Download - Variations in Genes
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Variations in Genes
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VariationVariation – is a change or difference in
something.Variations can happen to organisms
through: MutationsReproduction (Mitosis and Meiosis)Environment
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Mutations are a vital part of evolution, because they can change a gene, that changes the protein, to produce a totally new trait.• They occur constantly at a low rate.• Most are harmful, some are beneficial, and
some are neutral.
Mutations
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MutationsSpontaneous mutations
Can occur during DNA replication, recombination, or repair
MutagensAre physical or chemical agents that
can cause mutations.
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The change of a single nucleotide in the DNA’s template strand leads to the production of an abnormal protein
In the DNA, themutant templatestrand has an A where the wild-type template has a T.
The mutant mRNA has a U instead of an A in one codon.
The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).
Mutant hemoglobin DNAWild-type hemoglobin DNA
mRNA mRNA
Normal hemoglobin Sickle-cell hemoglobin
Glu Val
C T T C A T
G A A G U A
Example
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Types of Point MutationsPoint mutations within a gene can be divided into 3
general categoriesBase-pair substitutionBase-pair additionBase-pair deletion
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1. Silent – does not result in a change to the amino acid or protein.
2. Missense – causes one amino acid change and a change in the protein.
3. Nonsense - results in a stop codon instead of a codon for an amino acid. This results in translation being stopped before the protein is complete or may cause more than one amino acid in the chain to change.
4. Frameshift - When the number of nucleotides inserted or deleted is not a multiple of three, so that every codon beyond the mutation is read incorrectly during translation.
Results of Mutations
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SubstitutionsSubstitution
Is the replacement of one nucleotide and its partner with another pair of nucleotides
Can cause silent, missense or nonsense results
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Normal
A U G A A G U U U G G C U A AmRNA
Protein Met Lys Phe GlyStop
A U G A A G U U U G G U U A A
Met Lys Phe Gly
Base-pair substitution
Silent - No effect on amino acid sequence
U instead of C
Stop
A U G A A G U U U A G U U A A
Met Lys Phe Ser Stop
A U G U A G U U U G G C U A A
MetStop
Missense A instead of G
Nonsense
U instead of A
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Additions and DeletionsAdditions and Deletions
Are additions or losses of nucleotide pairs in a geneMay produce frame shift, missense, nonsense results
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mRNA
Protein
Normal
A U G A A G U U U G G C U A A
Met Lys Phe Gly
Amino end Carboxyl end
Stop
Base-pair insertion or deletion
Frameshift causing immediate nonsense
A U G U A A G U U U G G C U A
A U G A A G U U G G C U A A
A U G U U U G G C U A A
MetStop
U
Met Lys Leu Ala
Met Phe GlyStop
MissingA A G
Missing
Extra U
Frameshift causing extensive missense
Insertion or deletion of 3 nucleotides:no frameshift but extra or missing amino acid