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BI10.1-7 | Molecular Biology — Part 3
Gene Mutations and Associated Disorders
Types of Gene Mutations and Clinical Examples
| Mutation Type | Mechanism | Effect on Protein | Clinical Example |
|---|---|---|---|
| Silent | Single base change | No amino acid change (codon degeneracy) | Often clinically silent |
| Missense | Single base change | Different amino acid | Sickle cell anaemia (Glu→Val) |
| Nonsense | Single base change | Premature stop codon → truncated protein | Beta-thalassaemia (some forms) |
| Frameshift | Insertion/deletion (not multiple of 3) | Entire downstream sequence altered | Cystic fibrosis (ΔF508) |
| Trinucleotide repeat expansion | Unstable repeat amplification | Toxic gain of function or gene silencing | Fragile X, Huntington disease |
A mutation is any permanent change in the DNA sequence. Mutations can be:
Figure: Gene Mutations and Associated Disorders
By type:
- Point mutations (single base changes):
- Silent: No amino acid change (due to codon degeneracy)
- Missense: Different amino acid (e.g., sickle cell anaemia — GAG → GTG, Glu → Val in beta-globin)
- Nonsense: Premature stop codon → truncated, non-functional protein
- Frameshift mutations: Insertion or deletion of bases (not in multiples of 3) shifts the entire reading frame
- Trinucleotide repeat expansions: Abnormal amplification of repeat sequences (e.g., Huntington disease — CAG repeats; Fragile X syndrome — CGG repeats)
By origin:
- Spontaneous: Replication errors, spontaneous deamination
- Induced: UV light (thymine dimers), chemicals (alkylating agents), radiation (strand breaks)
In the Indian context, common genetic disorders include thalassaemia (point mutations and deletions in globin genes — carrier frequency up to 3-17% in different populations) and sickle cell disease (prevalent in tribal populations of central India).
Regulation of Gene Expression
Not every gene is active in every cell at all times. Gene regulation ensures the right protein is made in the right cell at the right time. Key mechanisms include:
Figure: Regulation of Gene Expression
1. Transcriptional regulation (most important level):
- Promoters and enhancers: DNA sequences where transcription factors bind to activate gene expression
- Silencers and repressors: Shut down gene expression
- The lac operon (in bacteria) is the classic model: In the absence of lactose, a repressor blocks transcription. When lactose is present, it binds the repressor, releasing the block — an elegant on/off switch
2. Epigenetic regulation:
- DNA methylation: Adding methyl groups to cytosine residues (at CpG islands) silences genes without changing the DNA sequence
- Histone modification: Acetylation opens chromatin (gene ON), deacetylation compacts it (gene OFF)
- Epigenetic changes can be inherited through cell divisions and are implicated in cancer
3. Post-transcriptional regulation:
- mRNA stability (longer poly-A tail = more stable)
- microRNA (miRNA): Small non-coding RNAs that bind to mRNA and block translation or trigger degradation
- RNA interference (RNAi): A natural defence mechanism now being explored as therapy
4. Translational and post-translational regulation:
- Control of translation initiation (phosphorylation of initiation factors)
- Protein modification (phosphorylation, ubiquitination for degradation)
SELF-CHECK — DNA Repair & Gene Regulation
A child presents with severe sunburn after minimal sun exposure and develops multiple skin cancers by age 5. The most likely underlying defect is in:
A. Mismatch repair
B. Base excision repair
C. Nucleotide excision repair
D. Homologous recombination
Reveal Answer
Answer: C. Nucleotide excision repair
Which epigenetic mechanism silences genes without altering the DNA sequence?
A. Point mutation
B. DNA methylation
C. Frameshift mutation
D. Trinucleotide repeat expansion
Reveal Answer
Answer: B. DNA methylation
Recombinant DNA Technology
Recombinant DNA technology (genetic engineering) involves combining DNA from different sources to create new genetic combinations. The basic toolkit includes:
Figure: Recombinant DNA Technology
1. Restriction enzymes (molecular scissors):
- Cut DNA at specific palindromic sequences (e.g., EcoRI cuts at GAATTC)
- Produce sticky ends (overhanging bases that can pair with complementary ends) or blunt ends
2. DNA ligase (molecular glue):
- Joins DNA fragments from different sources
3. Vectors (delivery vehicles):
- Plasmids, bacteriophages, cosmids, or BACs that carry the foreign DNA into host cells
- Must have: origin of replication, selectable marker (e.g., antibiotic resistance), cloning site
4. Host organisms:
- E. coli (most common), yeast, mammalian cell lines
Applications in medicine:
- Recombinant insulin (Humulin) — first recombinant drug approved (1982). Before this, diabetic patients relied on pig or cow insulin, which occasionally caused allergic reactions
- Recombinant hepatitis B vaccine — produced in yeast, safer than plasma-derived vaccine
- Recombinant erythropoietin (EPO) — for anaemia in chronic kidney disease
- Growth hormone — for growth hormone deficiency in children