8.1 Altering sequence of bases alters the protein structure

• What is a gene mutation?

  A change in the base sequence of DNA potentially resulting in formation of a new allele

  - Can arise spontaneously during DNA replication (interphase)
• What is a mutagenic agent?

  A factor that increases rate of mutation, eg. ultraviolet (UV) light or alpha particles.
• Explain how a gene mutation can lead to the production of a non-functional

  protein or enzyme (general).

  - Changes sequence of base triplets in DNA so changes sequence of codons on mRNA

  - So changes sequence of amino acids in the encoded polypeptide

  - So changes position of hydrogen / ionic / disulphide bonds (between amino acids)

  - So changes tertiary structure (shape) of protein

  - Enzymes - active site changes shape so substrate can't bind, enzyme-substrate complex can't form
• Describe substitution.

  A base / nucleotide is replaced by a different base / nucleotide in DNA
• Describe addition.

  1 or more bases / nucleotides are added to the DNA base sequence
• Describe deletion.

  1 or more bases / nucleotides are lost from the DNA base sequence
• Describe duplication.

  A sequence of DNA bases / nucleotides is repeated / copied
• Describe inversion.

  A sequence of bases / nucleotides detaches from the DNA sequence, then rejoins at the same position in the reverse order
• Describe translocation.

  A sequence of DNA bases / nucleotides detaches and is inserted at a different location within the same or a different chromosome
• Explain why not all gene mutations affect the order of amino acids.

  - Some substitutions change only 1 triplet code / codon which could still code for the same amino acid

  as the genetic code is degenerate (an amino acid can be coded for by more than one triplet)

  - Some occur in introns which do not code for amino acids
• Explain why a change in amino acid sequence is not always harmful.

  - May not change tertiary structure of protein (if position of ionic / disulphide / H bonds don't change)

  - May positively change the properties of the protein, giving the organism a selective reproductive advantage
• Explain what is meant by a frameshift.

  - A frameshift occurs when gene mutations (eg. addition, deletion, duplication or translocation) change the number of nucleotides / bases by any number not divisible by 3

  - This shifts the way the genetic code is read, so all the DNA triplets / mRNA codons shift from the point of mutation
• When will addition/deletion not lead to frameshift?

  If a multiple of 3 bases is added / removed there won't be a frameshift, but extra / less triplets will result in extra / less amino acids in the encoded polypeptide.

  
  

  OR

  
  

  A frameshift could also lead to production of a stop codon (that doesn't code for amino acids so terminates translation), resulting in a shorter polypeptide.
• Alteration of the sequence of bases in DNA can alter the structure of proteins

  Explain how a single base substitution causes a change in the structure of this polypeptide

  (haemoglobin). Do not include details of transcription and translation in your answer.

  1. Change in (sequence of) amino acid(s)/primary structure

  2. Change in hydrogen/ionic/disulfide bonds

  3. Alters tertiary structure