Chapter 15 — Molecular Genetics (Zoology) Exercise Solutions
This chapter focuses on applications of molecular genetics. Know the techniques and their specific uses precisely.
How to Use This Page
- Review the Key Concepts table first to refresh the terminology and definitions before attempting questions.
- Work through each section sequentially — MCQs, Fill in the Blanks, True or False, Match the Following, short answers, and then detailed answers.
- For 5-mark answers, ensure you include all sub-points mentioned in the model answers.
- Pay special attention to Common Mistakes to Avoid — many students confuse VNTR with SNP, or somatic vs germline gene therapy.
- Use the Exam Checklist as a last-minute revision guide to confirm you have covered all key topics.
Key Concepts (Quick Recap)
| Term | Definition |
|---|---|
| Human Genome Project (HGP) | International project (1990–2003) to sequence the entire human genome of ~3 billion base pairs |
| ELSI | Ethical, Legal, and Social Implications — a component of HGP addressing genetic privacy and discrimination |
| DNA fingerprinting | Technique to identify individuals based on unique DNA patterns (VNTRs and STRs) |
| VNTR | Variable Number Tandem Repeat — regions of DNA with varying numbers of tandem repeats between individuals |
| STR | Short Tandem Repeat — short repeating sequences (2–6 bp) used in DNA profiling; also called microsatellites |
| Southern blotting | Technique to transfer DNA fragments from gel to a membrane for probe hybridisation |
| Autoradiography | Visualisation of DNA bands on X-ray film after radioactive probe hybridisation |
| Gene therapy | Introduction of a functional gene into cells to correct a genetic disorder |
| Somatic gene therapy | Gene therapy targeting body (somatic) cells; the correction is NOT inherited by offspring |
| Germline gene therapy | Gene therapy targeting gametes or early embryos; the correction IS inherited by offspring |
| ADA deficiency | Adenosine deaminase deficiency — first successful gene therapy target; causes SCID |
| Bioinformatics | Application of computational tools to manage, store, and analyse biological data |
| BLAST | Basic Local Alignment Search Tool — used for comparing nucleotide or protein sequences |
| Genetic code | Set of rules by which 64 codons specify 20 amino acids; degenerate, universal, non-overlapping |
| Codon | Triplet of nucleotides on mRNA that specifies a particular amino acid |
Human Genome Project — Key Facts
| Feature | Data |
|---|---|
| Total base pairs | ~3 billion |
| Protein-coding genes | ~20,000–25,000 |
| Protein-coding DNA | Only 1.5% of total DNA |
| Repetitive sequences | ~50% of genome |
| Duration | 1990–2003 |
| Led by | DOE and NIH (USA) |
| India's contribution | CDFD (Hyderabad) |
ELSI — Ethical, Legal, and Social Implications:
- Privacy of genetic information
- Genetic discrimination in insurance/employment
- Ownership of genetic data
- Genetic enhancement vs treatment
DNA Fingerprinting — Steps
1. Extraction of DNA from sample (blood, saliva, hair root). 2. Digestion with restriction enzymes. 3. Gel electrophoresis — fragments separated by size. 4. Southern blotting — transfer to membrane. 5. Hybridisation with radioactive/fluorescent probes targeting VNTR regions. 6. Autoradiography — X-ray film reveals band pattern. 7. Comparison of band patterns.
VNTRs — regions that vary in the number of repeats between individuals. Inherited in Mendelian fashion.
Gene Therapy — Types
| Type | Target | Effect on offspring |
|---|---|---|
| Somatic gene therapy | Body cells | Not inherited |
| Germline gene therapy | Egg/sperm/early embryo | Inherited |
ADA deficiency gene therapy (first approved, 1990):
- ADA gene deficiency → SCID (Severe Combined Immunodeficiency).
- Functional ADA gene cloned into retrovirus vector.
- Vector infects patient's lymphocytes.
- Cells reintroduced — ADA produced.
Bioinformatics
- Management, storage, and analysis of large biological data sets.
- Tools: BLAST (sequence alignment), NCBI databases.
- Applications: Drug target identification, protein structure prediction, genome annotation.
I. Choose the Best Answer (MCQs)
1. The Human Genome Project was completed in the year: (a) 1990 (b) 1995 (c) 2003 (d) 2010
2. Approximately what percentage of the human genome codes for proteins? (a) 50% (b) 25% (c) 10% (d) 1.5%
3. VNTR stands for: (a) Variable Number Transcription Repeats (b) Variable Number Tandem Repeats (c) Variable Nucleotide Tandem Repeats (d) Variable Nucleotide Transcription Repeats
4. The first successful gene therapy was performed for: (a) Cystic fibrosis (b) Haemophilia (c) ADA deficiency (SCID) (d) Sickle cell anaemia
5. Which of the following is used as a vector in gene therapy for ADA deficiency? (a) Adenovirus (b) Retrovirus (c) Plasmid (d) Bacteriophage
6. In DNA fingerprinting, the step after gel electrophoresis is: (a) Restriction digestion (b) Autoradiography (c) Southern blotting (d) Hybridisation
7. Somatic gene therapy differs from germline gene therapy in that: (a) It uses different vectors (b) The corrected gene is not inherited by offspring (c) It targets only blood cells (d) It is more effective
8. BLAST is a tool used in bioinformatics for: (a) Sequence alignment and comparison (b) Protein folding (c) Gene cloning (d) Restriction mapping
9. The number of codons in the genetic code is: (a) 20 (b) 32 (c) 61 (d) 64
10. The stop codons in the genetic code are: (a) AUG, UAA, UAG (b) UAA, UAG, UGA (c) UAA, UGG, UGA (d) AUG, UGA, UAG
11. In germline gene therapy, the target is: (a) Skin cells (b) Liver cells (c) Gametes or early embryo (d) T-lymphocytes
II. Fill in the Blanks
1. The Human Genome Project sequenced approximately 3 billion base pairs of human DNA. 2. The ELSI component of HGP stands for Ethical, Legal, and Social Implications. 3. DNA fingerprinting is based on variations in VNTR (Variable Number Tandem Repeats) regions. 4. The technique of transferring DNA fragments from gel to a membrane is called Southern blotting. 5. The first successful gene therapy was done for ADA (Adenosine deaminase) deficiency which causes SCID. 6. In gene therapy, when the correction is made in body cells and is not passed to offspring, it is called somatic gene therapy. 7. The total number of codons in the genetic code is 64, of which 61 code for amino acids and 3 are stop codons. 8. The start codon AUG codes for methionine. 9. India's contribution to HGP was through CDFD (Centre for DNA Fingerprinting and Diagnostics) located in Hyderabad. 10. The genetic code is degenerate because multiple codons can code for the same amino acid.
III. True or False (with corrections for false statements)
1. True. The Human Genome Project was an international collaborative effort that sequenced the entire human genome.
2. False. — The HGP sequenced the entire human genome, not just the protein-coding regions. Only about 1.5% of the genome actually codes for proteins, but the entire 3 billion base pairs were sequenced.
3. True. VNTR patterns are unique to each individual (except identical twins), making DNA fingerprinting a powerful identification tool.
4. False. — Somatic gene therapy does not affect the patient's offspring because it targets only body (somatic) cells, not gametes. Only germline gene therapy changes are inherited.
5. True. The genetic code is universal — the same codon specifies the same amino acid in nearly all organisms.
6. False. — DNA fingerprinting can use any nucleated cell as a DNA source, not just blood. Sources include saliva, hair roots, skin cells, and semen.
7. True. BLAST (Basic Local Alignment Search Tool) is used for comparing nucleotide or protein sequences against databases.
8. False. — Germline gene therapy is not currently approved for use in humans due to ethical concerns about heritable genetic changes. Only somatic gene therapy is approved.
IV. Match the Following
| Column A | Column B |
|---|---|
| 1. HGP completion year | (e) 2003 |
| 2. VNTR | (h) Variable Number Tandem Repeats |
| 3. Southern blotting | (a) Transfer of DNA from gel to membrane |
| 4. ADA deficiency | (g) First gene therapy target — SCID |
| 5. BLAST | (c) Sequence alignment tool |
| 6. Somatic gene therapy | (f) Correction not inherited by offspring |
| 7. ELSI | (b) Ethical, Legal, and Social Implications |
| 8. Autoradiography | (d) X-ray film visualisation of DNA bands |
V. Answer Briefly (2-mark)
VI. Answer in Detail (5-mark)
Worked Examples
Example 1 — Reading the genetic code (codon → amino acid)
An mRNA reads 5'-AUG GCU UAA-3'. Interpret it.
Example 2 — Number of codons
Example 3 — DNA profiling logic
Why can DNA fingerprinting identify individuals?
Everyone (except identical twins) has unique VNTR patterns — the number of tandem repeats at given loci differs between people. Comparing band patterns on a gel gives a near-unique profile, used in forensics and paternity tests.
Common Mistakes
- Confusing VNTR (Variable Number Tandem Repeat) with SNP (Single Nucleotide Polymorphism) — both used in DNA profiling but different.
- Saying HGP sequenced only protein-coding DNA — it sequenced the entire genome.
- Forgetting that somatic gene therapy does not affect offspring.
- Stating DNA fingerprinting is done only from blood — it can use any nucleated cell.
Common Mistakes to Avoid
- ❌ Saying HGP sequenced only protein-coding genes → ✅ HGP sequenced the entire human genome (~3 billion base pairs); only ~1.5% actually codes for proteins.
- ❌ Confusing VNTR with SNP → ✅ VNTR (Variable Number Tandem Repeat) — regions where the number of tandem repeats varies between individuals; SNP (Single Nucleotide Polymorphism) — variation at a single nucleotide position. Both are used in DNA profiling but are different.
- ❌ Stating that somatic gene therapy changes are inherited → ✅ Somatic gene therapy targets body cells; corrections are not inherited by offspring. Only germline gene therapy changes are inherited.
- ❌ Writing that DNA fingerprinting can only use blood samples → ✅ DNA fingerprinting can use any nucleated cell — blood, saliva, hair root, skin cells, semen, or bone marrow.
- ❌ Saying there are 20 codons for 20 amino acids → ✅ There are 64 codons (4³ = 64), of which 61 code for amino acids and 3 are stop codons. The code is degenerate.
- ❌ Confusing Southern blotting with Northern blotting → ✅ Southern blotting transfers DNA from gel to membrane; Northern blotting transfers RNA. DNA fingerprinting uses Southern blotting.
- ❌ Stating germline gene therapy is commonly used → ✅ Germline gene therapy is not approved for use in humans due to ethical concerns about heritable genetic changes. Only somatic gene therapy is currently practised.
- ❌ Writing that the genetic code is ambiguous → ✅ The genetic code is unambiguous — each codon specifies only one amino acid. It is degenerate (multiple codons for the same amino acid), not ambiguous.
Exam Checklist
- [ ] Know HGP timeline (1990–2003), goals, and key findings (~20,000–25,000 genes, 1.5% coding)
- [ ] Define ELSI and explain why ethical considerations are important in genome research
- [ ] List all steps of DNA fingerprinting in correct sequence (extraction → restriction → electrophoresis → Southern blot → hybridisation → autoradiography)
- [ ] Differentiate VNTR from STR and from SNP
- [ ] Define somatic vs germline gene therapy and state which is inherited
- [ ] Explain ADA deficiency gene therapy step by step (first successful gene therapy, 1990)
- [ ] Know all 6 properties of the genetic code: degenerate, universal, non-overlapping, commaless, unambiguous, has start/stop codons
- [ ] Remember: 64 codons total, 61 code for amino acids, 3 stop codons (UAA, UAG, UGA), start codon AUG
- [ ] Explain bioinformatics tools: BLAST (sequence alignment), NCBI databases
- [ ] Differentiate Southern blotting (DNA) from Northern blotting (RNA) and Western blotting (protein)
- [ ] Know India's contribution: CDFD, Hyderabad
- [ ] State why germline gene therapy is not approved: ethical concerns about heritable changes