DG6- mutations

• mutations are changes in the DNA that can result in no protein or an altered protein being synthesised
• Single gene mutations involve the alteration of a DNA nucleotide sequence as a result of substitution, insertion or deletion of nucleotides
• In that substitutions only affect one codon they are often reffered to as point mutations
• Nucleotide substitutions fall into three categories- missense, nonsense and splice-site mutations
• Missense mutations result in one amino acid being changed for another
• The change of one amino acid may alter the shape of the protein and result in a non-functional protein or it may have little effect on the protein
• Nonsense mutations result in a stop codon being produced earlier in the sequence which results in a shorter protein
• A substitution mutation within a protein coding gene may not always lead to a change in the amino acid sequence of the encoded protein. This is described as a silent mutation
• In a splice site mutations the substitution affects the boundaries between exons and introns (splice sites). They result in some introns being retained and/or some exons not being included in the mature transcript
• Nucleotide insertions or deletions result in frame-shift mutations
• As a result of a frame-shift mutation;

  • all of the codons after the mutation will change
  • all of the amino acids after the mutation may change
• Frame shift mutations have a major effect on the structure of the protein produced which is usually non functional
• Chromosome structure mutations are the result of alterations to the structure of one or more whole chromosomes
• Chromosome structure mutations can be one of four types;

  • duplication
  • deletion
  • inversion
  • translocation
• Duplication is where a section of a chromosome is added from its homologous partner
• Deletion is where a section of chromosome is removed
• Inversion is where a section of a chromosome is reversed
• Translocation is where a section of a chromosome is added to a chromosome which is NOT its homologous partner
• The substantial changes caused by chromosome structure mutations often make them lethal
• Gene duplication is very important for the evolution of species because it can facilitate the creation of new genes
• Duplication allows potential beneficial mutations to occur in a duplicated gene whilst the original gene can still be expressed to produce its protein