This document discusses different types of mutations, how they arise, and their effects. It distinguishes between adaptations to the environment versus heritable changes due to mutations in genetic material. Mutations can be defined by their location (e.g. gene, chromosome), type (e.g. point, frameshift), and effects (e.g. missense, nonsense). They can occur spontaneously or be induced by mutagens and may have varying consequences depending on whether they are in somatic or germ-line cells.

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 β-
hemoglobin gene leads to an amino acid
substitution in the β-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 and its Effect on TranslationA Nonsense Mutation and its Effect on Translation

12. Types of Base-Pair Substitution MutationsTypes of Base-Pair Substitution Mutations

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-Pair Substitution MutationsTypes of Base-Pair Substitution Mutations

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 suppressorMechanism of action of an intergenic nonsense suppressor
mutation that results from mutation of a tRNA genemutation that results from mutation of a tRNA gene

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 generation of addition andSpontaneous generation of addition and
deletion mutants by DNA looping-outdeletion mutants by DNA looping-out
errors during replicationerrors during replication

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%