Protein Structure & Mutations
- A gene mutation is a change in the sequence of bases in a DNA molecule; this may result in a new allele
- Mutations occur all the time and occur randomly
- Mutations are copying errors that take place when DNA is replicated during S phase of interphase
- As the DNA base sequence determines the sequence of amino acids that make up a polypeptide, mutations in a gene can sometimes lead to a change in the polypeptide for which the gene codes
- Most mutations are harmful or neutral (have no effect) but some can be beneficial
- Inheritance of mutations:
- Mutations present in normal body cells are not inherited; they are eliminated once the affected cells die
- Mutations within gametes are inherited by offspring, so can lead to heritable genetic conditions
- Point mutations are mutations where one base in the DNA sequence is altered; this can result in a changed amino acid at this location
Example of a point mutation: sickle cell disease
- A small change to a gene can have serious consequences for an organism
- Sickle cell disease is a genetic disorder caused by a single point mutation within the gene that codes for the alpha-globin polypeptide in haemoglobin (Hb)
- Most humans have the allele HbA
- The mutation results in a new allele HbS
The sickle cell mutation
- Within the haemoglobin gene a point mutation changes the DNA triplet GAG to GTG on the coding strand
- The resulting DNA triplet (CAC) on the template strand is transcribed into the mRNA codon GUG, instead of GAG
- During translation the amino acid valine (Val) replaces the original amino acid glutamic acid (Glu)
- This occurs at the sixth position of the polypeptide
Sickle cell anaemia point mutation diagram
A base substitution on the DNA molecule results in a change in the amino acid at position 6 of the haemoglobin polypeptide, altering the overall structure and function of the protein
The effects
- The protein haemoglobin S is produced instead of haemoglobin A; this causes a distortion in the shape of red blood cells, resulting in a sickle shape
- Sickle-shaped red blood cells:
- Have a limited oxygen-carrying capacity
- Block the capillaries and limit the flow of normal red blood cells
- People with sickle cell anaemia suffer from acute pain, fatigue and anaemia
- There is a correlation between the global distribution of sickle cell disease and malaria
- In areas with increased malaria cases there is an increased frequency of sickle cell alleles; this is thought to be due to increased resistance to the malaria parasite in individuals with the HbS allele
Sickled cells diagram
Sickled cells can block the flow of blood through the capillaries, restricting oxygen supply to the tissues
- You will cover more on mutations later in the course; see this link