3.8.1 The alteration of the sequence of bases in DNA can alter the structure of proteins

Define a gene mutation. (2 marks)

- A change in the base sequence of DNA on a chromosome.

- Can occur spontaneously during DNA replication in interphase.

Define a mutagenic agent. (2 marks)

- A factor that increases the rate of mutation.

- Such as ultraviolet light or alpha particles.

Explain how a gene mutation can produce a non-functional protein or enzyme. (5 marks)

- Alters the sequence of base triplets in DNA, changing the codons in mRNA.

- This changes the sequence of amino acids in the polypeptide.

- This alters the position of hydrogen, ionic, or disulphide bonds between amino acids.

- Changes the tertiary structure of the protein.

- If an enzyme is affected, the active site changes shape so the substrate cannot bind, preventing enzyme-substrate complex formation.

Describe different types of gene mutations. (6 marks)

- Substitution occurs when one base is replaced by a different base in the DNA sequence.

- Addition occurs when one or more bases are inserted into the DNA sequence.

- Deletion occurs one or more bases are removed from the DNA sequence.

- Duplication occurs when a sequence of bases is copied and repeated.

- Inversion occurs when a sequence of bases detaches and rejoins in reverse order.

- Translocation occurs when a sequence of bases detaches and attaches in a different location on the same or another chromosome.

Explain why not all gene mutations change the order of amino acids. (2 marks)

- A substitution may code for the same amino acid if the genetic code is degenerate.

- A mutation in an intron will not affect amino acid order because introns are removed during splicing.

Explain why a change in amino acid sequence may not be harmful. (2 marks)

- The tertiary structure may not be altered if bonding positions remain unchanged.

- The change could improve the protein's properties, giving a selective advantage.

Define a frameshift mutation. (2 marks)

- Caused by addition, deletion, duplication, or translocation of bases when the number of bases added or removed is not a multiple of three.

- Alters how the genetic code is read, changing all codons downstream from the mutation.

Explain how mutations can lead to the production of shorter polypeptides. (3 marks)

- Deletion or translocation can remove triplets or codons, so amino acids are missing.

- Substitution, addition, deletion, duplication, inversion, or translocation can create a premature stop codon.

- Which ends translation early so amino acids are missing from the end of the chain