Athough DNA replication is a very accurate process accasionally mistakes occur producing changes or mutations. These changes can also occur due to other factors such as radiation, ultraviolet light or chemicals . Mutations in gene sequences or in the sequences which negulate gene expression (transcription and translation) may alter the amino acid sequence in the protein encodedby that gene. In some cases protein function will be mained; taines; in other cases it will change or cease, perhaps producing a clinical disorder. Many different types of mutation occur.
This is the simplest type of change and involves the substitution of one nucleotide for another, so changing the codon in a coding sequence. For example, the triplet AAA, which codes for lysine, may be mutated to AGA, which codes for arginine. Whether a substitution produces a clinical disorder or not depends on whether it changes a critical part of the protein molecule produced. Fortunately, many substitutions have no effect on the function or stability of the proteins produced as several codons code for the same amino acid. However, some mutations may have a severe effect; for example, in sickle cell disease a mutation in the J3-globin gene changes one codon from GAG to GTG so that instead of glutamic acid, valine is incorporated into the polypeptide chain, which radically alters its properties.
Insertion or deletion
Insertion or deletion of one or more bases is a more serious change, as it results in the alteration of the rest of the following sequence to give a frame-shift mutation. For example, if the original code was:
TAA GGA GAG TTT
and an extra nucleotide (A) IS inserted, the sequence becomes:
TAA AGG AGA GTT T
or the third nucleotide (A) IS deleted, the sequence becomes:
TAG GAG AGT TT
and in both cases different amino acids are incorporated into the polypeptide chain. This type of change is responsible for some forms of thalassaemia. Insertions and deletions can involve many hundreds of base pairs of DNA; for example some large deletions in the dystrophin gene remove coding sequences and this results in Duchenne muscular dystrophy. Recently an insertion/deletion (m) polymorphism in the angiotensin converting enzyme (ACE) gene has been shown to result in the genotypes II, lD and DD. The deletion is of a 287 bp repeat sequence and DD is associated with higher concentrations of circulating ACE and cardiac disease.
If the DNA sequences which direct the splicing of introns from mRNA are mutated then abnormal splicing may occur. In this case the processed mRNA which is translated into protein by the ribosomes may carry intronsequences, so altering which amino acids are incorporated into the polypeptide chain.
Normal polypeptide chain termination occurs when the ribosomes processing the mRN A reach one of the chain termination or ‘stop’ codons (see above). Mutations involving these codons will result in either late or premature termination. For example in haemoglobin Constant Spring-a haemoglobin variant-instead of the ‘stop’ sequence, a single base change allows the insertion of an extra amino acid.