Alteration of the sequence of bases in DNA can alter the structure of proteins Flashcards
What is a mutation
The change in the base sequence of DNA
Can arise spontaneously during DNA replication, when interphase occurs in the cell cycle
What is a mutagenic agent
Mutagenic agents increase the rate of gene mutation (above the rate of naturally occurring mutations)
e.g.
Ionising radiation (gamma and X-rays)
Carcinogens e.g. mustard gas
Some viruses
Substitution and its effect on amino acid sequences
1 base is replaced by another
Either no change due to degenerate nature of genetic code or 1 triplet or codon changes, in which case 1 amino acid changes
Addition and its effect on amino acid sequences
1 or more bases added to base sequence
Frameshift; a shift in the base triplets/codons change downstream of mutation and the amino acid sequence changes
Or if multiple of 3 bases added – no frameshift, but extra triplets/codons, leading to extra amino acids
Deletion and its effect on amino acid sequences
1 or more bases lost from base sequence
Frameshift; a shift in the base triplets/codons change downstream of mutation and the amino acid sequence changes
Or if multiple of 3 bases lost – no frameshift, but missing triplets/codons, leading to missing amino acids
Inversion and its effect on amino acid sequences
A sequence of bases is separated from DNA and inserted at the same position, backwards
No frameshift because the number of bases stays the same
Triplets/codons in inverted region change and the sequence of amino acids is encoded by the inverted region change
Duplication and its effect on amino acid sequences
A sequence of bases is inserted twice, or multiple times
Frameshift; a shift in the base triplets/codons change downstream of mutation and the amino acid sequence changes
Or if multiple of 3 bases added – no frameshift, but extra triplets/codons, leading to extra amino acids
Translocation and its effect on amino acid sequences
Sequence of bases is taken out and inserted at a different position on the same, or a different chromosome
Significant impact on gene expression and amino acid sequences at original and new location
Effects of mutations: not all affect the order of amino acids in the polypeptide
Some gene mutations (substitution)
change only 1 codon
New codon might still code for same
amino acid because genetic code is
degenerate (meaning the same
amino acid can be coded for by more
than one triplet), as shown in fig. 8.1.
Also, some gene mutations occur in
the introns (non-coding sequences
within genes) and therefore won’t
affect amino acid sequences
Effects of mutations: frame shift
A frameshift occurs when gene mutations such as insertion or deletion change the number of nucleotides by any number not divisible by 3
This shifts the way the genetic code is read, so all the DNA triplets/mRNA codons downstream from the mutation change
The sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant
Stop codons
There are 3 stop codons in the genetic code (UAA, UGA, UAG)
Unlike other codons, these don’t code for amino acids, so they terminate translation
A mutation (substitution or frameshift) may create a premature stop codon and result in the production of a shorter and often non-functional polypeptide
How mutations can lead to the production of a non-functional protein/enzyme
- Change in base/triplet sequence of DNA/gene
- Changes sequence of codons on mRNA
- Changes sequence of amino acids in primary structure of polypeptide
- Changes position of hydrogen/ionic/disulphide bonds in protein tertiary structure
- Changes tertiary structure/shape of protein and in the case of enzymes, the active site will change shape
- In the case of enzymes, the substrate will be unable to bind to active site and form an enzyme-substrate complex
