Biology · Molecular Basis of Inheritance · NEET
A point mutation is a change in just one base pair at a single position, so usually only one codon (and at most one amino acid) changes. A frameshift mutation is caused by insertion or deletion of one or two bases; this shifts the reading frame from that point, so every codon after it is read wrongly. Point = one spot change; frameshift = whole downstream frame shifts.
The mRNA is read in groups of three bases (codons) with no gaps. If you insert or delete three bases, or any multiple of three, you add or remove whole codons, so the reading frame stays the same after that point. Only insertions/deletions that are NOT multiples of three (one or two bases) shift the frame. NCERT states this clearly with the 'RAM HAS CAP' word game.
Every codon from the point of deletion to the end of the reading frame is altered, because the frame shifts. In the NEET 2017 problem, a 333-amino-acid protein has base 901 deleted; base 901 sits in codon number 301 (ceil of 901/3), so codons 301 to 333 all change, which is 333 minus 301 plus 1 = 33 codons.
It is a point mutation, not a frameshift. A single base pair changes in the beta-globin gene, so the sixth codon changes from GAG (glutamate) to GUG (valine). Only one amino acid is affected, so the reading frame is not shifted. This is the classic NCERT example of a point mutation.
NCERT uses a sentence of three-letter words to show the reading frame. 'RAM HAS EDC AP' after inserting a letter reads as gibberish, and 'RAM HAS DCA P' after deleting shows the same shift. It teaches that adding or removing bases not in multiples of three makes all the following 'words' (codons) meaningless, which is exactly what a frameshift mutation does to a gene.
A particular mRNA, in a hypothetical case, codes for a protein with 333 amino acids, and the base at position 901 is deleted such that the length of the RNA becomes 998 bases, how many codons will be altered?
The sixth mutant codon of beta globin gene causing polymerization of Haemoglobin and change in RBC shape is ______.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A point mutation is a change of a single base pair in DNA. The classic NCERT example is sickle-cell anaemia, where the beta-globin gene has one base pair changed, altering the sixth codon from GAG to GUG and changing glutamate to valine.
A frameshift mutation is caused by insertion or deletion of one or two bases (any number not a multiple of three). It shifts the reading frame from that point, so all codons after it are read wrongly and the protein is usually non-functional.
No. Because the genetic code is degenerate (more than one codon can code the same amino acid), some point mutations are silent and do not change the amino acid. Frameshift mutations, however, almost always change many amino acids downstream.
The mRNA is read in a fixed frame of continuous triplets with no punctuation. This is why deleting or adding bases not in multiples of three destroys the frame. It links directly to why frameshift mutations are so damaging for NEET.
Look at what changed. If one base is replaced (substituted), it is a point mutation. If bases are added or removed and the number is not a multiple of three, it is a frameshift. If added or removed in multiples of three, the frame stays and it is not a frameshift.
