Gene mutations

The word mutation means a change in the genetic composition of a cell. Mutations can be divided into two main groups. The first group is gene mutations. These are chemical changes in the DNA of the cell.

Section of a DNA molecule

DNA exists as two helical strands held together through the base pairs by hydrogen bonding.

Gene Mutations

Original gene structure

This shows the normal order of the bases for one of the strands in the area highlighted in the DNA molecule. Only one of the strands of DNA is involved in protein synthesis. A change from the normal order of bases leads to different types of gene mutation.

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Sickle Cell Anaemia

The diagram above shows the difference in the synthesis of haemoglobin in normal DNA and abnormal DNA. The top strand shows the normal DNA, mRNA and amino acid sequences. A single substitution mutation has changed the base sequence in the DNA. The base sequence on the mRNA produced by the DNA is altered. As a result, a codon on the mRNA is altered. A different amino acid is inserted into the protein chain. The amino acid valine is inserted instead of glutamic acid, which is shown by the bottom strand of DNA. The protein synthesised is altered and no longer functions normally. These sequences result in sickle-shaped red blood cells. This blood disorder is known as sickle cell anaemia.

Phenylketonuria

Phenylketonuria

The diagram represents part of the normal metabolic pathway involving the amino acid phenylalanine within human cells. The gene controls the synthesis of the enzyme and the enzyme converts the amino acid phenylalanine to tyrosine.

If a gene mutation occurs then individuals with this mutation can no longer produce the normal enzyme. This in turn means that they can no longer convert phenylalanine to tyrosine. The failure to bring about the normal enzyme conversion is called a metabolic block.

In newly born babies this metabolic block leads to the condition phenylketonuria (PKU). The effects of the condition if undetected and treated at birth are that:

• phenylalanine concentration builds up within the body of the new born baby
• phenylaline is converted into toxic substances
• the toxic substances cause irreversible damage to the developing brain