6.1.1 (a) - types of gene mutations

spec points

types of gene mutations and their possible effects on protein production and function To include substitution, insertion or deletion of one or more nucleotides AND the possible effects of these gene mutations (i.e. beneficial, neutral or harmful).

protein synthesis key events

transcription in the nucleus

• DNA in region of gene unzips (H bonds broken)

• Free ribonucleotides complementary base pair with exposed bases on the template strand.

• RNA polymerase catalyses formation of sugar-phosphate backbone.

• mRNA copy of the coding strand produced separates from the DNA and leaves nucleus via nuclear pore.

translation on the ribosomes

• mRNA binds to grove on ribosome.

• RNA is read in triplets (codons)

• 2 codons fit into ribosome at one time.

• tRNA with complementary anticodon binds to 1^st codon on mRNA and brings a specific amino acid with it.

• 2^nd tRNA with a 2^nd amino acid binds at 2^nd codon.

• Peptide bond forms

• Ribosome moves along by 1 codon, 1^st tRNA is released and 3^rd tRNA brings 3^rd amino acid and binds to 3^rd codon.

• Process repeats until stop codon.

• Primary structure of polypeptide created.

mutations

• Mutation is a random process and genes tend to mutate at their own rate.
  New mutations can create new alleles for a gene.

• New mutation must occur in gamete-forming cells to be inherited.

• Gametes formed from cells w/ mutations fuse during sexual reproduction, all cells of that new organism will have a copy of that mutant allele.

• Mutations in somatic cells NOT passed on but can contribute to ageing and cancer development.

  • ultraviolet exposure causes mutations, meaning skin ages faster

example: maize

• Maize grains = naturally purple (dominant allele).

• Yellow is a recessive mutant allele. Dominant allele mutates into recessive allele at rate of 1: 1X10⁵ every generation (1 in 100,000)

• If purple is dominant and only mutates every 100,000 grains, why is all the sweetcorn we eat yellow?

  • bred them to be homozygous recessive. selective breeding only the yellow sweetcorn

increasing the mutation rate

Mutagens increase the rate:

• e.g. X-rays/UV/gamma radiation, physically damage DNA so has to be repaired + can lead to mistakes.

• Chemicals (nitrous acid, 5 bromouracil) → changes of 1 DNA base for another.

  • e.g. Cytosine (C) converted to Uracil (U) so bonds to adenine (A) instead of guanine (G).

• Carcinogens (Benzene, tar chemicals)

types of mutations

gene mutations

• DNA/Gene mutations: change to sequence of nucleotide bases in DNA strand as result of deletion, insertion or substitution of base/bases. can result in:

  • Silent, Missense, Nonsense + frame shift mutations

    • silent mutation: doesn’t affect amino acids despite change to nucleotide base sequence in gene

    • missense mutation: change to single amino acid within primary structure of protein. due to change in one codon within the nucleotide base sequence

    • nonsense mutation: production of stop codon so produces shortened (truncated polypeptide)

    • frame shift mutation: insertion/deletion of nucleotides within gene changes (all) codons downstream of the mutation + therefore the primary structure of the polypeptide produces

  • change to primary structure polypeptide (protein) produced by that gene - may affect folding of protein + so its function.

substitution mutation example

causes sickle-cell disease,

plenary: the effect of genotype and the environment on phenotype

• Most rabbits show pigmentation in their fur; but some, called Himalayan; are mostly white with some black.

• Siamese cats are also like this.

• The gene that is mutated is called tyrosinase and is involved in the formation of melanin.

• Can you suggest why this mutation leads to the results below:

  • Baby rabbits are pure white whilst they remain in the burrow

  • Adult rabbits will develop colour on their extremities after they leave the burrow