MCQs: Nucleotide Metabolism, Gout, Genome Structure & Central Dogma

Quick Revision Notes for GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC & State Pharmacist Exams

Nucleotides are essential biomolecules that serve as the building blocks of DNA and RNA. They also participate in energy transfer, cellular signaling, and several metabolic reactions. The metabolism of nucleotides includes their biosynthesis, salvage, and degradation.

For competitive examinations, special attention should be given to purine and pyrimidine synthesis, uric acid formation, gout, Lesch-Nyhan syndrome, orotic aciduria, SCID, DNA replication, transcription, translation, and chromatin organization.

1. Nucleotide Structure and Functions

A nucleotide consists of three components:

Nitrogenous base + Pentose sugar + Phosphate

Nitrogenous Bases

Purines:

• Adenine (A)

• Guanine (G)

Pyrimidines:

• Cytosine (C)

• Thymine (T)

• Uracil (U)

Easy Memory Trick

"Pure As Gold" → Purines = Adenine + Guanine

"CUT the PY" → Pyrimidines = Cytosine + Uracil + Thymine

Nucleoside vs Nucleotide

• Nucleoside = Base + Sugar

• Nucleotide = Base + Sugar + Phosphate

Important Functions

Nucleotides are involved in:

• Formation of DNA and RNA

• Energy transfer: ATP and GTP

• Cell signaling: cAMP and cGMP

• Coenzyme structure: NAD⁺, FAD, and CoA contain nucleotide components

• Activation of metabolic intermediates

2. Purine Nucleotide Biosynthesis

Purine nucleotides are synthesized by two major pathways:

1. De novo synthesis

2. Salvage pathway

A. De Novo Purine Synthesis

In de novo synthesis, the purine ring is gradually constructed directly on a ribose phosphate molecule.

The starting material is:

Ribose-5-phosphate → PRPP → IMP → AMP/GMP

The first fully formed purine nucleotide is IMP (inosine monophosphate).

IMP serves as the common precursor for:

• AMP (adenosine monophosphate)

• GMP (guanosine monophosphate)

Easy Memory Trick

"IMP is the IMPortant junction"

IMP → AMP or GMP

Important Point

The purine ring is built on PRPP.

This is a very common competitive-exam question.

Rate-Limiting/Committed Step

PRPP → 5-Phosphoribosylamine

Enzyme:

Glutamine-PRPP amidotransferase

It is inhibited by the end products:

IMP, AMP, and GMP

Thus, purine synthesis is regulated by feedback inhibition.

3. Purine Salvage Pathway

The salvage pathway recycles free purine bases to form nucleotides.

Important enzymes:

HGPRT

Hypoxanthine + PRPP → IMP

Guanine + PRPP → GMP

APRT

Adenine + PRPP → AMP

Clinical Importance: Lesch-Nyhan Syndrome

Complete deficiency of HGPRT causes Lesch-Nyhan syndrome.

Important features include:

• Severe hyperuricemia

• Gout-like symptoms

• Neurological dysfunction

• Dystonia

• Intellectual disability

• Characteristic self-injurious behavior

Easy Memory Trick

"HGPRT → HyperGout + Psychiatric/neurological Problems + Repeated self-injury"

Remember:

HGPRT deficiency = Lesch-Nyhan syndrome

4. Pyrimidine Nucleotide Biosynthesis

The major pyrimidine nucleotides are:

• CMP

• UMP

• TMP

Unlike purines, the pyrimidine ring is synthesized first and then attached to PRPP.

Easy Comparison

Purines: Ring built on PRPP

Pyrimidines: Ring built first, then attached to PRPP

The pathway can be summarized as:

Carbamoyl phosphate → Orotate → OMP → UMP

UMP is the central precursor for other pyrimidine nucleotides.

Important Enzyme

Carbamoyl phosphate synthetase II (CPS-II)

Location:

Cytoplasm

Function:

Produces carbamoyl phosphate for pyrimidine synthesis.

High-Yield Difference

CPS-I

• Mitochondria

• Urea cycle

• Uses ammonia

CPS-II

• Cytoplasm

• Pyrimidine synthesis

• Uses glutamine as nitrogen donor

Easy Memory Trick

"CPS-I = I am for urea"

"CPS-II = I make pyrimidines"

5. Orotic Aciduria

Orotic aciduria is caused by deficiency of:

UMP synthase

This enzyme is required for conversion of orotic acid to UMP.

Features

• Increased urinary orotic acid

• Megaloblastic anemia

• Growth retardation

• No significant hyperammonemia

Treatment

Uridine supplementation

Uridine bypasses the metabolic block and restores pyrimidine nucleotide synthesis.

Exam Trap

Orotic aciduria → UMP synthase deficiency

Do not confuse it with OTC deficiency, which also causes increased orotic acid but is associated with hyperammonemia.

6. Catabolism of Purine Nucleotides

Purine degradation ultimately produces uric acid in humans.

Adenine Pathway

AMP → Adenosine → Inosine → Hypoxanthine → Xanthine → Uric acid

Guanine Pathway

GMP → Guanosine → Guanine → Xanthine → Uric acid

The enzyme xanthine oxidase catalyzes:

Hypoxanthine → Xanthine

and

Xanthine → Uric acid

Easy Memory Trick

"HX → X → UA"

Hypoxanthine → Xanthine → Uric Acid

Why Do Humans Produce Uric Acid?

Humans lack functional uricase, the enzyme that converts uric acid into the more soluble compound allantoin.

Therefore:

Purines → Uric acid = Final major product in humans

7. Hyperuricemia

Hyperuricemia means an abnormally increased concentration of uric acid in the blood.

It may occur due to:

1. Increased Uric Acid Production

Examples:

• Increased purine breakdown

• High purine intake

• Rapid cell destruction

• Tumor lysis syndrome

• Certain genetic disorders

2. Decreased Uric Acid Excretion

This is a common cause of hyperuricemia.

It may occur due to:

• Renal impairment

• Reduced renal urate excretion

• Certain medications

Clinical Consequences

Persistent hyperuricemia may lead to:

• Gout

• Urate crystal deposition

• Kidney stones

• Uric acid nephropathy

8. Gout

Gout is an inflammatory disorder caused by deposition of monosodium urate crystals in joints and tissues.

Classic Features

• Severe joint pain

• Swelling

• Redness

• Inflammation

• Often begins in the first metatarsophalangeal joint

This is called:

Podagra

Pathogenesis

Hyperuricemia → Monosodium urate crystal deposition → Inflammation → Gout

Urate crystals activate inflammatory pathways and attract neutrophils, producing intense inflammation.

9. Important Drugs Used in Gout

A. Allopurinol

Mechanism:

Xanthine oxidase inhibitor

It decreases:

Hypoxanthine → Xanthine → Uric acid

Used mainly for long-term control of hyperuricemia.

Remember:

Allopurinol = Allopurine inhibits Xanthine Oxidase

B. Febuxostat

• Selective xanthine oxidase inhibitor

• Reduces uric acid synthesis

Remember:

Allopurinol + Febuxostat → Decrease uric acid production

C. Probenecid

• Uricosuric drug

• Increases renal excretion of uric acid

Remember:

Probenecid → Promotes urate excretion

D. Colchicine

Colchicine does not primarily lower uric acid levels.

It reduces inflammation by inhibiting microtubule polymerization and decreasing neutrophil migration.

It is particularly useful in acute gout attacks.

Important Exam Comparison

• Drug Main Action

• Allopurinol Inhibits xanthine oxidase

• Febuxostat Inhibits xanthine oxidase

• Probenecid Increases uric acid excretion

• Colchicine Reduces inflammation in acute gout

10. Tumor Lysis Syndrome and Hyperuricemia

Rapid destruction of cancer cells can release large quantities of nucleic acids.

The sequence is:

Rapid cell destruction → Purine release → Purine degradation → Increased uric acid → Hyperuricemia

This may result in:

• Acute kidney injury

• Uric acid nephropathy

• Electrolyte abnormalities

Therefore, tumor lysis syndrome is an important clinical cause of acute hyperuricemia.

11. Genome Structure and Organization

The genome is the complete genetic material of an organism.

In mammals, the genome is mainly located in the nucleus, with a small amount of DNA present in mitochondria.

Mammalian Nuclear Genome

DNA is organized into:

DNA → Nucleosomes → Chromatin → Chromosomes

Nucleosome

The nucleosome is the basic structural unit of chromatin.

It consists of DNA wrapped around a core of eight histone proteins:

• 2 × H2A

• 2 × H2B

• 2 × H3

• 2 × H4

Histone H1 binds linker DNA between nucleosomes.

Easy Memory Trick

"The 4 Histones Make an 8"

H2A + H2B + H3 + H4, each present twice = 8 histone proteins

12. Euchromatin and Heterochromatin

Euchromatin

• Less condensed

• Transcriptionally active

• Accessible to transcription machinery

Heterochromatin

• Highly condensed

• Generally transcriptionally inactive or less active

Easy Memory Trick

EU = Expressed

HETERO = Heavily packed

13. Chromosomes and Telomeres

Chromosomes contain DNA and associated proteins.

Important chromosome regions include:

• Centromere: Important for chromosome segregation

• Telomeres: Protect chromosome ends

Telomeres contain repetitive DNA sequences and prevent:

• Chromosome degradation

• End-to-end chromosome fusion

The enzyme telomerase maintains telomere length.

High-Yield Point

Telomerase activity is increased in:

• Germ cells

• Stem cells

• Many cancer cells

14. Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information:

DNA → RNA → Protein

The three major processes are:

DNA Replication: DNA → DNA

Transcription: DNA → RNA

Translation: RNA → Protein

Easy Memory Trick

"Copy → Write → Translate"

• Replication = Copy DNA

• Transcription = Write RNA

• Translation = Make Protein

15. DNA Replication

DNA replication is the process by which DNA makes an identical copy of itself.

It occurs during the S phase of the cell cycle.

Important Characteristics

DNA replication is:

• Semiconservative

• Bidirectional

• Template-directed

Semiconservative Replication

Each daughter DNA molecule contains:

One parental strand + One newly synthesized strand

Important Enzymes

Helicase
→ Unwinds DNA double helix

Primase
→ Synthesizes RNA primer

DNA polymerase
→ Synthesizes new DNA

DNA ligase
→ Joins DNA fragments

Topoisomerase
→ Relieves DNA supercoiling

Direction of DNA Synthesis

DNA polymerase synthesizes DNA only in the:

5′ → 3′ direction

16. Leading and Lagging Strands

Leading Strand

• Synthesized continuously

• Requires one RNA primer

Lagging Strand

• Synthesized discontinuously

• Forms Okazaki fragments

• Requires multiple RNA primers

DNA ligase joins the Okazaki fragments.

Easy Memory Trick

"Leading = Continuous"

"Lagging = Little pieces"

17. Transcription

Transcription is the synthesis of RNA using DNA as a template.

DNA → RNA

In eukaryotic cells, transcription mainly occurs in the nucleus.

Important RNA Polymerases

RNA Polymerase I
→ Major rRNA synthesis

RNA Polymerase II
→ mRNA synthesis

RNA Polymerase III
→ tRNA and 5S rRNA synthesis

Easy Memory Trick

"I = rRNA, II = mRNA, III = tRNA"

18. Eukaryotic mRNA Processing

The primary RNA transcript undergoes processing before becoming mature mRNA.

Important steps:

1. 5′ Capping

A 7-methylguanosine cap is added to the 5′ end.

Functions:

• Protects mRNA from degradation

• Helps nuclear export

• Helps translation initiation

2. Splicing

Introns are removed and exons are joined.

Splicing is carried out by the spliceosome.

3. Polyadenylation

A poly-A tail is added to the 3′ end.

Functions:

• Increases mRNA stability

• Protects mRNA from degradation

• Helps efficient translation

Easy Memory Trick

"Cap → Cut → Tail"

5′ Cap → Remove introns → 3′ Poly-A tail

19. Translation

Translation is the process by which the information in mRNA is used to synthesize a protein.

mRNA → Protein

It occurs on ribosomes.

Three Main Stages

1. Initiation

2. Elongation

3. Termination

Initiation Codon

AUG

AUG generally codes for:

Methionine

In bacteria, the initiating amino acid is:

N-formylmethionine (fMet)

Stop Codons

UAA

UAG

UGA

These do not code for amino acids.

Easy Memory Trick

"AUG = Start"

"UAA, UAG, UGA = Stop"

20. tRNA and Anticodon

Transfer RNA (tRNA) carries amino acids to the ribosome.

Each tRNA contains an anticodon that pairs with the complementary codon on mRNA.

Remember:

mRNA = Codon

tRNA = Anticodon

The ribosome reads the mRNA codons and links amino acids together through peptide bonds.

The catalytic activity responsible for peptide bond formation is primarily associated with ribosomal RNA (rRNA).

Thus, the ribosome functions as a ribozyme.

21. High-Yield Comparison: Replication vs Transcription vs Translation

• Feature Replication Transcription Translation

• Basic process DNA → DNA DNA → RNA RNA → Protein

• Main product DNA RNA Protein

• Main machinery DNA polymerase RNA polymerase Ribosome

• Template DNA DNA mRNA

• Main location in eukaryotes Nucleus Nucleus Cytoplasm/Rough ER

• Primer required? Yes No No

• Direction 5′→3′ 5′→3′ mRNA read 5′→3′

22. Must-Remember Clinical and Exam Associations

Purine Metabolism

HGPRT deficiency
→ Lesch-Nyhan syndrome

ADA deficiency
→ Severe combined immunodeficiency (SCID)

Xanthine oxidase
→ Hypoxanthine → Xanthine → Uric acid

Uricase absent in humans
→ Uric acid is the final major product of purine degradation

Allopurinol / Febuxostat
→ Xanthine oxidase inhibitors

Probenecid
→ Uricosuric drug

Colchicine
→ Reduces inflammation in acute gout

Pyrimidine Metabolism

UMP synthase deficiency
→ Orotic aciduria

Orotic aciduria
→ Megaloblastic anemia + increased urinary orotic acid

Treatment
→ Uridine

CPS-II
→ Pyrimidine synthesis

CPS-I
→ Urea cycle

Genome and Central Dogma

Nucleosome
→ Basic unit of chromatin

H1
→ Linker DNA

Euchromatin
→ Active transcription

Heterochromatin
→ Condensed and relatively inactive

Telomere
→ Protects chromosome ends

Helicase
→ Unwinds DNA

Primase
→ Makes RNA primer

DNA ligase
→ Joins Okazaki fragments

RNA polymerase II
→ mRNA synthesis

Spliceosome
→ Removes introns

AUG
→ Start codon

UAA, UAG, UGA
→ Stop codons

Final Rapid Revision Formula

Purine

PRPP → IMP → AMP/GMP → Uric Acid

Pyrimidine

Carbamoyl phosphate → Orotate → UMP → Other pyrimidines

Gout

↑ Uric Acid → Urate Crystals → Inflammation

Lesch-Nyhan

↓ HGPRT → ↑ Purine degradation → ↑ Uric Acid

Orotic Aciduria

↓ UMP Synthase → ↑ Orotic Acid → Megaloblastic Anemia

Central Dogma

DNA → RNA → Protein

DNA Replication

Helicase → Primase → DNA Polymerase → Ligase

mRNA Processing

5′ Cap → Splicing → 3′ Poly-A Tail

Translation

AUG → Initiation → Elongation → Stop Codon

One-Line Exam Takeaways

• Purines are built on PRPP; pyrimidine rings are built first.

• IMP is the common precursor of AMP and GMP.

• HGPRT deficiency causes Lesch-Nyhan syndrome.

• ADA deficiency causes SCID.

• UMP synthase deficiency causes hereditary orotic aciduria.

• Xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid.

• Allopurinol and febuxostat inhibit xanthine oxidase.

• Probenecid increases uric acid excretion.

• Colchicine reduces inflammation in acute gout.

• A nucleosome is the basic unit of chromatin.

• Euchromatin is transcriptionally active.

• DNA replication is semiconservative, and DNA synthesis occurs 5′→3′.

• RNA polymerase II synthesizes mRNA.

• Spliceosomes remove introns.

• AUG is the start codon; UAA, UAG, and UGA are stop codons.

• The central dogma is DNA → RNA → Protein.

Below is an MCQ set designed in the style of questions commonly encountered in GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC, and State Pharmacist examinations.

1. A patient with severe gout is prescribed allopurinol. The primary biochemical basis for its therapeutic action is inhibition of:

A. Adenylosuccinate synthetase
B. Xanthine oxidase
C. Hypoxanthine-guanine phosphoribosyltransferase
D. Carbamoyl phosphate synthetase II

Answer: B. Xanthine oxidase

Explanation: Allopurinol is a structural analogue of hypoxanthine and inhibits xanthine oxidase, reducing the conversion of hypoxanthine → xanthine → uric acid. It is therefore useful in chronic hyperuricemia and gout.

2. A child presents with self-mutilation, dystonia, and hyperuricemia. The most likely enzyme deficiency is:

A. Adenosine deaminase
B. Hypoxanthine-guanine phosphoribosyltransferase
C. Xanthine oxidase
D. Thymidylate synthase

Answer: B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

Explanation: Complete HGPRT deficiency causes Lesch-Nyhan syndrome. Defective purine salvage increases PRPP and decreases IMP/GMP formation, stimulating de novo purine synthesis and increasing uric acid production.

3. Which reaction represents a key regulatory step in de novo purine nucleotide synthesis?

A. IMP → AMP
B. Ribose-5-phosphate → PRPP
C. PRPP → 5-Phosphoribosylamine
D. GMP → Uric acid

Answer: C. PRPP → 5-Phosphoribosylamine

Explanation: Glutamine-PRPP amidotransferase catalyzes the committed step of de novo purine synthesis. It is inhibited by purine nucleotides such as IMP, AMP, and GMP.

4. Which statement correctly distinguishes purine and pyrimidine biosynthesis?

A. The purine ring is synthesized first and then attached to ribose
B. The pyrimidine ring is synthesized directly on PRPP
C. The purine ring is assembled stepwise on a ribose phosphate scaffold
D. Both purine and pyrimidine rings are synthesized exclusively in mitochondria

Answer: C. The purine ring is assembled stepwise on a ribose phosphate scaffold

Explanation: In purine synthesis, the ring is built sequentially on PRPP. In pyrimidine synthesis, the pyrimidine ring is synthesized first and subsequently attached to PRPP.

5. The immediate precursor of uric acid in purine catabolism is:

A. Adenine
B. Hypoxanthine
C. Xanthine
D. Guanine

Answer: C. Xanthine

Explanation: Purine degradation proceeds through hypoxanthine → xanthine → uric acid, with xanthine oxidase catalyzing both oxidation steps.

6. A patient receiving 6-mercaptopurine develops severe myelosuppression after starting azathioprine. Which enzyme is most important for the metabolism of these drugs?

A. Xanthine oxidase
B. HGPRT
C. Dihydrofolate reductase
D. Thymidine kinase

Answer: B. HGPRT

Explanation: 6-Mercaptopurine is activated by HGPRT to nucleotide derivatives that inhibit purine synthesis. Its metabolism also involves xanthine oxidase and TPMT, making drug interactions clinically important.

7. Which enzyme catalyzes the conversion of ribonucleotides into deoxyribonucleotides?

A. DNA polymerase
B. Ribonucleotide reductase
C. RNA polymerase
D. Thymidylate synthase

Answer: B. Ribonucleotide reductase

Explanation: Ribonucleotide reductase converts ribonucleoside diphosphates into deoxyribonucleotides, providing precursors required for DNA synthesis.

8. A patient with gout is started on febuxostat. Compared with allopurinol, febuxostat primarily acts by:

A. Inhibiting HGPRT
B. Inhibiting xanthine oxidase
C. Increasing uric acid excretion by inhibiting URAT1
D. Inhibiting purine salvage

Answer: B. Inhibiting xanthine oxidase

Explanation: Febuxostat is a selective xanthine oxidase inhibitor. It reduces uric acid formation and is used for chronic hyperuricemia associated with gout.

9. The major end product of purine catabolism in humans is uric acid because humans lack:

A. Xanthine oxidase
B. Adenosine deaminase
C. Uricase
D. HGPRT

Answer: C. Uricase

Explanation: Humans lack functional uricase, the enzyme that converts uric acid to the more soluble allantoin. Consequently, uric acid accumulates as the final product of purine degradation.

10. Which condition is most directly associated with deposition of monosodium urate crystals in joints?

A. Rheumatoid arthritis
B. Osteoarthritis
C. Gout
D. Osteoporosis

Answer: C. Gout

Explanation: Gout results from deposition of needle-shaped monosodium urate crystals in joints and tissues due to hyperuricemia, triggering intense inflammatory responses.

11. A patient develops acute gout after chemotherapy for leukemia. The most likely cause is:

A. Increased pyrimidine synthesis
B. Tumor lysis syndrome
C. Increased urea cycle activity
D. Reduced DNA degradation

Answer: B. Tumor lysis syndrome

Explanation: Rapid destruction of tumor cells releases nucleic acids, increasing purine degradation and uric acid production. This can cause acute hyperuricemia and urate nephropathy.

12. Which drug promotes uric acid excretion primarily by inhibiting renal tubular urate reabsorption?

A. Probenecid
B. Allopurinol
C. Colchicine
D. Febuxostat

Answer: A. Probenecid

Explanation: Probenecid is a uricosuric agent that reduces renal tubular reabsorption of uric acid, increasing urinary urate excretion.

13. A deficiency of adenosine deaminase is classically associated with:

A. Lesch-Nyhan syndrome
B. Severe combined immunodeficiency
C. Gout
D. Orotic aciduria

Answer: B. Severe combined immunodeficiency (SCID)

Explanation: ADA deficiency causes accumulation of deoxyadenosine and dATP, which inhibits ribonucleotide reductase and impairs lymphocyte development, producing SCID.

14. Orotic aciduria is most directly associated with deficiency of:

A. UMP synthase
B. HGPRT
C. Adenosine deaminase
D. Xanthine oxidase

Answer: A. UMP synthase

Explanation: UMP synthase deficiency causes hereditary orotic aciduria. Orotic acid accumulates, resulting in megaloblastic anemia and growth retardation without hyperammonemia.

15. Which intermediate is common to the biosynthesis of both AMP and GMP?

A. UMP
B. IMP
C. PRPP
D. Carbamoyl phosphate

Answer: B. IMP

Explanation: IMP (inosine monophosphate) is the first fully formed purine nucleotide and serves as the precursor for both AMP and GMP.

16. In mammalian cells, the majority of genomic DNA is located in:

A. Ribosomes
B. Mitochondria
C. Nucleus
D. Lysosomes

Answer: C. Nucleus

Explanation: Most mammalian genomic DNA is located in the nucleus, organized into chromosomes. A small, distinct genome is also present in mitochondria.

17. The fundamental repeating unit of chromatin is:

A. Centrosome
B. Nucleosome
C. Spliceosome
D. Ribosome

Answer: B. Nucleosome

Explanation: A nucleosome consists of approximately 147 base pairs of DNA wrapped around a histone octamer containing H2A, H2B, H3, and H4.

18. Which histone protein is primarily associated with linker DNA between nucleosomes?

A. H1
B. H2A
C. H3
D. H4

Answer: A. H1

Explanation: Histone H1 binds linker DNA and promotes higher-order chromatin organization.

19. Which form of chromatin is generally transcriptionally active?

A. Heterochromatin
B. Euchromatin
C. Centromeric chromatin
D. Constitutive heterochromatin

Answer: B. Euchromatin

Explanation: Euchromatin is relatively less condensed and generally accessible to transcription machinery. Heterochromatin is more condensed and transcriptionally inactive or less active.

20. Telomeres are important because they:

A. Initiate transcription
B. Protect chromosome ends from degradation and fusion
C. Encode ribosomal proteins
D. Initiate translation

Answer: B. Protect chromosome ends from degradation and fusion

Explanation: Telomeres consist of repetitive DNA sequences that protect the ends of chromosomes. Telomerase maintains telomere length in germ cells, stem cells, and many cancer cells.

21. DNA replication is described as semiconservative because:

A. Only one strand is copied
B. Each daughter DNA molecule contains one parental and one newly synthesized strand
C. Both daughter strands are newly synthesized
D. DNA replication occurs only during mitosis

Answer: B. Each daughter DNA molecule contains one parental and one newly synthesized strand

Explanation: In semiconservative replication, each daughter DNA molecule contains one original parental strand and one newly synthesized complementary strand.

22. Which enzyme synthesizes the RNA primer required for DNA replication?

A. DNA polymerase
B. DNA ligase
C. Primase
D. Topoisomerase

Answer: C. Primase

Explanation: Primase, an RNA polymerase, synthesizes short RNA primers that provide the free 3′-OH group required by DNA polymerase.

23. The major difference between leading and lagging strand synthesis is that:

A. Leading strand synthesis is discontinuous
B. Lagging strand synthesis is discontinuous
C. Both are synthesized independently of primers
D. Lagging strand synthesis occurs in the 3′→5′ direction

Answer: B. Lagging strand synthesis is discontinuous

Explanation: DNA polymerase synthesizes DNA only in the 5′→3′ direction. Therefore, the lagging strand is synthesized discontinuously as Okazaki fragments.

24. Which enzyme joins Okazaki fragments?

A. DNA helicase
B. DNA ligase
C. Primase
D. Topoisomerase II

Answer: B. DNA ligase

Explanation: DNA ligase seals nicks in the sugar-phosphate backbone by forming phosphodiester bonds between adjacent DNA fragments.

25. A drug inhibits DNA topoisomerase II. Which process is most directly affected?

A. Removal of supercoiling during DNA replication
B. Formation of peptide bonds
C. RNA splicing
D. Amino acid activation

Answer: A. Removal of supercoiling during DNA replication

Explanation: Topoisomerases control DNA supercoiling. Topoisomerase II introduces transient double-strand breaks to relieve torsional stress and separate intertwined DNA molecules.

26. The enzyme responsible for synthesis of messenger RNA in eukaryotic cells is:

A. RNA polymerase I
B. RNA polymerase II
C. RNA polymerase III
D. DNA polymerase α

Answer: B. RNA polymerase II

Explanation: RNA polymerase II synthesizes precursor mRNA and several small nuclear RNAs. RNA polymerase I mainly synthesizes rRNA, while RNA polymerase III synthesizes tRNA and 5S rRNA.

27. Which modification is added to the 5′ end of eukaryotic mRNA?

A. Poly-A tail
B. 7-Methylguanosine cap
C. Intron
D. Anticodon

Answer: B. 7-Methylguanosine cap

Explanation: The 5′ cap protects mRNA from degradation and assists in nuclear export and translation initiation.

28. The poly-A tail of eukaryotic mRNA is added to the:

A. 5′ end
B. 3′ end
C. Middle of the transcript
D. Promoter region

Answer: B. 3′ end

Explanation: The poly-A tail is added to the 3′ end of most eukaryotic mRNAs. It contributes to mRNA stability and efficient translation.

29. Introns are removed from eukaryotic pre-mRNA by:

A. Ribosomes
B. Spliceosomes
C. DNA polymerases
D. Proteasomes

Answer: B. Spliceosomes

Explanation: Spliceosomes, composed of small nuclear RNAs and proteins, remove introns and join exons during RNA splicing.

30. Which sequence is most directly involved in initiating transcription?

A. Promoter
B. Anticodon
C. Poly-A tail
D. Stop codon

Answer: A. Promoter

Explanation: The promoter is a DNA sequence recognized by transcription machinery and transcription factors, determining where RNA synthesis begins.

31. During translation, the anticodon of tRNA pairs with the:

A. Promoter
B. Coding strand of DNA
C. Codon of mRNA
D. Ribosomal RNA

Answer: C. Codon of mRNA

Explanation: The tRNA anticodon base-pairs with the complementary mRNA codon, ensuring incorporation of the correct amino acid into the growing polypeptide.

32. The universal initiation codon for translation is:

A. UAA
B. AUG
C. UGA
D. UAG

Answer: B. AUG

Explanation: AUG generally serves as the start codon and codes for methionine. In bacteria, the initiating amino acid is formylmethionine (fMet).

33. Which of the following is a termination codon?

A. AUG
B. UGG
C. UGA
D. GUG

Answer: C. UGA

Explanation: UGA, along with UAA and UAG, is a stop codon. These codons do not specify amino acids and signal termination of translation.

34. The catalytic activity responsible for peptide bond formation is primarily associated with:

A. DNA
B. Ribosomal RNA
C. Transfer RNA
D. Messenger RNA

Answer: B. Ribosomal RNA

Explanation: The ribosome functions as a ribozyme. Its peptidyl transferase activity is primarily associated with rRNA in the large ribosomal subunit.

35. A mutation changes a codon for an amino acid into UAA. This type of mutation is called:

A. Missense mutation
B. Silent mutation
C. Nonsense mutation
D. Frameshift mutation

Answer: C. Nonsense mutation

Explanation: A nonsense mutation converts an amino acid codon into a premature stop codon, resulting in early termination of protein synthesis.

36. Which sequence correctly represents the central dogma of molecular biology?

A. RNA → DNA → Protein
B. DNA → RNA → Protein
C. Protein → RNA → DNA
D. DNA → Protein → RNA

Answer: B. DNA → RNA → Protein

Explanation: The classical central dogma describes information flow from DNA to RNA by transcription and from RNA to protein by translation.

37. A mutation occurs in the promoter region of a gene but leaves the coding sequence intact. The most likely consequence is:

A. Altered amino acid sequence
B. Altered transcriptional regulation
C. Altered genetic code
D. Direct alteration of ribosomal structure

Answer: B. Altered transcriptional regulation

Explanation: A promoter controls transcription initiation. A mutation may reduce or increase gene expression without necessarily changing the amino acid sequence of the encoded protein.

38. Which of the following correctly pairs the process with its cellular location in a typical eukaryotic cell?

A. Transcription—cytoplasm; translation—nucleus
B. Transcription—nucleus; translation—cytoplasm
C. Both transcription and translation—nucleus
D. Both transcription and translation—mitochondria

Answer: B. Transcription—nucleus; translation—cytoplasm

Explanation: In eukaryotic cells, transcription occurs mainly in the nucleus, whereas translation occurs on cytoplasmic ribosomes or ribosomes associated with rough ER.

39. A researcher observes that a gene is highly expressed despite its DNA being relatively accessible and less condensed. This DNA is most likely present as:

A. Heterochromatin
B. Euchromatin
C. Centromeric DNA
D. Telomeric DNA

Answer: B. Euchromatin

Explanation: Euchromatin is less condensed and generally more accessible to transcription factors and RNA polymerase, favoring active gene expression.

40. Which combination is correctly matched?

A. Allopurinol—Inhibits HGPRT
B. Probenecid—Xanthine oxidase inhibitor
C. Colchicine—Reduces neutrophil-mediated inflammation in acute gout
D. ADA deficiency—Hyperuricemia as the primary clinical feature

Answer: C. Colchicine—Reduces neutrophil-mediated inflammation in acute gout

Explanation: Colchicine inhibits microtubule polymerization and reduces neutrophil migration and inflammatory responses to urate crystals. Allopurinol inhibits xanthine oxidase, while ADA deficiency is classically associated with SCID.

Quick Revision: High-Yield Exam Traps

• Topic Key Point

• Purine synthesis Purine ring is built on PRPP

• Pyrimidine synthesis Ring is synthesized before attachment to PRPP

• IMP Common precursor of AMP and GMPH

• GPRT deficiency Lesch-Nyhan syndrome

• ADA deficiency SCID

• UMP synthase deficiency Orotic aciduria

• Xanthine oxidase Hypoxanthine → Xanthine → Uric acid

• Allopurinol/Febuxostat Xanthine oxidase inhibitors

• Probenecid Uricosuric drug

• Colchicine Controls inflammation in acute gout

• Human uric acid Final purine degradation product due to lack of functional uricase

• Nucleosome Basic unit of chromatin

• Histone H1 Linker DNA

• Euchromatin Transcriptionally active

• Primase Makes RNA primer

• DNA polymerase DNA synthesis 5′→3′

• DNA ligase Joins Okazaki fragments

• RNA polymerase II mRNA synthesis

• 5′ cap 7-methylguanosine

• Poly-A tail Added at 3′ end

• Spliceosome Removes introns

• AUG Start codon

• UAA, UAG, UGA Stop codons

• Ribosome Peptidyl transferase activity is primarily rRNA-based

Dr. Alok Singh