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•  New alleles are formed by mutation

A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait

  • New alleles are formed by mutation

Gene mutations can be beneficial, detrimental or neutral

  • Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
  • Detrimental mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait
  • Neutral mutations have no effect on the functioning of the specific feature (silent mutations)

Types of Gene Mutations


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•  The cause of sickle cell anaemia, including a base substitution mutation, a change to the base sequence of

    mRNA transcribed from it and a change to the sequence of a polypeptide in haemoglobin

Sickle cell anaemia is an example of a disorder caused by a gene mutation

  • The disease allele arose from a base substitution mutation – where a single base was changed in the gene sequence

Cause of Sickle Cell Anaemia

Sickle cell anaemia results from a change to the 6th codon for the beta chain of haemoglobin

  • DNA:  The DNA sequence changes from GAG to GTG on the non-transcribed strand (CTC to CAC on the template strand)
  • mRNA:  The mRNA sequence changes from GAG to GUG at the 6th codon position
  • Polypeptide:  The sixth amino acid for the beta chain of haemoglobin is changed from glutamic acid to valine (Glu to Val)

sickle cell cause

Consequence of Sickle Cell Anaemia

The amino acid change (Glu  Val) alters the structure of haemoglobin, causing it to form insoluble fibrous strands

  • The insoluble haemoglobin cannot carry oxygen as effectively, causing the individual to feel constantly tired

The formation of fibrous haemoglobin strands changes the shape of the red blood cell to a sickle shape

  • The sickle cells may form clots within the capillaries, blocking blood supply to vital organs and causing myriad health issues
  • The sickle cells are also destroyed more rapidly than normal cells, leading to a low red blood cell count (anaemia)

sickle cell consequence