Question
Classify mutations into point mutations, frameshift mutations, and chromosomal mutations. Give examples and explain which type is most harmful and why.
(NEET + CBSE Class 12)
Solution — Step by Step
A change in a single nucleotide in DNA:
| Type | Change | Effect | Example |
|---|---|---|---|
| Silent | Base changes but amino acid stays same | No effect | Due to codon degeneracy |
| Missense | Base changes, different amino acid | Altered protein | Sickle cell anaemia (GAG to GUG, Glu to Val) |
| Nonsense | Base changes to a stop codon | Truncated (shortened) protein | Some forms of thalassemia |
Insertion or deletion of one or more bases (not in multiples of 3) shifts the entire reading frame from that point onward.
Normal: AUG-GCA-UGA-CUU… After insertion of U: AUG-GUC-AUG-ACU-U…
Every codon downstream changes — resulting in a completely different (and usually non-functional) protein. Frameshift mutations are generally more harmful than point mutations because they affect ALL amino acids after the mutation site.
| Type | What happens | Example |
|---|---|---|
| Deletion | Part of chromosome lost | Cri du chat syndrome (5p deletion) |
| Duplication | Segment copied | Bar eyes in Drosophila |
| Inversion | Segment flipped 180 degrees | Usually no phenotypic effect |
| Translocation | Segment moves to non-homologous chromosome | Chronic myeloid leukemia (Philadelphia chromosome) |
| Aneuploidy | Gain or loss of whole chromosome | Down syndrome (trisomy 21), Turner syndrome (45,X) |
Mutation Classification Tree
flowchart TD
A["Mutations"] --> B["Gene-level"]
A --> C["Chromosomal-level"]
B --> D["Point mutation — single base change"]
B --> E["Frameshift — insertion or deletion"]
D --> D1["Silent: no amino acid change"]
D --> D2["Missense: different amino acid"]
D --> D3["Nonsense: premature stop codon"]
E --> E1["Insertion: extra base added"]
E --> E2["Deletion: base removed"]
C --> C1["Structural: deletion, duplication, inversion, translocation"]
C --> C2["Numerical: aneuploidy, polyploidy"]Why This Works
Mutations are classified by scale: gene-level (affecting one or few bases) vs chromosomal (affecting large segments or whole chromosomes). The severity depends on how much protein function is disrupted.
Silent mutations have zero effect because of codon degeneracy (multiple codons code for the same amino acid). Missense mutations range from harmless to lethal depending on the amino acid change. Frameshift and nonsense mutations are usually severe because they produce non-functional proteins.
Common Mistake
Students assume all mutations are harmful. Many mutations are neutral (silent mutations, mutations in non-coding DNA). Some are even beneficial — they provide raw material for evolution. Sickle cell trait (heterozygous) actually provides malaria resistance. NEET asks: “Are all mutations harmful?” The answer is no — mutations can be neutral, harmful, or beneficial depending on context.