A classic example of missense mutation –
Correct Answer: Sickle cell disease
Description: In sickle cell anemia there is missense type of point mutation.
Mutations
A mutation is a permanent change in the DNA.
Mutations that affect germ cells (sperm or ovum) are transmitted to progeny and may give rise to Inherited disease.
Mutations that affect somatic cells are not transmitted to progeny but are important in the genesis of cancers and congenital malformations.
Mutations may be classified into three categories –
1. Gene mutations
The vast majority of mutations associated with the hereditary disease are gene mutations.
These may of different types depending on whether it involves complete gene or single base -
(a) Point mutation
A single nucleotide base is substituted by a different base.
When a pyrimidine base is substituted by other pyrimidine base or a purine base is substituted by other purine → Transition.
When a purine is substituted by a pyrimidine or vice-versa → Transversion.
This may alter the code in a triplet of bases, i.e. in codon and leads to the replacement of one aminoacid by another in the gene product.
Because these mutations alter the meaning of the genetic code, they are often termed missense mutation.
An example is a sickle mutation in which CTC codon in beta-chain of hemoglobin that codes for glutamic acid are changed to CAC codon that codes for valine.
Another type of point mutation is a nonsense mutation in which a point mutation may change an amino acid codon to a stop codon.
Example is beta-thalassemia in which CAG codon in p-chain of hemoglobin that code for Glutamine is changed to stop codon UAG Acer point mutation.
b. Deletion and insertions
Deletion or insertion of one or two bases lead to alteracations in the reading frame of the DNA
Strand → frameshift mutation.
If the number of base pairs involved is three or a multiple of three frameshift does not occur (Because codon is triplet), instead an abnormal protein missing one or more amino acids is synthesized.
c. Trinucleotide repeat mutation
Normally a codon is triplet i.e., trinucleotide.
In this type of mutation a codon, i.e., trinucleotide sequence undergoes amplification and the Same codon is repeated continuously so many times in the chain,
For example in fragile X-syndrome, CGG codon is repeated 250-4000 times, ie. There are 250- 4000 tandem repeats of CGG
2. Chromosome mutation
The result from the rearrangement of genetic material that gives rise to visible structural changes in the Chromosome.
3. Genome mutation Involves loss
Category:
Pathology
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