This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'A classic example of missense mutation - \nSelect and explain. \nA. Sickle cell disease, B. Thalassemia, C. Sideroblas, D. Hemochromatosis', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In sickle cell anemia there is missense type of point mutation.\nMutations\n\nA mutation is a permanent change in the DNA.\nMutations that affect germ cells (sperm or ovum) are transmitted to progeny and may give rise to Inherited disease.\nMutations that affect somatic cells are not transmitted to progeny but are important in the genesis of\xa0cancers and congenital malformations.\nMutations may be classified into three categories –\n\n1. Gene mutations\n\nThe vast majority of mutations associated with the hereditary disease are gene mutations.\nThese may of different types depending on whether it involves complete gene or single base -\n\n(a) Point mutation\n\nA single nucleotide base is substituted by a different base.\nWhen a pyrimidine base is substituted by other pyrimidine base or a purine base is\xa0substituted by other purine → Transition.\nWhen a purine is substituted by a pyrimidine or vice-versa → Transversion.\nThis 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.\nBecause these mutations alter the meaning of the genetic code, they are often termed missense mutation.\nAn 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.\nAnother type of point mutation is a nonsense mutation in which a point mutation may change an amino acid codon to a stop codon.\nExample 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.\n\nb. Deletion and insertions\n\nDeletion or insertion of one or two bases lead to alteracations in the reading frame of the DNA\n\nStrand → frameshift mutation.\n\nIf the number of base pairs involved is three or a multiple of three frameshift does not occur\xa0(Because codon is triplet), instead an abnormal protein missing one or more amino acids is synthesized.\n\nc. Trinucleotide repeat mutation\n\nNormally a codon is triplet i.e., trinucleotide.\nIn 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,\nFor example in fragile X-syndrome, CGG codon is repeated 250-4000 times, ie. There are 250- 4000 tandem repeats of CGG\n\n2. Chromosome mutation\nThe result from the rearrangement of genetic material that gives rise to visible structural changes in the Chromosome.\n3. Genome mutation Involves loss', 'role': 'assistant'}]