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Mutation questions give you a DNA or mRNA strand, change one letter or remove one, and ask what happens to the protein. Students who know the definitions still lose marks because they do not actually translate the strand. This post takes one short mRNA, mutates it in five ways, and translates each version so you can see the difference instead of memorising it.
Where is this in the syllabus?
The NEET (UG) 2026 syllabus lists mutation under evolution and the genetic code under molecular basis of inheritance, and the CBSE 2026-27 Biology curriculum has the same chapters. NCERT covers mutation twice. The Principles of Inheritance and Variation chapter defines a point mutation as a change in a single base pair, with sickle-cell anaemia as the classic example, and says deletions and insertions of base pairs cause frame-shift mutations. The Molecular Basis of Inheritance chapter explains the reading frame. The CBSE 2026-27 sample paper (Question 31) gives a DNA strand with two replication errors and asks which type of mutation each is; its marking scheme answers "Point Mutation" for one and "Deletion / Frameshift" for the other.
What is a point mutation?
NCERT's sickle-cell example is precise. A single base substitution at the sixth codon of the beta globin gene changes GAG to GUG, so glutamate (Glu) at the sixth position of the beta globin chain is replaced by valine (Val). The faulty haemoglobin polymerises at low oxygen and the red cells become sickle-shaped. The chemical difference between normal and sickle-cell haemoglobin was reported by V. M. Ingram in Nature in 1957.
One changed base can have three different outcomes, depending on the new codon:
Silent: the new codon codes for the same amino acid.
Missense: a different amino acid, as in sickle-cell anaemia.
Nonsense: the new codon is a stop codon, so the protein is cut short.
These three labels go beyond NCERT's wording, but they make the effects easy to sort.
What is a frameshift mutation?
The ribosome reads mRNA three bases at a time from the start codon, with no gaps. NCERT shows this with the sentence RAM HAS RED CAP: insert one letter and every word after it turns to nonsense, but insert three letters and the rest of the sentence reads normally again. In NCERT's words, insertion or deletion of one or two bases "changes the reading frame from the point of insertion or deletion", while three or its multiples add or remove whole codons and the reading frame "remains unaltered".
Worked example: one mRNA, five mutations
Start with this mRNA (5' to 3'), read in codons:
AUG UUU GGC UAC AAA UGA
Using the standard genetic code: Met, Phe, Gly, Tyr, Lys, then stop. Normal protein: Met-Phe-Gly-Tyr-Lys (5 amino acids).
Change C to U in codon 4 (UAC to UAU). UAU also codes for Tyr. Protein unchanged: silent.
Change G to A in codon 3 (GGC to GAC). GAC codes for Asp. Protein: Met-Phe-Asp-Tyr-Lys. One amino acid swapped: missense, the same type as sickle-cell.
Change C to A in codon 4 (UAC to UAA). UAA is a stop codon. Protein: Met-Phe-Gly, only 3 amino acids: nonsense.
Delete the U at position 5. The sequence now reads AUG UUG GCU ACA AAU GA... which is Met-Leu-Ala-Thr-Asn. Every codon after the deletion is different, and the original stop codon is no longer in frame: frameshift.
Delete all of codon 3 (GGC). The sequence reads AUG UUU UAC AAA UGA: Met-Phe-Tyr-Lys. One amino acid missing, everything else normal: in-frame deletion.
Compare 4 and 5. Deleting one base did far more damage than deleting three, which is exactly the point NCERT's RAM HAS RED CAP example makes.
Which mistakes cost marks?
Using the DNA template as if it were mRNA. If the question gives the 3' to 5' template, write the complementary mRNA first. In the sample paper, the template 3'-TAC...-5' becomes 5'-AUG...-3'.
Calling every substitution harmful. Because the code is degenerate, some substitutions change nothing.
Forgetting the stop codon. Count amino acids up to, not including, the stop.
Calling a three-base deletion a frameshift. It removes one amino acid; the frame stays.
Mixing up point and chromosomal mutations. NCERT treats deletions and duplications of chromosome segments as chromosomal aberrations, separate from single-base changes.
If codon reading still feels slow, revise the genetic code itself in our post on the genetic code and translation, and see where mutations sit in the wider chapter in molecular basis of inheritance. When you cannot see why your translation went wrong, the Fermi Problem Library has a step-by-step solver that explains the reasoning rather than printing an answer.
A 20-minute practice routine
Write any 15-base mRNA starting with AUG and ending with a stop codon.
Make five mutants: one silent, one missense, one nonsense, one single-base deletion and one three-base deletion.
Translate all six versions with a codon table and label each.
Swap with a friend and classify each other's mutants without looking at the labels.
Before your next test, redo sample-paper Question 31 by writing out the mRNA and translating it, not by guessing from the letters.
