Biochemistry
MBBS Biochemistry question bank: biomolecules, enzymes, metabolism, molecular biology, vitamins, nutrition and clinical biochemistry.
12chapters144questions75High-Yield
Definition
Purine metabolism covers the synthesis and degradation of purine nucleotides (adenine, guanine); its end product is uric acid, whose excess causes gout.
Synthesis
- De novo (from ribose-5-P, amino acids)
- Salvage pathway (HGPRT)
- Rate-limiting: PRPP amidotransferase
Degradation
- Purines → hypoxanthine / xanthine → uric acid
- By xanthine oxidase
- Uric acid excreted in urine
Gout
- Hyperuricaemia → urate crystals in joints
- Acute arthritis (classically big toe)
- Treated by allopurinol (xanthine oxidase inhibitor)
Purines break down to uric acid; its excess deposits as gout. Feature Detail End product Uric acid Enzyme Xanthine oxidase Gout drug Allopurinol Applied
- Gout, uric-acid stones
- Lesch-Nyhan (HGPRT deficiency)
🔑KEY POINTS TO REMEMBER- Purine end product = uric acid (via xanthine oxidase).
- Salvage by HGPRT; de novo rate enzyme PRPP amidotransferase.
- Excess → gout; treat with allopurinol.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Pyrimidine metabolism is the synthesis and degradation of pyrimidine nucleotides (cytosine, thymine, uracil).
Synthesis
- De novo — ring built first, then ribose added
- Starts with carbamoyl phosphate (cytoplasm)
- Regulated by aspartate transcarbamoylase
Degradation
- Yields soluble products (β-alanine, CO₂, ammonia)
- No insoluble end product (unlike purines)
The pyrimidine ring is built first, then attached to ribose. Feature Purine Pyrimidine Synthesis Ring on ribose Ring first End product Uric acid Soluble Applied
- Orotic aciduria
- 5-FU (blocks thymidylate synthase)
🔑KEY POINTS TO REMEMBER- Pyrimidine ring built first, then ribose added.
- Degradation gives soluble products (no uric acid).
- 5-FU blocks thymidylate synthase.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
DNA replication is the semiconservative copying of DNA before cell division, producing two identical daughter molecules.
Features
- Semiconservative (each strand a template)
- Bidirectional from an origin
- Needs DNA polymerase, primer, dNTPs
Key Enzymes
- Helicase — unwinds the helix
- Primase — lays an RNA primer
- DNA polymerase — synthesis (5′→3′)
- Ligase — joins fragments
Strands
- Leading — continuous
- Lagging — Okazaki fragments
The helix is unwound and each strand copied to give two identical molecules. Enzyme Function Helicase Unwinds Polymerase Synthesis Ligase Joins Applied
- Semiconservative (Meselson-Stahl)
- Anticancer drugs target replication
🔑KEY POINTS TO REMEMBER- Replication: semiconservative, bidirectional.
- Enzymes: helicase, primase, polymerase, ligase.
- Leading (continuous) vs lagging (Okazaki).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Transcription is the synthesis of RNA from a DNA template by RNA polymerase.
Steps
- Initiation — RNA polymerase binds the promoter
- Elongation — RNA synthesised (5′→3′)
- Termination
- Uses one DNA strand as template
Post-transcriptional (eukaryotes)
- 5′ capping
- 3′ poly-A tail
- Splicing (remove introns)
RNA polymerase copies one DNA strand into RNA, which is then processed. RNA Function mRNA Codes protein tRNA Carries amino acid rRNA Forms ribosome Applied
- Rifampicin (bacterial RNA polymerase)
- α-amanitin (RNA pol II)
🔑KEY POINTS TO REMEMBER- Transcription: DNA → RNA (RNA polymerase).
- Steps: initiation, elongation, termination.
- Eukaryotic processing: cap, poly-A tail, splicing.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Translation is the synthesis of protein from mRNA on ribosomes, decoding the genetic code (codons).
Steps
- Initiation — ribosome + mRNA + initiator tRNA
- Elongation — peptide bonds form (codon by codon)
- Termination — at a stop codon
Genetic Code Features
- Triplet codons (64 total)
- Degenerate (multiple codons per amino acid)
- Universal, non-overlapping
- Start = AUG (Met); stop = UAA/UAG/UGA
Ribosomes read codons and assemble amino acids into a protein. Codon Role AUG Start (Met) UAA/UAG/UGA Stop Applied
- Antibiotics target translation
- Point mutations → disease
🔑KEY POINTS TO REMEMBER- Translation: mRNA → protein on ribosomes.
- Steps: initiation, elongation, termination.
- Code: triplet, degenerate, universal (AUG start).
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
The Watson–Crick model describes DNA as a double helix of two antiparallel strands held by base pairing.
Features
- Two antiparallel strands (5′→3′ and 3′→5′)
- Right-handed double helix
- Sugar-phosphate backbone outside, bases inside
- Base pairs: A-T (2 H-bonds), G-C (3 H-bonds)
Two complementary strands twist into a double helix held by base pairs. Base pair H-bonds A-T 2 G-C 3 Applied
- Basis of replication
- Chargaff’s rule (A=T, G=C)
🔑KEY POINTS TO REMEMBER- DNA = antiparallel double helix.
- A-T (2 bonds), G-C (3 bonds).
- Basis of replication; Chargaff’s rule.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
The genetic code is the set of rules by which nucleotide triplets (codons) specify amino acids during translation.
Features
- Triplet (3 bases = 1 codon)
- 64 codons (61 code for amino acids)
- Degenerate (redundant)
- Non-overlapping, unambiguous, universal
- AUG = start; UAA/UAG/UGA = stop
Codons are read three bases at a time to specify each amino acid. Feature Detail Codon 3 bases Start AUG Stop UAA/UAG/UGA Applied
- Mutations alter codons
- Frameshift mutations
🔑KEY POINTS TO REMEMBER- Genetic code: triplet, degenerate, universal, non-overlapping.
- 64 codons; AUG start, 3 stop codons.
- Mutations change codons.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Mutations are permanent changes in the DNA sequence that may alter the protein product.
Types
- Point mutations — single base (silent, missense, nonsense)
- Frameshift — insertion / deletion
- Chromosomal — large-scale changes
A DNA change alters codons and hence the protein produced. Mutation Effect Silent No change Missense New amino acid Nonsense Stop codon Applied
- Sickle cell (missense)
- Cancer, genetic diseases
🔑KEY POINTS TO REMEMBER- Point (silent/missense/nonsense) & frameshift mutations.
- Frameshift = insertion/deletion.
- Sickle cell = missense.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Polymerase chain reaction (PCR) is a technique to amplify a specific DNA segment exponentially in vitro.
Steps (per cycle)
- Denaturation (~95°C) — strands separate
- Annealing (~55°C) — primers bind
- Extension (~72°C) — Taq polymerase extends
- Repeated → exponential amplification
Cycles of heating and cooling copy the target DNA exponentially. Step Temperature Denaturation ~95°C Annealing ~55°C Extension ~72°C Applied
- Diagnosis (infections, genetics)
- Forensics, COVID testing
🔑KEY POINTS TO REMEMBER- PCR amplifies DNA in vitro.
- Cycle: denaturation, annealing, extension (Taq).
- Used in diagnosis, forensics.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Lesch–Nyhan syndrome is an X-linked disorder due to deficiency of HGPRT, impairing purine salvage.
Mechanism
- HGPRT deficiency → no purine salvage
- ↑ De novo purine synthesis
- ↑ Uric acid (hyperuricaemia)
Features
- Hyperuricaemia, gout
- Self-mutilation (biting)
- Intellectual disability, spasticity
Losing purine salvage floods the body with uric acid and causes the syndrome. Feature Detail Enzyme HGPRT Inheritance X-linked Applied
- Allopurinol for uric acid
- Classic self-mutilation
🔑KEY POINTS TO REMEMBER- Lesch-Nyhan = HGPRT deficiency (X-linked).
- No purine salvage → ↑ uric acid.
- Self-mutilation, gout, disability.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
DNA repair mechanisms correct errors and damage in DNA to maintain genomic integrity.
Mechanisms
- Mismatch repair — replication errors
- Base excision repair — a damaged base
- Nucleotide excision repair — UV damage (thymine dimers)
- Direct repair
Damaged DNA is recognised, cut out, resynthesised and sealed. Repair Corrects Mismatch Replication error NER UV damage Applied
- Xeroderma pigmentosum (NER defect)
- HNPCC (mismatch repair)
🔑KEY POINTS TO REMEMBER- Repair: mismatch, base excision, nucleotide excision, direct.
- NER fixes UV thymine dimers.
- Defects → xeroderma pigmentosum, HNPCC.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.Definition
Reverse transcription is the synthesis of DNA from an RNA template by the enzyme reverse transcriptase.
Features
- RNA → DNA (opposite of transcription)
- Enzyme: reverse transcriptase
- Found in retroviruses (HIV)
Reverse transcriptase copies RNA into DNA — the reverse of transcription. Feature Detail Direction RNA → DNA Enzyme Reverse transcriptase Applied
- HIV (antiretroviral targets)
- cDNA in molecular biology
🔑KEY POINTS TO REMEMBER- Reverse transcription: RNA → DNA.
- Enzyme = reverse transcriptase (retroviruses).
- Target of HIV drugs.
📚SOURCES: Textbook of Biochemistry (U. Satyanarayana); Harper’s Illustrated Biochemistry; Lehninger Principles of Biochemistry.