The Central Dogma Flow
To understand mutations, we must follow the flow of information:
DNA
→ Transcription →
mRNA
→ Translation →
Protein
*Remember: In mRNA, Thymine (T) is replaced by Uracil (U).
1. Transition Mutation
A "Like-for-Like" swap on the DNA level. Purine swaps for Purine (A ↔ G).
--- NORMAL ---
DNA (Coding Strand)
...TCTCAAAATTT...
mRNA (Codons)
...UCUCAAAAUUU...
▼ MUTATION (A → G) ▼
--- MUTATED ---
DNA (Coding Strand)
...TCTCAAGATTT...
mRNA (Codons)
...UCUCAAGAUUU...
2. Transversion Mutation
A "Like-for-Different" swap. Purine swaps for Pyrimidine (C ↔ G). This distorts the DNA width.
--- NORMAL ---
DNA
...TCTCAAAAA...
mRNA
...UCUCAAAAA...
▼ MUTATION (C → G) ▼
--- MUTATED ---
DNA
...TCTGAAAAA...
mRNA
...UCUGAAAAA...
3. Missense Mutation
Effect: The DNA change creates a different codon in the mRNA, which calls for a different amino acid.
DNA Mutation
...CAA AAA... → ...CAA GAA...
↓ Transcription ↓
mRNA Codon Change
...CAA AAA... → ...CAA GAA...
↓ Translation ↓
Protein Result
GlnLys
→
GlnGlu
Lysine replaced by Glutamic Acid!
4. Nonsense Mutation
Effect: The mRNA codon becomes a STOP signal (UAA, UAG, or UGA). Construction halts.
DNA Mutation
...AAA ATT... → ...AAA TTT...
↓ Transcription ↓
mRNA Codon Change
...AAA AUU... → ...AAA UAA...
↓ Translation ↓
Protein Result
LysIle
→
LysSTOP
5. Silent Mutation
Effect: The DNA changes, the mRNA codon changes, but... it codes for the SAME amino acid.
DNA Mutation
...CAA AAA... → ...CAA AAG...
↓ Transcription ↓
mRNA Codon Change
...CAA AAA... → ...CAA AAG...
↓ Translation ↓
Protein Result
GlnLys
→
GlnLys
No change in protein!
6. The Concept of "Reading Frame"
DNA is read in strict groups of 3 (Codons). If you add or delete a number of bases not divisible by 3, everything shifts!
1. Normal Sequence:
THE
FAT
CAT
ATE
RAT
12345
↓ DELETION of 'F' from FAT ↓
2. Frameshift Effect:
The letters shift left to fill the gap. Now the words make no sense.
THE
ATC
ATA
TER
AT...
⚠️ EXAM TIP: Insertion/Deletion of 3 bases (or multiples of 3) is NOT a frameshift. It just adds/removes one whole amino acid. The rest of the chain stays normal!
7. Biological Frameshift (Insertion)
Effect: Adding one base shifts how the ribosome reads the mRNA groups. Total scrambling.
DNA (Insertion of G)
TCT CAA GAA AAT TTA...
↓ Transcription ↓
Shifting mRNA Reading Frame
UCU CAA GAA AAU UUA...
(Notice how the groups of 3 are now different)
↓ Translation ↓
Scrambled Protein
Ser
Gln
Glu
Ile
Leu
...