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Mutation
 Mutation: is a change in the genetic material
 Mutations can be neutral, beneficial, or harmful
 Mutagen: Agent that causes mutations
 Spontaneous mutations: Occur in the absence of a mutagen
1- Missense mutation
 Base substitution (point mutation)
 Change in one base
 Result in change in amino acid
2- Nonsense Mutation
 Results in a nonsense codon
 The remaining codons of the mRNA
are not translated into amino proteins
because the stop codon is
prematurely reached during
translation.
 This can yield a truncated abbreviated
protein product, which lacks the
functionality of the normal protein.
Figure 8.17a,
c
3- Frameshift Mutation
 Insertion or deletion of one or more
nucleotide pairs
 Due to the triplet nature of gene
expression by codons this can
change the reading resulting in a
completely different translation from
the original.
 The earlier in the sequence the
deletion or insertion occurs, the more
altered the protein produced is.
Figure 8.17a,
d
The Frequency of Mutation
 Spontaneous mutation rate = 1 in 109 replicated base pairs or 1
in 106 replicated genes
 Mutagens increase to 10–5 or 10–3 per replicated gene
Chemical Mutagens
 Nitrous acid convert A to form
that pair with C instead of T.
 Analogs are molecules that are
structurally similar to normal
bases but have different basepairing prosperities.
Radiation
1- Ionizing radiation (X rays and gamma rays) causes the
formation of ions that can react with nucleotides and
the deoxyribose-phosphate backbone
Radiation
2. UV radiation causes thymine
dimers
 Photolyases separate thymine
dimers
 Nucleotide excision repair
Figure 8.20
Selection
 Positive (direct) selection detects
mutant cells because they grow
or appear different
 Negative (indirect) selection
detects mutant cells because
they do not grow
 Replica plating
 Auxotroph: a mutant
microorganism having a
nutritional requirement that is
absent in the parent cells
Ames Test for Chemical Carcinogens
•Salmonella can not produce
histadine
•Develop mutant that enable
them to produce histidine
•Adding mutagen can revert
mutant to original form
Figure 8.22
Genetic Recombination
 Vertical gene transfer: Occurs
during reproduction between
generations of cells.
 Horizontal gene transfer: The
transfer of genes between cells
of the same generation.
Genetic Recombination
 Exchange of genes between
two DNA molecules
 Crossing over occurs when
two chromosomes break and
rejoin
Figure 8.23
Genetic Recombination
 Genetic recombination is a process by
which a molecule of nucleic acid is
broken and then joined to a different
one.
 Recombination is a common method of
DNA repair in both bacteria and
eukaryotes.
 In genetic engineering, recombination
can also refer to artificial and deliberate
recombination of disparate pieces of
DNA, often from different organisms,
creating what is called recombinant DNA