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Summary Mutation and recombination
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BLGY1232 Mutation and recombinationMutation
Mis-sense mutations cause a codon to change its coding specify from one amino acid
to another this change might cause the protein structure or activity to be altered
resulting in a novel phenotype
Non-sense mutations occur when a codon changes from encoding an amino acid to
specifying chain termination which may have no or partial activity depending on
where the mutation occurs in the coding sequence
Silent mutations result from changes in triplets that do not result in a change of
amino acid in the protein encoded by the gene Such mutations can occur because
the triplet code is degenerate: often these changes occur in third-base positions
Neutral mutations are changes that cause an alteration in protein-coding that has no
consequences for protein structure of function For example, a change that results
in the replacement of an amino acid with another that has similar chemical
properties
Frame-shifts occur when one or two bases are inserted or deleted in a coding
sequence Because the code is read in groups of three bases, this will alter the
“reading-frame” of the gene and its mRNA, so that an altered protein is synthesized
the analysis of this led to the discovery of the triplet code
Other causes in mutation
Chemical changes;
Changes to bases can occur that alter their properties
Deamination of cytosine (which has an amino group on the 4-position of the
pyrimidine ring): the replacement of this amine group by =O turns it into
Uracil - normally found only in RNA This would result in DNA replication
altering a C-G base-pair to a U-A and then a T-A base pair in successive
rounds, if it were not that DNA repair usually recognizes the U as
inappropriate in DNA if the deaminated cytosine had been methylated in
the genome (a common modification in DNA), then the chemical change to
the base results in it being altered to a Thymine residue (again, a C-G to a T-A
base pair change) Because methylated C residues occur commonly within
genomes, they represents “hot-spots” for mutational change
Depurination of DNA - the removal of purine residues from the DNA
sequence, leaving a gap. When the depurinated strand acts as a template in
DNA replication, any of the 4 possible nucleotides can be incorporated into
the new strand by DNA polymerase
Many chemical agents modify DNA and are used by geneticists to generate
mutations at random throughout the genome
Among the most commonly used chemical mutagens, the alkylating agents
(add –CH3 onto =O) ethylmethane sulphonate (EMS) and methylmethane
sulphonate (MMS) are the most popular targets adenine (AT to GC base
pair change) and guanine (GC to AT base pair change)
Nitrous acid targets cytosine (CG to TA base pair change) and adenine (AT to
GC base pair change)
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