AQA A Level Biology复习笔记8.1.2 The Effect of Genetic Mutations

The Effect of Genetic Mutations

 

  • Mutations occur spontaneously and randomly during DNA replication
  • As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the amino acid sequence coded for by the gene
  • Most mutations do not alter the polypeptide or only alter it slightly so that its structure or function is not changed
  • As the genetic code is degenerate (more than one triplet code codes for the same amino acid) some mutations will not cause a change in the amino acid sequence
    • Substitution mutations are the mutations that usually have a smaller effect on the resultant polypeptide

     

  • Some gene mutations change all base triplets downstream from (after) the mutation, this will result in a non-functional polypeptide
    • Insertion and deletion mutations result in a frameshift

     

Causes of mutations

  • The rate that mutations occur can be estimated as around one mutation per 100 000 genes per generation
  • Exposure to mutagenic agents can increase the rate of mutation, they include
    • High energy ionising radiation, such as alpha, beta or gamma radiation
    • Chemicals, such as nitrogen dioxide or benzopyrene from tobacco smoke

     

 The effect of gene mutations on polypeptides 

  • Most mutations do not alter the polypeptide or only alter it slightly so that its appearance or function is not changed
  • However, a small number of mutations code for a significantly altered polypeptide with a different shape
  • This may affect the ability of the protein to perform its function. For example:
    • If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
    • A structural protein (like collagen) may lose its strength if its shape changes

     

The effect of gene mutations on phenotype

  • Polypeptides / proteins affect the phenotype of an organism via specific cellular mechanisms
  • If a mutation causes a major alteration in a polypeptide then cellular mechanisms could be affected, which may impact the phenotype of the organism
  • For example, a mutation in the TYR gene in humans affects the structure of an enzyme that is needed for the production of the pigment melanin
    • The phenotype of the human is affected by the lack of melanin
    • Individuals with the mutation have albinism; very pale skin and hair

     

Exam Tip

Mutagens are factors which increase the rate of mutation. Many mutagens are also categorised as carcinogens if they lead to the development of cancerous cells, however, this is not true of all mutagens. There are also carcinogens which are not mutagens, they work by increasing cell division but do not actually cause changes in the DNA.Also, in the exam you will be expected to relate the nature of a gene mutation to its effect on the encoded polypeptide.

 

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