A couple from a tribal district of Chhattisgarh, both carriers of sickle cell trait, request prenatal diagnosis. Chorionic villus sampling is done at 11 weeks. Which technique is best suited to detect the known single-base change in the fetal beta-globin gene?

PCR followed by restriction digestion

FISH with a probe for chromosome 11p15

Western blotting of fetal protein

A hospital molecular laboratory in Kochi runs real-time PCR for hepatitis B viral DNA. For three runs in a row the no-template control has turned positive late in the run, and several patients who were negative before now show low-level positives. The positive control and the internal control perform as expected. What is the most appropriate corrective action?

Control amplicon carryover with one-way workflow and UNG

Report patient results, as the control is only advisory

Increase the cycle number to strengthen true signals

Lower the annealing temperature to improve priming

A 45-year-old man in Lucknow with chronic myeloid leukaemia has been on imatinib for a year. Every three months his response is monitored by quantitative PCR for the BCR::ABL1 transcript, reported on the International Scale. Why is this test performed on RNA (converted to cDNA) rather than on genomic DNA?

Genomic breakpoints vary, but the mRNA junction is constant

RNA is far more stable than DNA in stored blood samples

The fusion gene is absent from the DNA of the CML cells

DNA polymerase cannot copy across a translocation junction

Molecular biology and genetic techniques

21 NEET-PG practice questions on Molecular biology and genetic techniques, part of Biochemistry. The first 21 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
A couple from a tribal district of Chhattisgarh, both carriers of sickle cell trait, request prenatal diagnosis. Chorionic villus sampling is done at 11 weeks. Which technique is best suited to detect the known single-base change in the fetal beta-globin gene?
1. AFISH with a probe for chromosome 11p15
2. BPCR followed by restriction digestion
3. CG-banded karyotyping
4. DWestern blotting of fetal protein
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Answer: B · PCR followed by restriction digestion
The sickle mutation is a single-base substitution in codon 6 of the beta-globin gene (GAG to GTG, glutamate to valine). This change abolishes a restriction site, so the segment amplified by PCR is cut differently in normal and mutant alleles, separating AA, AS and SS genotypes on a gel. Allele-specific PCR (ARMS) or sequencing can also be used. Chorionic villi give fetal DNA at 10-13 weeks, allowing an early decision with counselling.
- A: FISH detects deletions or copy number of large segments, not a point mutation.
- C: Karyotyping detects whole-chromosome or large structural changes, not one base.
- D: Western blotting detects proteins, and fetal haemoglobin at 11 weeks is mostly not adult beta-globin.
Question 2 · Hard
A hospital molecular laboratory in Kochi runs real-time PCR for hepatitis B viral DNA. For three runs in a row the no-template control has turned positive late in the run, and several patients who were negative before now show low-level positives. The positive control and the internal control perform as expected. What is the most appropriate corrective action?
1. AReport patient results, as the control is only advisory
2. BIncrease the cycle number to strengthen true signals
3. CLower the annealing temperature to improve priming
4. DControl amplicon carryover with one-way workflow and UNG
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Answer: D · Control amplicon carryover with one-way workflow and UNG
A positive no-template control with new low-level positives in patients means contamination, most often by amplified product (amplicon) carried over from earlier runs. The run must be rejected, work areas and equipment decontaminated (for example with bleach), reagents replaced with fresh aliquots, and a one-way workflow kept from clean pre-PCR rooms to post-PCR areas. Using dUTP in the reaction with uracil-N-glycosylase destroys carried-over amplicons before cycling. Patient results from the affected runs are retested after the problem is fixed.
- A: A failed negative control invalidates the run; results cannot be released.
- B: More cycles would amplify the contaminant further and add late false positives.
- C: A lower annealing temperature reduces specificity and adds non-specific products.
Question 3 · Hard
A 45-year-old man in Lucknow with chronic myeloid leukaemia has been on imatinib for a year. Every three months his response is monitored by quantitative PCR for the BCR::ABL1 transcript, reported on the International Scale. Why is this test performed on RNA (converted to cDNA) rather than on genomic DNA?
1. AGenomic breakpoints vary, but the mRNA junction is constant
2. BRNA is far more stable than DNA in stored blood samples
3. CThe fusion gene is absent from the DNA of the CML cells
4. DDNA polymerase cannot copy across a translocation junction
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Answer: A · Genomic breakpoints vary, but the mRNA junction is constant
In t(9;22) the breaks occur at different points within long introns of BCR and ABL1 in different patients, so a single pair of primers cannot amplify every genomic junction. After transcription and splicing, however, the fusion mRNA joins a few fixed exons (usually e13a2 or e14a2), so one assay on cDNA works for nearly all patients and measures the number of leukaemic transcripts. Results relative to a control gene, on the International Scale, define major molecular response and guide whether imatinib is working. Rising levels prompt testing for ABL1 kinase domain mutations.
- B: RNA is less stable than DNA, so samples must be processed promptly.
- C: The fusion gene is present in the DNA; it is the variable genomic breakpoint that makes DNA assays impractical.
- D: DNA polymerase can copy across a junction if primers flank it; the problem is the variable breakpoint.
Question 4 · Medium
A family of four in the hills of Manipur cooks wild mushrooms collected from the forest. About ten hours later all develop severe vomiting and watery diarrhoea, which settles, and on the third day they become jaundiced with a prolonged INR. The toxin responsible acts by inhibiting which enzyme?
1. ARNA polymerase I
2. BDNA polymerase delta
3. CRNA polymerase II
4. DRNA polymerase III
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Answer: C · RNA polymerase II
Late-onset gastroenteritis followed by liver failure is typical of poisoning by Amanita species, whose alpha-amanitin binds tightly to RNA polymerase II. This enzyme makes the precursors of messenger RNA (and most small nuclear RNAs), so protein synthesis stops and hepatocytes, which take up the toxin, die. RNA polymerase III is inhibited only at higher concentrations and RNA polymerase I is resistant. Treatment is supportive with activated charcoal, measures to cut the toxin's uptake and, in liver failure, transplantation.
- A: RNA polymerase I makes ribosomal RNA and is resistant to alpha-amanitin.
- B: Alpha-amanitin does not affect DNA polymerases.
- D: RNA polymerase III makes tRNA and 5S rRNA and is inhibited only at high toxin concentrations.
Question 5 · Easy
A 40-year-old woman with a complicated urinary tract infection is started on ciprofloxacin after culture shows a sensitive E. coli. Which bacterial enzyme does this drug inhibit?
1. ADNA-dependent RNA polymerase
2. BDNA gyrase
3. CDihydropteroate synthase
4. DPeptidyl transferase
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Answer: B · DNA gyrase
Fluoroquinolones such as ciprofloxacin inhibit bacterial DNA gyrase (topoisomerase II) and topoisomerase IV. Gyrase introduces negative supercoils and relieves the positive supercoiling ahead of the replication fork; blocking it traps the enzyme on cut DNA and kills the bacterium. In Gram-negative bacteria gyrase is the main target, and in Gram-positive bacteria topoisomerase IV. Resistance arises from mutations in gyrA and parC, and is now common among Indian E. coli isolates.
- A: DNA-dependent RNA polymerase is the target of rifampicin.
- C: Dihydropteroate synthase is the target of sulphonamides.
- D: Peptidyl transferase on the 50S ribosome is blocked by chloramphenicol.
Question 6 · Medium
A 34-year-old woman with triple-negative breast cancer has a next-generation sequencing panel that reports a pathogenic BRCA1 variant. The laboratory confirms the variant by Sanger sequencing before reporting. In this method, why does a growing DNA chain stop when a dideoxynucleotide is added?
1. AThe dideoxynucleotide has no 5'-phosphate group
2. BThe template strand breaks at that position
3. CThe fluorescent dye blocks the polymerase site
4. DThe dideoxynucleotide has no 3'-hydroxyl group
Answer and explanation in LearnFree account
Question 7 · Easy
A 36-year-old schoolteacher in Kozhikode is diagnosed with adenocarcinoma of the caecum. His mother had endometrial cancer at 45 and his maternal uncle colon cancer at 40. The tumour shows high microsatellite instability. Which pathway is most likely defective?
1. ABRCA-mediated recombination
2. BNucleotide excision repair
3. CDNA mismatch repair
4. DAPC-beta-catenin signalling
Answer and explanation in LearnFree account
Question 8 · Easy
A research team in a medical college wants to know whether a tumour cell line expresses the messenger RNA of a particular gene. They separate total RNA by size on a gel, transfer it to a membrane and hybridise it with a labelled DNA probe. Which technique are they using?
1. AWestern blotting
2. BDot blot hybridisation
3. CNorthern blotting
4. DSouthern blotting
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Question 9 · Medium
A 14-year-old boy from Kolkata, still walking independently, has calf hypertrophy, mild proximal weakness and a serum CK of 4,800 U/L. MLPA shows a deletion of exons 45-47 of the dystrophin gene that keeps the reading frame. Why is his disease milder than Duchenne muscular dystrophy?
1. AAn in-frame deletion yields a shorter dystrophin
2. BA triplet repeat expansion partly silences the gene
3. CA frameshift deletion causes a premature stop codon
4. DA promoter mutation lowers transcription slightly
Answer and explanation in LearnFree account
Question 10 · Hard
A 2-year-old with severe anaemia and no HbA on HPLC is evaluated in a Mumbai thalassaemia clinic. Sequencing of his beta-globin gene shows that codon 39, normally CAG (glutamine) on the coding strand, has changed to TAG on both alleles. Which statement best describes the mutation and its effect?
1. AMissense mutation, so an unstable chain is made
2. BFrameshift mutation, so a longer chain is made
3. CSilent mutation, so the protein is unchanged
4. DNonsense mutation, so no beta chains are made
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Question 11 · Hard
A 15-year-old boy from Mysuru has moderate intellectual disability, a long face, large ears and enlarged testes. His maternal uncle has a similar problem. Standard PCR across the FMR1 CGG repeat amplifies no allele in the boy, while his mother shows one normal allele and one allele of about 90 repeats. Which test best confirms the full mutation and its methylation status in the boy?
1. AChromosomal microarray
2. BSouthern blot analysis of FMR1
3. CWhole-exome sequencing
4. DKaryotype in folate-deficient culture
Answer and explanation in LearnFree account
Question 12 · Medium
A 3-year-old boy is on monthly transfusions for beta-thalassaemia major at a thalassaemia day-care centre in Ahmedabad. Gene analysis shows he is homozygous for IVS1-5 (G>C), a mutation within intron 1 of the beta-globin gene that is common in India. How does a mutation in an intron reduce beta-globin production?
1. AIt disrupts normal splicing of pre-mRNA
2. BIt deletes the entire beta-globin gene
3. CIt destroys the TATA box of the promoter
4. DIt creates a premature stop codon in exon 1
Answer and explanation in LearnFree account
Question 13 · Medium
A 19-year-old with transfusion-dependent beta-thalassaemia is told about a gene therapy in which his own stem cells are edited outside the body with CRISPR-Cas9 to disrupt the erythroid enhancer of BCL11A, so that fetal haemoglobin rises. What does the Cas9 protein do in this process?
1. ADegrades the matching messenger RNA in the cytoplasm
2. BAdds nucleotides to the 3' ends of chromosomes
3. CCuts both DNA strands at a site set by its guide RNA
4. DMethylates CpG islands to silence the target gene
Answer and explanation in LearnFree account
Question 14 · Medium
During DNA replication one strand is made continuously, while the other, the lagging strand, is made as short Okazaki fragments that are later joined. Why is the lagging strand made discontinuously?
1. ADNA polymerase can add nucleotides only 5' to 3'
2. BLigase must join the strand every few bases
3. CHelicase unwinds only one parental strand at a time
4. DPrimase can work only on the leading strand
Answer and explanation in LearnFree account
Question 15 · Medium
A postgraduate student revising laboratory techniques before her rotation in the molecular laboratory lists four pairs: 1. Southern blot: DNA. 2. Northern blot: RNA. 3. Western blot: protein detected with an antibody. 4. Southwestern blot: DNA-binding protein detected with a labelled DNA probe. Which pairs are correctly matched?
1. A2 and 3 only
2. B1, 2, 3 and 4
3. C1 and 2 only
4. D1, 3 and 4 only
Answer and explanation in LearnFree account
Question 16 · Easy
A 32-year-old man with cough for six weeks gives a sputum sample at a district TB centre. The cartridge-based nucleic acid amplification test reports Mycobacterium tuberculosis detected, rifampicin resistance detected. The test detects mutations in the gene for which target?
1. ACatalase-peroxidase (katG)
2. BDNA gyrase A subunit (gyrA)
3. CRNA polymerase beta subunit (rpoB)
4. DEnoyl-ACP reductase (inhA)
Answer and explanation in LearnFree account
Question 17 · Medium
A 12-year-old boy from Guwahati has nail dystrophy, lacy reticulate pigmentation on his neck and white patches on his tongue, and now has pancytopenia from marrow failure. Genetic testing shows a defect in the enzyme complex that maintains telomere length. What type of polymerase is this enzyme?
1. ADNA-dependent DNA polymerase
2. BRNA-dependent DNA polymerase
3. CDNA-dependent RNA polymerase
4. DRNA-dependent RNA polymerase
Answer and explanation in LearnFree account
Question 18 · Medium
A 28-year-old man attending an ART centre has been on dolutegravir-based treatment for six months. Under NACO guidance his HIV-1 plasma viral load is measured by real-time PCR. Which enzyme carries out the first step of this assay on the viral RNA?
1. AReverse transcriptase
2. BTaq DNA polymerase
3. CRNA-dependent RNA polymerase
4. DDNA ligase
Answer and explanation in LearnFree account
Question 19 · Hard
A newborn in a Hyderabad NICU has low birth weight, clenched hands with overlapping fingers, rocker-bottom feet, a small jaw and a large ventricular septal defect. The cardiac surgeons need to know within one to two days whether he has a lethal trisomy before deciding on surgery. Which test gives the answer fastest?
1. AG-banded karyotype on cultured lymphocytes
2. BChromosomal microarray on blood DNA
3. CWhole-genome sequencing
4. DInterphase FISH on uncultured cells
Answer and explanation in LearnFree account
Question 20 · Medium
A 4-year-old boy is seen in a developmental clinic in Chennai with global developmental delay, autistic features and mild facial dysmorphism; his G-banded karyotype is normal. The geneticist orders a SNP-based chromosomal microarray. Four statements are discussed: 1. It detects submicroscopic deletions and duplications. 2. It detects balanced reciprocal translocations. 3. It can show long stretches of homozygosity that suggest uniparental disomy. 4. It detects single-base point mutations within a gene. Which statements are correct?
1. A3 and 4 only
2. B1 and 3 only
3. C1, 2 and 3 only
4. D1 and 2 only
Answer and explanation in LearnFree account
Question 21 · Easy
Parents from Barmer, Rajasthan, bring their 8-year-old son to the skin OPD. Since he was a baby, a few minutes outdoors has left him blistered and red; his cheeks and forearms are now covered in dark lentigines, and a pearly nodule on his nose is a basal cell carcinoma on biopsy. Which DNA repair process is defective?
1. AMismatch repair
2. BHomologous recombination repair
3. CBase excision repair
4. DNucleotide excision repair
Answer and explanation in LearnFree account

Molecular biology and genetic techniques

A couple from a tribal district of Chhattisgarh, both carriers of sickle cell trait, request prenatal diagnosis. Chorionic villus sampling is done at 11 weeks. Which technique is best suited to detect the known single-base change in the fetal beta-globin gene?

A family of four in the hills of Manipur cooks wild mushrooms collected from the forest. About ten hours later all develop severe vomiting and watery diarrhoea, which settles, and on the third day they become jaundiced with a prolonged INR. The toxin responsible acts by inhibiting which enzyme?

A 40-year-old woman with a complicated urinary tract infection is started on ciprofloxacin after culture shows a sensitive E. coli. Which bacterial enzyme does this drug inhibit?

A 34-year-old woman with triple-negative breast cancer has a next-generation sequencing panel that reports a pathogenic BRCA1 variant. The laboratory confirms the variant by Sanger sequencing before reporting. In this method, why does a growing DNA chain stop when a dideoxynucleotide is added?

A 36-year-old schoolteacher in Kozhikode is diagnosed with adenocarcinoma of the caecum. His mother had endometrial cancer at 45 and his maternal uncle colon cancer at 40. The tumour shows high microsatellite instability. Which pathway is most likely defective?

A research team in a medical college wants to know whether a tumour cell line expresses the messenger RNA of a particular gene. They separate total RNA by size on a gel, transfer it to a membrane and hybridise it with a labelled DNA probe. Which technique are they using?

A 14-year-old boy from Kolkata, still walking independently, has calf hypertrophy, mild proximal weakness and a serum CK of 4,800 U/L. MLPA shows a deletion of exons 45-47 of the dystrophin gene that keeps the reading frame. Why is his disease milder than Duchenne muscular dystrophy?

A 2-year-old with severe anaemia and no HbA on HPLC is evaluated in a Mumbai thalassaemia clinic. Sequencing of his beta-globin gene shows that codon 39, normally CAG (glutamine) on the coding strand, has changed to TAG on both alleles. Which statement best describes the mutation and its effect?

A 19-year-old with transfusion-dependent beta-thalassaemia is told about a gene therapy in which his own stem cells are edited outside the body with CRISPR-Cas9 to disrupt the erythroid enhancer of BCL11A, so that fetal haemoglobin rises. What does the Cas9 protein do in this process?

During DNA replication one strand is made continuously, while the other, the lagging strand, is made as short Okazaki fragments that are later joined. Why is the lagging strand made discontinuously?

A 32-year-old man with cough for six weeks gives a sputum sample at a district TB centre. The cartridge-based nucleic acid amplification test reports Mycobacterium tuberculosis detected, rifampicin resistance detected. The test detects mutations in the gene for which target?

A 12-year-old boy from Guwahati has nail dystrophy, lacy reticulate pigmentation on his neck and white patches on his tongue, and now has pancytopenia from marrow failure. Genetic testing shows a defect in the enzyme complex that maintains telomere length. What type of polymerase is this enzyme?

A 28-year-old man attending an ART centre has been on dolutegravir-based treatment for six months. Under NACO guidance his HIV-1 plasma viral load is measured by real-time PCR. Which enzyme carries out the first step of this assay on the viral RNA?