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5.2 Positive, negative and neutral DNA mutations by substitution, deletion and addition
DNA is a double helix molecule, and its structure is very stable and designed to prevent mistakes in its genetic code.
However, despite is marvellous stable structure, sometimes errors get through. DNA mutations can occur during DNA
replication (before nuclear division), either by mitosis or meiosis. When a DNA mutation occurs as a cell divides by mitosis
the mutation is not passed on, but the mutation may contribute to the ageing process or lead to cancer. Mutations within
the DNA may well be inherited from cells that have divided by meiosis (reproduction of the gametes).
There are three main types of DNA mutation:
substitutions (also known as point mutations) involve one base pair replacing another, so only one base has changed
insertions occur where an extra nucleotide base is inserted into the genetic code
deletions occur where one base is removed from the genetic code
Both insertions and deletions cause what is known as a frameshift. This is where the inserted or deleted base pair
changes the entire coding of the DNA molecule as the triplet codons all change. Take a look at the DNA sequence below:
A T G G C T A C G C T A T C A C C G T A C A T T
Met Ala Thr Leu Ser Pro Tyr Ileu
The sequence above is the original DNA sequence, also showing the polypeptide chain it will code for. See what happens
when there is a substitution mutation:
A T G G C T A C G C T T T C A C C G T A C A T T
Met Ala Thr Leu Ser Pro Tyr Ileu
Substitutions can be silent mutations. This means although one base has been changed, the triplet still codes for the
same amino acid, and so there is no effect on the polypeptide produced. Sometimes, however, a substitution can change
the amino acid the codon codes for. Now see what happens if there is a base insertion:
A T G G C T A C G C T G A T C A C C G T A C A T
Met Ala Thr Leu Pro Thr Val His
Clearly, the insertion of the guanine base in the fourth codon shown affects the entire polypeptide: it would make no
sense, as that's not the polypeptide that should be co ded for. The affected amino acids have been highlighted in blue. You
can see that an insertion moves every base along one slot, so each triplet is different to what it should be. Now let's take
a look at what happens when there is a base deletion:
A T G G C T A C G C T T C A C C G T A C A T T G
Met Ala Thr Leu His Arg Thr Leu
When there's a base deletion, a similar thing happens. Again, all the nucleotide bases shift one spot (just in the opposite
direction). Most likely the majority of amino acids coded for will therefore change, as can be seen above with the removal
of the adenine from the original sequence. This is a frameshift, as with the insertion.
Silent, neutral and affective mutations
A silent mutation is one where there is a change in the DNA base sequence, but the amino acid coded for remains the
same (so there is no effect). However, not all mutations are silent: insertions and deletions are certainly not silent. There
are many point mutations that do actually change the structure of a protein, but the same polypeptides are produced, so
there has been little effect. These are neutral mutations, so long as the resultant differentiation gives the organism no
particular advantage or disadvantage.
Some mutations do change the protein structure and actually give the organism an advantage or disadvantage as a result.
These are known as positive and negative (or harmful and beneficial) mutations, respectively.