mutations

• mutations in a base sequence in genes (DNA):

  may not affect the structure of a polypeptide as:

  • triplets may code for the same amino acid
  • mutation could occur in the non-coding base sequence
• mutations are changes in the the DNA base sequence
• types of genetic mutations:

  1. gene mutation
  2. chromosome mutation
• how gene mutations occur:

  1. substitution
  2. deletion
  3. insertion
• gene mutations can lead to the formation of new alleles
• substitution mutation:

  • one base is substituted for another base
  • so only one amino acid is altered
  • rest of the protein stays the same -> can form functional/non-functional protein
  • non-frame shift mutation
• deletion mutation:

  • one base is deleted from the sequence
  • reading frame is altered
  • rest of the protein is incorrect -> forms non-functional protein
  • frame shift mutation
• insertion mutation:

  • a base is added to the sequence
  • reading frame is altered
  • rest of the protein is incorrect -> forms non-functional protein
  • frame shift mutation
• in gene mutations multiple bases can be substituted, deleted or inserted
• silent mutation:

  • substitution occurs without changing the triplet code
  • triplet codes for the same amino acid
  • forms functional proteins
• missense mutation:

  • substitution occurs
  • only one amino acid changes in sequence
  • forms functional/non-functional proteins
• nonsense mutation:

  • substitution occurs
  • triplet codon now codes for a premature stop codon
  • so polypeptide chain is incomplete
  • forms non-functional proteins
• frameshift mutation:

  • insertion or deletion occurs
  • so reading frame is altered
  • sequence of amino acids changes
  • which can form new alleles
  • forms non-functional proteins
• functionality of a protein depends on its shape
• silent mutations

  occur when:

  • mutations occur in non-coding DNA
  • mutated gene is not expressed
  • mutation does not change the amino acid coded for
• mutations in coding DNA that is expressed will change the cell's phenotype
• how does the cell's phenotype change:

  1. change in the DNA base sequence forms different triplets
  2. different mRNA codons are formed
  3. different amino acids are coded for
  4. primary structure changes
  5. secondary and tertiary structure changes (changes in the hydrogen/ionic bonds formed between R groups of AA's)
  6. protein forms a different shape
  7. protein's function changes
  8. cell's function changes (phenotype)