gene mutation

1.  Discuss the difference between
adaptation to environmental mutations
and heritable changes due to mutations
 List several different types of mutations
and discuss how mutations arise in cells

2.  The genetic material can be changed
through mutations, which are changes in
the nucleotide sequences of genes
 Changes that are not repaired are called
mutations
 Early in this century, some geneticists
supported the theory that environmental
conditions could cause all heritable
changes in the genetic material
(adaptations)

3.  A mutation is a change in a DNA base-pair
or region of chromosome, of which
there are many causes
 A somatic mutation affects the individual
in which it happens and is not passed on
to the succeeding generation
 Germ-line mutations may be transmitted
by the gametes to the next generation,
producing an individual with mutations in
both the somatic and germ-line cells
 Germ-line mutations are heritable

4.  A chromosomal mutation or aberration is
a change in the structure or number of
chromosomes
 A gene mutation is a change in the DNA
sequence of a particular gene
 Mutations can occur spontaneously or be
induced by a mutagen, which is a
chemical or physical agent that increases
the frequency of mutational events

5.  Spontaneous mutations occur naturally
 A point-mutation is a base-pair substitution
mutation, in which one base pair is
replaced by another base pair (AT to GC)

6.  Point mutations include:
 Transition mutations, in which the change is
from one purine-pyrimidine base pair to
the other purine-pyrimidine base pair (AT
to GC, GC to AT, TA to CG, and CG to TA)
 Transitions can be caused by
oxidative deamination.

7.  Transversion mutations involve a change
from a purine-pyrimidine base pair to a
pyrimidine-purine base pair (AT to TA,
GC to CG, AT to CG, and GC to TA)
 the consequences of this change tend
to be more severe than those of
transitions.
 Transversions can be caused by
ionizing radiation.

9.  Mutations can be defined according to
their effects on amino acid sequences in
proteins and include:
 Missense mutations, a gene mutation in
which a change in the DNA causes a
change in an mRNA codon so that a
different amino acid is inserted into a
polypeptide during biosynthesis

10.  In sickle-cell anemia: a single nucleotide
base-pair change in codon 6 of the b-
hemoglobin gene leads to an amino acid
substitution in the b-hemoglobin chain
 Nonsense mutations, in which an mRNA
codon is changed from an amino acid into
a stop codon (UAG, UAA, or UGA)

11. A Nonsense Mutation aanndd iittss EEffffeecctt oonn TTrraannssllaattiioonn

12. Types of Base-PPaaiirr SSuubbssttiittuuttiioonn MMuuttaattiioonnss

13.  Neutral mutations, which produce no
detectable change in the function of the
protein translated from the message
 It is a subset of missense mutations where
the new codon codes for a different
amino acid which is chemically equivalent
to the original one i.e Lys to Arg (protein
function is not affected)

14.  Silent mutations are mutations in which a
base pair change transforms a codon into
another codon for the same amino acid,
with no detectable change in the resulting
protein
 Frameshift mutations, which result when
the reading frame of a gene is shifted due
to the addition or deletion of one or two
more base pairs in a gene
 It results in a non-functional protein

15. Types of Base-PPaaiirr SSuubbssttiittuuttiioonn MMuuttaattiioonnss

17.  Point mutations are of two classes:
 Forward mutations
 Reverse mutations
 Forward mutations cause the genotype to
change from wild-type to mutant
 A reversion is a mutational event that
changes a mutant phenotype back to
wild-type

18.  The effects of a mutation may be diminished
or abolished by a suppressor mutation
(secondary or second site mutation)
 A suppressor mutation is a mutation at a
different site from an original mutation that
does not result in a reversal of the original
mutation but instead masks or compensates
for the effects of the initial mutation

19.  There are two major classes of suppressor
mutations:
 Intragenic suppressors occur within the same
mutated gene
mRNA codon aa
WT 5’CGT3’ CGU Arg
3’GCA5’
Mut 5’AGT3’ AGU Ser
3’TCA5’
Intragenic suppressor
5’AGA3’ AGA Arg
3’TCT5’
 Intergenic suppressors occur in different genes

20. Mechanism of action of an intergenic nonsense ssuupppprreessssoorr
mmuuttaattiioonn tthhaatt rreessuullttss ffrroomm mmuuttaattiioonn ooff aa ttRRNNAA ggeennee

21.  Mutation rates and mutation frequencies are
terms describing the quantitative measure of
the occurrence of mutations
 Mutation rate presents the probability of a
particular kind of mutation as a function of
time
 i.e. number per nucleotide pair per
generation or per gene per generation ex:
spontaneous mutation rate in Eukaryote is
10-4 to 10-6 per gene/generation

22.  Mutation frequency is the number of
occurrences of a particular kind of
mutation expressed as the proportion of
cells or individuals in a population i.e. the
number per 100,000 organisms or number
per 1 million gametes

23.  Mutations arise in DNA spontaneously as a
result of natural cellular processes
 Looping-out during the DNA replication
can lead to deletion of bases on the
newly synthesized strand or on the
template strand.
 This will lead to shifted frame on the DNA.
 Most spontanous errors are corrected by
cellular repair systems

24. Spontaneous ggeenneerraattiioonn ooff aaddddiittiioonn aanndd
ddeelleettiioonn mmuuttaannttss bbyy DDNNAA llooooppiinngg--oouutt
eerrrroorrss dduurriinngg rreepplliiccaattiioonn

25.  Mutagenic chemicals or radiation can
create induced mutations in DNA

26.  Mutations in the genetic code
Can be induced in somatic cells
by:
› Chemicals: cancerogen
› Radiation: X-ray, UV
› Some viruses
 Heredity - 5%