosis can be responsible for this because of the crossing
over process can result in new grouping of alleles. However, meiosis reshuffles
existing alleles into new combinations. Therefore, only mutation can create new
alleles and new genes. Once this variation is produce, then selection, drift, and
migration can act.
5.1. Where New Alleles Come From
•The Nature of Mutation
oMutation is any change in DNA. Genes are made of DNA, so changes in
DNA create changes in genes
oConsider the mutation for the human gene for hemoglobin that results in
In 1958, Vernon Ingram showed that the difference between
normal and sickle-cell hemoglobin was due to a single amino acid
change at position number 6 in the protein chain.
Instead of having glutamic acid in this position, the sickle cell
allele has valine; caused by a single base substitution in the
A change like this is called a point mutation which is a change in a
single base pair in DNA and occurs when errors in DNA replication
or repair are not fixed properly.
oPoint mutations are caused by one of two processes:
Random errors in DNA synthesis
Random errors in the repair of sites damaged by chemical
mutagens or high-energy radiation
oIf DNA polymerase mistakenly substitutes a purine for another purine or
a pyrimidine for another, the resulting point mutation is known as
oIf a purine is substituted for a pyrimidine or a pyrimidine for a purine,
the mutation that results is called a transversion.
Transitions are most common
oChanges in the first or second position of a codon almost always change
the amino acid specified by the resulting mRNA; however, due to the
redundancy of the genetic code, changes in the third position frequently
produce no change at all.
Point mutations that results in an amino acid change are called
Those that result in no change in the amino acid coded are silent
Both types result in new alleles
oRead about this research done on page 148
oThe research suggests that the mutation rate per cell divisions is
approximately equal in most or all organisms
oSuggested that natural selection had led to a single, common mutation
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