Classic example of missense mutation -
Ans. is i.e., Sickle cell diseaseIn sickle cell anemia there is missense type of point mutation.Mutationso A mutation is a permanent change in the DNA.o Mutations that affect germ cells (sperm or ovum) are transmitted to progeny and may give rise to inherited disease.o Mutations that affect somatic cells are not transmitted to progeny but are important in the genesis of cancers and congenital malformations.o Mutations may be classified into three categories -1. Gene mutationso The vast majority of mutations associated with hereditary disease are gene mutations.o These may of different types depending w'hether it involves complete gene or single base -(a) Point mutationo A single nucleotide base is substituted by a different base.o When a pyrimidine base is substituted by other pyrimidine base or a purine base is substituted by other purine - Transition.o When a purine is substituded by a pyrimidine or vice-versa - Transversion.o This may alter the code in a triplet of bases, i.e. in codon and leads to replacement of one aminoacid by another in the gene product.o Because these mutations alter the meaning of the genetic code, they are often termed missense mutation.o Example is sickle mutation in which CTC codon in [3-chain of hemoglobin that codes for glutamic acid is changed to CAC codon that codes for valine.o Another type of point mutation is nonsense mutation in which a point mutation may change an amino acid codon to a stop codon.o Example is [3-thalassemia in which CAG codon in p-chain of hemoglobin that code for glutamin is changed to stop codon UAG after point mutation.Deletion and insertionso Deletion or insertion of one or two base lead to alterations in the readi ng frame of the DNA strand - frame shift mutation.o If the number of base pairs invoved is three or a multiple of three ffamshift does not occur (because codon is triplet), instead an abnormal protein missing one or more amino acids is synthesized.Trinucleotide repeat mutationo Normally a codon is triplet ie trinucleotide.o In this type of mutation a codon, ie trinucleotide sequence undergoes amplification and the same codon is repeated continuously so many times in the chain,o For example in fragile X-syndrome, CGG codon is repeated 250-4000 times, ie. there are 250-4000 tandem repeates of CGG.Chromosome mutationo Result from rearrangement of genetic material that give rise to visible structural changes in the chromosome..Genome mutationo Involves loss or gain of whole chromosome, e.g. monosomy - Turner syndrome, trisomy - Down syndrome.
Thalassemia
Sickle cell disease
Sideroblastic anemia
Hemochromatosis
Mendelian Disorders: Single-Gene Defects