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

What is a genetic mutation?

Changes to the base sequence or quantity of DNA in a gene or section of DNA that occurs during DNA replication

Describe the different types of genetic mutation

• Substitution: a base is replaced by a different base

• Addition: 1 or more bases are added to the DNA base sequence

• Deletion: 1 or more bases are lost from the DNA base sequence

• Duplication: a sequence of DNA bases is repeated

• Inversion: a sequence of bases detaches from the DNA sequence, then rejoins at the same position in reverse order

• Translocation: a sequence of DNA bases detaches and is inserted at a different location within the same or different chromosome

What is a mutagenic agent?

a factor that increases the rate of mutation

What are the different types of mutagenic agents (and give an example)?

• Physical; ionising radiation that breaks DNA strands

• Chemical: deaminating agents that cause base substitution

• Biological: viruses insert DNA into host cells

What is a silent mutation?

A mutation that causes no change to the resulting amino acid sequence

What is a missense mutation?

A mutation that alters a single amino acid in the polypeptide chain

What is a nonsense mutation?

A mutation that creates a premature stop codon, causing the polypeptide chain produced to be incomplete so the final protein is non-functioning

Why do not all gene mutations affect the order of amino acids?

• Some substitutions only change 1 codon which could still code for the same amino acid

• As genetic code is degenerate

• Some mutations occur in introns which do not code for amino acids

How can a gene mutation lead to the production of a non-functional protein/enzyme?

• A mutation causes the sequence of base triplets in DNA to change, so the sequence of codons is changed

• So amino acid sequence in encoded polypeptide is changed

• So position of hydrogen/ionic/disulphide bonds between amino acids is changed

• So tertiary structure of the protein is changed

• In enzymes, active site changes shape, so substrate cannot bind and enzyme-substrate complex cannot be formed

Why may a change in amino acid sequence not always be harmful?

• May not change the tertiary structure of the protein

• May positively change the properties of the protein, giving the organism a selective advantage

What is a frameshift?

• Occurs when gene mutations change the number of bases by any number not divisible by 3

• This shifts the way genetic code is read, so all DNA triplets downstream of the mutation change

• This causes the sequence of amino acids to change, so this has significant effects on the encoded polypeptide