Most patients with primary gout have hyperuricaemia mainly because of:

Reduced renal excretion of uric acid

A high dietary intake of purines alone

In Southern blotting, the gel is soaked in sodium hydroxide before the DNA is transferred to the membrane. Why?

To make the DNA single-stranded for the probe

To stain the DNA bands so they can be seen

To remove RNA and protein from the gel

To cut the DNA at its restriction sites

Nucleic acid metabolism and molecular biology

34 Kerala MSc MLT Entrance practice questions on Nucleic acid metabolism and molecular biology, part of Biochemistry. The first 25 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 · Easy
During DNA replication, which enzyme lays down the short RNA primers from which each Okazaki fragment is extended?
1. ATopoisomerase II
2. BDNA ligase
3. CPrimase
4. DHelicase
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Answer: C · Primase
DNA polymerases cannot start a new strand; they can only add nucleotides to an existing 3'-OH end. Primase, an RNA polymerase, makes a short RNA primer at the start of the leading strand and of every Okazaki fragment on the lagging strand. The primers are later removed and replaced with DNA, and DNA ligase seals the nicks. In eukaryotes, primase works as part of DNA polymerase alpha.
- A: Topoisomerase II relieves supercoiling ahead of the fork.
- B: DNA ligase joins fragments after the primers are replaced.
- D: Helicase unwinds the double helix ahead of the fork.
Question 2 · Easy
Which enzyme is deficient in hereditary orotic aciduria, with a megaloblastic anaemia that does not respond to vitamin B12 or folate?
1. AOrnithine transcarbamoylase
2. BThymidylate synthase
3. CUMP synthase
4. DDihydrofolate reductase
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Answer: C · UMP synthase
UMP synthase converts orotic acid to UMP in the last steps of pyrimidine synthesis. When it is deficient, orotic acid accumulates and is excreted, and the shortage of pyrimidines slows DNA synthesis, giving megaloblastic anaemia. Treatment with uridine bypasses the block. OTC deficiency also raises urinary orotic acid but causes hyperammonaemia, not anaemia.
- A: OTC deficiency raises orotic acid with hyperammonaemia, not megaloblastic anaemia.
- B: Thymidylate synthase acts after UMP and does not cause orotic acid build-up.
- D: Dihydrofolate reductase deficiency does not cause orotic aciduria.
Question 3 · Medium
Alpha-amanitin, the toxin of the death cap mushroom, kills liver cells mainly by inhibiting which enzyme?
1. ADNA polymerase delta
2. BRNA polymerase II
3. CRNA polymerase I
4. DPeptidyl transferase
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Answer: B · RNA polymerase II
Alpha-amanitin binds RNA polymerase II very tightly and blocks the synthesis of messenger RNA. Without new mRNA, protein synthesis stops and hepatocytes die, causing acute liver failure. RNA polymerase I, which makes ribosomal RNA, is resistant, and RNA polymerase III is inhibited only at high concentrations. This differing sensitivity was used to tell the three polymerases apart.
- A: Alpha-amanitin does not act on DNA replication.
- C: RNA polymerase I is resistant to alpha-amanitin.
- D: Peptidyl transferase is inhibited by antibiotics such as chloramphenicol.
Question 4 · Medium
Most patients with primary gout have hyperuricaemia mainly because of:
1. AA high dietary intake of purines alone
2. BComplete deficiency of HGPRT
3. CReduced renal excretion of uric acid
4. DAn overactive PRPP synthetase
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Answer: C · Reduced renal excretion of uric acid
About 90% of primary gout is due to under-excretion: the kidney clears urate less efficiently than normal. Only a minority overproduce uric acid, through enzyme defects such as PRPP synthetase superactivity or partial HGPRT deficiency. Diet adds to the urate load but rarely causes gout by itself. A 24-hour urine uric acid helps separate under-excreters from over-producers.
- A: Diet contributes but is rarely the sole cause.
- B: Complete HGPRT deficiency causes Lesch-Nyhan syndrome, which is rare.
- D: PRPP synthetase superactivity is a rare cause of overproduction.
Question 5 · Medium
In Southern blotting, the gel is soaked in sodium hydroxide before the DNA is transferred to the membrane. Why?
1. ATo stain the DNA bands so they can be seen
2. BTo remove RNA and protein from the gel
3. CTo make the DNA single-stranded for the probe
4. DTo cut the DNA at its restriction sites
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Answer: C · To make the DNA single-stranded for the probe
Alkali breaks the hydrogen bonds between base pairs, so the separated fragments become single-stranded. Only single-stranded DNA on the membrane can hybridise with the labelled single-stranded probe. The gel is then neutralised and the DNA transferred by capillary action or vacuum. Restriction digestion and staining happen before this step.
- A: Ethidium bromide or similar dyes stain DNA; alkali does not.
- B: Purifying DNA is done before electrophoresis, not by alkali soaking.
- D: Restriction enzymes cut the DNA before electrophoresis.
Question 6 · Easy
The committed, rate-limiting step of de novo purine synthesis is catalysed by:
1. AAspartate transcarbamoylase
2. BAdenine phosphoribosyltransferase
3. CXanthine oxidase
4. DGlutamine PRPP amidotransferase
Answer and explanation in LearnFree account
Question 7 · Hard
A 5-month-old has recurrent pneumonia, oral thrush and chronic diarrhoea. Lymphocyte count 300/µL (reference for age 4,000-13,500), no thymic shadow on chest X-ray, and very low IgG, IgA and IgM. Red cell deoxyadenosine nucleotides (dATP) are greatly raised, and serum uric acid is normal. Which enzyme deficiency is most likely?
1. AHGPRT
2. BXanthine oxidase
3. CAdenosine deaminase
4. DPurine nucleoside phosphorylase
Answer and explanation in LearnFree account
Question 8 · Medium
Which structure removes introns from eukaryotic pre-mRNA?
1. ARestriction endonucleases
2. BThe spliceosome
3. CRNA polymerase III
4. DThe 40S ribosomal subunit
Answer and explanation in LearnFree account
Question 9 · Hard
The sickle mutation abolishes a restriction site in the beta-globin gene. A 380-bp PCR product from a normal allele is cut into 200- and 180-bp fragments, while the sickle allele stays uncut. A child's digest shows bands at 380, 200 and 180 bp; a normal control in the same run shows only 200 and 180 bp. The child is most likely:
1. AUninterpretable: incompletely digested
2. BHeterozygous (sickle cell trait)
3. CHomozygous for sickle cell disease
4. DA normal homozygote (HbAA)
Answer and explanation in LearnFree account
Question 10 · Hard
A 1-year-old has a large liver, fasting glucose 38 mg/dL, lactate 6.0 mmol/L (reference 0.5-2.2), triglyceride 450 mg/dL and uric acid 9.5 mg/dL (reference 2.0-5.5). Glucagon does not raise the glucose. What best explains the high uric acid?
1. AInhibition of HGPRT, so purine salvage fails
2. BRapid turnover of cells in the enlarged liver
3. CInduction of xanthine oxidase by low insulin
4. DExtra purine synthesis and lactate blocking urate excretion
Answer and explanation in LearnFree account
Question 11 · Hard
A PCR run for Mycobacterium tuberculosis DNA gives a band of the expected size in the patient sample, the positive control and the no-template control, which contains water instead of sample. How should the result be handled?
1. ARepeat with a higher annealing temperature
2. BReport the patient sample as negative
3. CReport the patient sample as positive
4. DReject the run and investigate contamination
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Question 12 · Hard
A 4-year-old boy has developmental delay, writhing movements and bites his lips and fingers. His mother noticed orange sand-like crystals in his nappies. Serum uric acid is 11.5 mg/dL (reference 2.0-5.5). Which enzyme is deficient?
1. AGlucose-6-phosphatase
2. BXanthine oxidase
3. CHGPRT
4. DAdenosine deaminase
Answer and explanation in LearnFree account
Question 13 · Medium
A man with gout passes uric acid stones. Making his urine more alkaline helps prevent new stones because uric acid:
1. AIs converted to allantoin in alkaline urine
2. BForms soluble calcium salts in alkaline urine
3. CIs more soluble as urate at higher pH
4. DIs less filtered by the glomerulus at higher pH
Answer and explanation in LearnFree account
Question 14 · Easy
In an adult man, hyperuricaemia is usually defined as a serum uric acid above about:
1. A4 mg/dL
2. B2 mg/dL
3. C7 mg/dL
4. D12 mg/dL
Answer and explanation in LearnFree account
Question 15 · Medium
A DNA fragment rich in guanine and cytosine melts at a higher temperature than an AT-rich fragment of the same length mainly because G-C pairs:
1. AAre joined by three hydrogen bonds
2. BLie outside the sugar-phosphate backbone
3. CContain uracil instead of thymine
4. DAre joined by covalent bonds
Answer and explanation in LearnFree account
Question 16 · Medium
Methotrexate blocks DNA synthesis in rapidly dividing cells mainly by inhibiting:
1. ADNA polymerase alpha
2. BDihydrofolate reductase
3. CThymidylate synthase
4. DRibonucleotide reductase
Answer and explanation in LearnFree account
Question 17 · Easy
Two nitrogen atoms of the purine ring, N3 and N9, are donated by the amide group of which amino acid?
1. AGlutamine
2. BAsparagine
3. CAspartate
4. DGlycine
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Question 18 · Medium
In a real-time PCR viral load assay, a sample with a lower cycle threshold (Ct) than another sample contains:
1. ALess target nucleic acid
2. BMore PCR inhibitors
3. CShorter amplified products
4. DMore target nucleic acid
Answer and explanation in LearnFree account
Question 19 · Medium
Hydroxyurea, used to raise fetal haemoglobin in sickle cell disease, blocks DNA synthesis by inhibiting:
1. ATopoisomerase II
2. BRibonucleotide reductase
3. CThymidylate synthase
4. DDihydrofolate reductase
Answer and explanation in LearnFree account
Question 20 · Easy
In Western blotting, the molecule separated on the gel and detected on the membrane is:
1. ACarbohydrate
2. BProtein
3. CRNA
4. DDNA
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Question 21 · Hard
A 45-year-old man with recurrent gout and one uric acid stone has serum uric acid 9.6 mg/dL and normal creatinine. On his usual diet, 24-hour urine uric acid is 1,050 mg/day (above 800 mg/day indicates overproduction). Which interpretation and drug choice fit?
1. AOverproduction; a xanthine oxidase inhibitor is preferred
2. BUnderexcretion; urine alkalinisation alone is enough
3. CUnderexcretion; a uricosuric drug is preferred
4. DNormal handling; no urate-lowering drug is needed
Answer and explanation in LearnFree account
Question 22 · Easy
After the RNA primers are replaced, the remaining nicks between Okazaki fragments are sealed by:
1. ATopoisomerase II
2. BDNA polymerase III
3. CDNA ligase
4. DHelicase
Answer and explanation in LearnFree account
Question 23 · Medium
In a PCR programme, the annealing temperature is chosen mainly according to the:
1. AMelting temperature of the primers
2. BLength of the whole template DNA
3. CNumber of cycles in the programme
4. DOptimum temperature of Taq polymerase
Answer and explanation in LearnFree account
Question 24 · Hard
A 14-year-old with acute lymphoblastic leukaemia and WBC 1,80,000/µL starts chemotherapy. Next day: uric acid 15.2 mg/dL, potassium 6.4 mmol/L, phosphate 9.1 mg/dL, calcium 6.8 mg/dL and creatinine 2.3 mg/dL. Which drug most rapidly lowers the uric acid?
1. AIntravenous rasburicase
2. BOral allopurinol
3. COral colchicine
4. DOral probenecid
Answer and explanation in LearnFree account
Question 25 · Medium
A DNA extract has an A260/A280 ratio of 1.4. This most likely indicates:
1. APure double-stranded DNA
2. BContamination with protein or phenol
3. CComplete degradation of the DNA
4. DContamination with RNA
Answer and explanation in LearnFree account

9 more Nucleic acid metabolism and molecular biology questions

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Nucleic acid metabolism and molecular biology

During DNA replication, which enzyme lays down the short RNA primers from which each Okazaki fragment is extended?

Which enzyme is deficient in hereditary orotic aciduria, with a megaloblastic anaemia that does not respond to vitamin B12 or folate?

Alpha-amanitin, the toxin of the death cap mushroom, kills liver cells mainly by inhibiting which enzyme?

Most patients with primary gout have hyperuricaemia mainly because of:

In Southern blotting, the gel is soaked in sodium hydroxide before the DNA is transferred to the membrane. Why?

The committed, rate-limiting step of de novo purine synthesis is catalysed by:

Which structure removes introns from eukaryotic pre-mRNA?

A 1-year-old has a large liver, fasting glucose 38 mg/dL, lactate 6.0 mmol/L (reference 0.5-2.2), triglyceride 450 mg/dL and uric acid 9.5 mg/dL (reference 2.0-5.5). Glucagon does not raise the glucose. What best explains the high uric acid?

A PCR run for Mycobacterium tuberculosis DNA gives a band of the expected size in the patient sample, the positive control and the no-template control, which contains water instead of sample. How should the result be handled?

A 4-year-old boy has developmental delay, writhing movements and bites his lips and fingers. His mother noticed orange sand-like crystals in his nappies. Serum uric acid is 11.5 mg/dL (reference 2.0-5.5). Which enzyme is deficient?

A man with gout passes uric acid stones. Making his urine more alkaline helps prevent new stones because uric acid:

In an adult man, hyperuricaemia is usually defined as a serum uric acid above about:

A DNA fragment rich in guanine and cytosine melts at a higher temperature than an AT-rich fragment of the same length mainly because G-C pairs:

Methotrexate blocks DNA synthesis in rapidly dividing cells mainly by inhibiting:

Two nitrogen atoms of the purine ring, N3 and N9, are donated by the amide group of which amino acid?

In a real-time PCR viral load assay, a sample with a lower cycle threshold (Ct) than another sample contains:

Hydroxyurea, used to raise fetal haemoglobin in sickle cell disease, blocks DNA synthesis by inhibiting:

In Western blotting, the molecule separated on the gel and detected on the membrane is:

A 45-year-old man with recurrent gout and one uric acid stone has serum uric acid 9.6 mg/dL and normal creatinine. On his usual diet, 24-hour urine uric acid is 1,050 mg/day (above 800 mg/day indicates overproduction). Which interpretation and drug choice fit?

After the RNA primers are replaced, the remaining nicks between Okazaki fragments are sealed by:

In a PCR programme, the annealing temperature is chosen mainly according to the:

A 14-year-old with acute lymphoblastic leukaemia and WBC 1,80,000/µL starts chemotherapy. Next day: uric acid 15.2 mg/dL, potassium 6.4 mmol/L, phosphate 9.1 mg/dL, calcium 6.8 mg/dL and creatinine 2.3 mg/dL. Which drug most rapidly lowers the uric acid?

A DNA extract has an A260/A280 ratio of 1.4. This most likely indicates: