CIE A Level Biology (9700) 2019-2021

Revision Notes

6.2.4 Nucleotides to Amino Acids

Nucleotides to Amino Acids

  • A gene is a sequence of nucleotides that forms part of a DNA molecule (one DNA molecule contains many genes)
  • This sequence of nucleotides codes for the production of a specific sequence of amino acids in a polypeptide
  • As the nucleotide sequence determines the sequence of amino acids, changes in this sequence (mutations) can sometimes lead to a change in the polypeptide that the gene codes for

Sickle cell anaemia

  • How important the nucleotide sequence is in determining the specific sequence of amino acids (and the resulting polypeptide that is formed) is demonstrated by the inherited blood disorder sickle cell anaemia
  • Haemoglobin is the red pigment in red blood cells which carries oxygen around the body
  • A haemoglobin molecule is made up of four polypeptide chains, each with one iron-containing haem group in the centre
  • Two of these polypeptide chains are called α-globin polypeptide chains and two are called β-globin polypeptide chains
  • The gene which codes for the amino acid sequence in the β-globin polypeptide chains is not the same in everyone
  • In most people, the β-globin polypeptide chains begin with the amino acid sequence:
    • Val-His-Leu-Thr-Pro-Glu-Glu-Lys-
  • This sequence of amino acids is coded by the HbA (normal) allele of the gene
  • In some people, there is a substitution mutation in the gene which codes for the amino acid sequence in the β-globin polypeptide chains:
    • The DNA base sequence CTT is replaced by CAT (the first thymine in the triplet is replaced by adenine)
    • CTT codes for the amino acid glutamine, whereas CAT codes for the amino acid valine
  • In these people, therefore, the β-globin polypeptide chains begin with the amino acid sequence:
    • Val-His-Leu-Thr-Pro-Val-Glu-Lys-
  • This sequence of amino acids is coded by the HbS (sickle cell) allele of the gene
  • Therefore, a single change in the amino acid sequence of the β-globin polypeptide results in the genetic disease sickle cell anaemia in individuals with two copies of the HbS allele

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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