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

When DNA replicates there is a chance of gene mutation. There are 6 types of mutation we need to know...

• Addition
• Insertion of an extra base. This results in a frame shift meaning that most triplets and therefore amino acids will be different - unless 3 bases are added.
• Deletion
• This is the loss of a nucleotide. This causes a frame shift meaning most triplets and therefore amino acids will be different. The polypeptides will therefore be different and a non-functioning protein will most likely result. A deletion at the end of a sequence is likely to have less impact.
• Substitution
• A nucleotide is replaced with (substituted by) another nucleotide. This could...
• Form a stop codon (very bad as polypeptide production would stop for the polypeptide being produced creating an incomplete protein)
• Form a different amino acid (the protein may differ in shape and therefore not function properly. This leads to conditions such as sickle cell anaemia
• The formation of a different codon which produces the same amino acid due to the degenerate genetic code.
• Inversion
• A group of bases separates from the DNA and rejoins in the same position but back-to-front
• Duplication
• One or more bases are duplicated. This has a similar affect to addition mutation therefore produces a frame shift
• Translocation of bases
• A group of bases separates from the DNA sequence on one chromosome and becomes inserted to a DNA sequence on a different chromosome. This often leads to an abnormal phenotype such as reduced fertility and development of certain cancers.

Causes of mutations

Mutations can arise spontaneously during DNA replication. This random and spontaneous occurrence can increase by outside factors known as mutagenic agents (mutagens) including:

• Chemicals
• For example nitrous oxide can directly alter the structure of DNA/interfere with transcription and bemnzopyrene (in tobacco smoke) can inactivate a tumour suppressor gene (TP53) leading to cancer.
• High energy ionizing radiation
• For example, alpha and beta particles and short length radiation such as X-rays and UV light. These can disrupt the structure of DNA.