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Ass. Lecturer : Khetam Qaid Mayee
Genetic Microbiology practice / 3red
Lecture / 5
Mutations
A mutation is any physical change in the genetic material (such as a gene or a chromosome). A gene that contains a mutation ( change in the base
sequence of the DNA) will produce an altered mRNA molecule that will produce an altered sequence of amino acids in the resulting protein
General Types of Mutations

Chromosomal Mutations
1.
2.

Changes in chromosome structure
 Deletion, duplication, inversion, or translocation.
Changes in chromosome number

Polyploidy, aneuploidy (autosomes or sex chromosomes).
a.
Changes in the DNA of a gene made by the substitution of a single base with another or by addition or deletion of one or
more
nucleotides.
Sickle
cell
disease
results
from
a
single
base
change
Point Mutations
Note : Remember! In RNA, the nucleotide base uracil replaces thymine.

Point Mutations : Changes in single DNA nucleotides.
o A missense mutation substitutes a different amino acid for the original one.
Example: Sickle cell disease results from a single base change
Remember! In RNA, the nucleotide base uracil replaces thymine.
TEMPLATE DNA code CTC (Glutamine - glu) -mutation->
TEMPLATE DNA code CAC (valine - val)
o
A nonsense mutation results in a stop codon being inserted someplace before the end of the gene.
TEMPLATE DNA code ATG (tyrosine - tyr) -mutation->
TEMPLATE DNA code ATT (STOP)
o
Silent mutations are point mutations that do not change the amino acid sequence of the protein. These are most likely to have no
effect. Redundancy of the Genetic Code reduces the chance that point mutations that result in a change in the third nucleotide of a
codon will alter the specified amino acid.
The mRNA codons GAA and GAG code for the amino acid Glutamic Acid (Glu).
The mRNA codons GCU, GCC, GCA, and GCG all code for the amino acid Alanine (Ala).
The mRNA codons GGU, GGC, GGA, and GGG all code for the amino acid Glycine (Gly).
o
Frameshift Mutations: Additions or deletions of one or more nucleotides.
Normal Sequence
Mutation 1
CTG / TTA / CGC
CTG / TTG / CGC
Silent
Mutation 2
CTG / TTT / CGC
Missense
Mutation 3
ATT / TTA / CGC
Nonsense
Template strand of the
DNA to be transcribed
Type of
Mutation
Mutation 4
CTA / GTT / ACG / C
Addition
(Frameshift)
Mutation 5
CT_T / TAC / GC
Deletion
(Frameshift)
Mutation 6
CTG / CTG / TTA / CGC
Expansion
Methods of detection mutations

Screening Methods : In general, target sequences are amplified by PCR before analysis. At present, Taq polymerase is widely used for
amplification. The error rate of Taq polymerase is in the range of 1024 to 1025 per nucleotide and is strongly affected by th e reaction
conditions (e.g., concentrations of magnesium chloride and dNTPs, pH, and temperature). Depending on the method of choice, polymerase
errors may contribute reasonably to unspecific background, limiting the level of detection.

Denaturing gradient gel electrophoresis (dgge), temperature gradient .
Double-stranded (ds) DNA is electrophoresed through a gradient of increasing concentration of a denaturing agent increasing concentration
of denaturant or temperature, domains in the DNA dissociate according to their melting temperature (Tm).


Chemical cleavage method (ccm).
Enzyme mismatch cleavage (emc).



RNase a cleavage method.
Cleavase fragment length polymorphism (cflp).
Protein truncation test (ptt).