8.1 alteration of base sequence alters protein structure

• gene mutation

  a change in the base sequence of DNA

  can arise spontaneously during DNA replication (interphase)
• mutagenic agent

  a factor that increases rate of mutation

  eg ultraviolet (UV) light or alpha particles
• 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

  so enzyme-substrate complex can’t form
• substitution

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

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

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

  a sequence of DNA bases / nucleotides is repeated / copied
• inversion

  a sequence of bases / nucleotides detaches from the DNA sequence then rejoins at the same position in the reverse order
• 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 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 downstream from the mutation change

  the sequence of amino acids encoded changes accordingly and the effects on the encoded polypeptide are significant
• frame shift extra information

  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

  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