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HMB265H1 (300)
Lecture

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Department
Human Biology
Course Code
HMB265H1
Professor
Stephen Wright

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LECTURE 17 MUTATION
Announcements:
oSolutions manual hard copy not yet available get it in the next
couple of weeks
oMidterm makeup done will soon post instructions to obtain copy of
original exam
8 lectures left
-started with Mendelian genetics, single-gene traits then quantitative traits, mapping QTL
Now molecular aspects of genetics today: mutations
Mutations
Process by which allele changes into another allele
New alleles can occur
Can mutate randomly, at any time, in any cell of organism
Can arise spontaneously, through natural causes
OR be induced by mutagen
Germline cell mutations are transmitted to progeny, not somatic mutations
Rate of forward mutation always higher than rate of reverse mutation
Forward: wildtype to mutant
Reverse: mutant back to wildtype
Classified by effect on underlying DNA
Substitution AKA point mutation
Indel mutations= insertion-deletion
Inversion: deleted, twisted around, reinserts = 180 rotation
oWithin same chromosomal location
Reciprocal translocation nonhomologous
Four types of bases
Substitution
Classified into two different types transition, transversion
Transition purine base changed into another purine; or pyrimidine into other
pyrimindine
oSame chemical category
Transversion purine changes into pyrimidine; vice versa
When one base changes on one strand, complementary change on opposite strand
during replication
Mutations are important because
Basis of genetic variation
Driving force behind evolution
Disrupt gene function so can study wildtype function of gene
www.notesolution.com

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Causes of mutation
Spontaneous naturally occurring no mutagen
oV. low rate according to studies
Induced via mutagen
Spontaneous mutations caused by
Depurination
oA and G are purines
oSometimes hydrolysis leads to break in glycosidic bond so G released
oResulting in apurinic site no purine, no base
oSometimes during DNA replication, a random base is inserted opposite
the apurinic site
o¾ time, mutation in the complementary strand of DNA
oThus MUTATION
Deamination
oOccurs when amino group spontaneously becomes uracil the bond
spontaneously breaks
oC to Uracil
oDuring replication, uracil binds to A, so mutant sequence occurring
oTransition mutation occurring ***
Induced mutations caused by
X-rays
oBreak sugar-phosphate backbone of DNA
oWhen two pieces incorrectly spliced together, see deletion sequence
UV light
oResult in thymine dimers
oTwo thymine residues adjacent to each other dimerize
oResult in disruption in readout of genetic information
Oxidation
oOxidative damage either naturally or from irradiative damage
oSuperoxide radicals (O2 minus) have extra electron
oThese active oxygen species can bind to any of the four bases THUS
altered base
oThis base then mispairs with A creating mutant sequence in replication
product
oG-C becomes T-A because of oxidative damage
Indel mutations
In regions of repeated bases
Prevailing hypothesis occur because of replication slippage
oThymine just slips right out (LH side of slide)
oThen T slips back in again
oOccurs with newly synthesized base pair strand
www.notesolution.com
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