Alteration of the sequence of bases in DNA can alter the structure of proteins

Question 1

what is a gene mutation

Answer 1

• Change in base sequence of DNA
• Can arise spontaneously during DNA replication, during interphase of the cell cycle

Question 2

what is a mutagenic agent

Answer 2

Increase the rate of gene mutation (above the rate of naturally occurring mutations)
* Examples:
* Ionising radiation (gamma and X-rays)
* Carcinogens e.g. mustard gas
* Some viruses

Question 3

what are the types of gene mutations and their effects on amino acid sequences

Answer 3

Substitution
1 base replaced with another
* No change due to degenerate nature of genetic code (see below table)
OR
* 1 triplet / codon change → 1 amino acid change

Addition
1 or more bases added to base sequence
* Frameshift; triplets / codons change
downstream of mutation → amino acid sequence changes (see below table)
OR
* If multiple of 3 bases added – no frameshift, but
extra triplets / codons → extra amino acids

Deletion
1 or more bases
lost from base
sequence
* Frameshift; triplets / codons change
downstream of mutation → amino acid
sequence changes (see below table)
OR
* If multiple of 3 bases lost – no frameshift, but
missing triplets / codons → missing amino acids

Inversion
A sequence of
bases is separated
from DNA and
inserted at the
same position,
backwards
* No frameshift because number of bases stays
the same
* Triplets / codons in inverted region change →
sequence of amino acids encoded by inverted
region change

Duplication
A sequence of
bases is inserted
twice, or multiple
times
* Frameshift; triplets / codons change
downstream of mutation → amino acid
sequence changes (see below table)
OR
* If multiple of 3 bases added – no frameshift, but
extra triplets / codons → extra amino acids

Translocation
Sequence of bases
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

Question 4

what are the effects of mutaions

Answer 4

• 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)
• Also, some gene mutations occur in
  the introns (non-coding sequences
  within genes) and therefore won’t
  affect amino acid sequences
• 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

Question 5

what happens if a mutation causes a stop codon

Answer 5

• 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

Question 6

How do mutations can lead to the production of a non-functional protein / enzyme

Answer 6

1. Change in base / triplet sequence of DNA / gene
2. Changes sequence of codons on mRNA
3. Changes sequence of amino acids in primary structure of polypeptide
4. Changes position of hydrogen / ionic / disulphide bonds in protein tertiary structure
5. Changes tertiary structure / shape of protein and in the case of enzymes, the active site
   will change shape
6. In the case of enzymes, the substrate will be unable to bind to active site and form an
   enzyme-substrate complex