Biology · Principles of Inheritance and Variation · NEET
A point mutation is a change in a SINGLE base pair of DNA. The most common type is substitution, where one base is swapped for another (for example A becomes T). Because only one base changes, usually only one codon changes, so at most one amino acid in the protein changes. NCERT's classic example is sickle cell anaemia: the beta globin codon GAG becomes GUG, so glutamic acid (Glu) is replaced by valine (Val).
A frameshift mutation happens when you INSERT or DELETE one or two bases in the DNA/mRNA. The ribosome reads mRNA in fixed groups of three bases (codons). If you add or remove one or two bases, all the triplet groupings after that point shift by one or two positions. This 'shift' in how the reading frame is grouped is why it is called a frameshift. NCERT (Ch 5) says: insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion.
Sickle cell anaemia is a POINT mutation, not a frameshift. It is a single base-pair substitution in the beta globin gene (GAG to GUG), changing only one amino acid (Glu to Val) at the 6th position. No bases are added or removed, so the reading frame is not shifted. NEET loves to test this, so remember it firmly.
The ribosome does not know where a codon ends — it just reads three bases, then the next three, and so on from a fixed starting point. If you delete one base, the ribosome still keeps reading in threes, but now it grabs different letters for every triplet after the deletion. So every codon downstream is re-grouped and usually codes for a wrong amino acid (or hits a stop codon early). This is why a frameshift is usually much more damaging than a single point mutation.
No. If you insert or delete THREE bases (or any multiple of 3), the reading frame stays the same after that point. You just add or remove one whole codon, so one whole amino acid is added or lost, but every codon after it is read correctly. Only insertion/deletion of 1 or 2 bases (not multiples of 3) causes a true frameshift. This is a favourite NEET trap.
When a single base is deleted at some position, find which codon that position falls in, then count from that codon to the end. Example (NEET 2017): a protein has 333 amino acids (codons cover bases 1 to 999), and base 901 is deleted. Base 901 is in codon number 301 (since 901 divided by 3, rounded up, is 301). From codon 301 to codon 333, the number altered = 333 minus 301 plus 1 = 33 codons.
A normal cell of an organism has a gene which 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?
Under which of the following conditions will there be NO change in the reading frame of the following mRNA? 5'-AACAGCGGUGCUAUU-3'
Select the correct statements about sickle cell anaemia. A. There is a change in gene for beta globin. B. In the beta globin, there is valine in the place of Lysine. C. It is an example of point mutation. D. In the normal gene U is replaced by A.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A point mutation can be harmful (like sickle cell anaemia), neutral (silent mutation, where the changed codon still codes for the same amino acid because of the degenerate genetic code), or rarely beneficial. It affects at most one amino acid, so its effect is often small compared with a frameshift.
A frameshift is usually more damaging. A point mutation changes at most one amino acid, but a frameshift re-reads every codon after the mutation, often producing a completely wrong protein or an early stop codon, giving a shortened, non-functional protein.
No. Insertions and deletions cause a frameshift ONLY when the number of bases added or removed is NOT a multiple of 3 (i.e. 1 or 2 bases). If 3, 6, 9 bases are added or removed, whole codons are added or lost but the reading frame stays intact.
The reading frame is the way mRNA is divided into consecutive non-overlapping triplets (codons) starting from a fixed point. The ribosome reads three bases at a time from the start codon. Shifting this grouping by one or two bases changes every codon after that point.
