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FIRST YEAR MBBS BIOCHEMISTRY - COMPLETE ANSWERS

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TISSUE PROTEINS AND BODY FLUIDS

Q1. Why does skin and blood vessels become stiff and less elastic in old age? (2021)

• Collagen is the most abundant structural protein. It is made of three polypeptide chains wound into a triple helix.
• With aging, there is increased cross-linking between collagen fibers. This cross-linking is due to the accumulation of Advanced Glycation End products (AGEs), formed by non-enzymatic glycation (Maillard reaction) between glucose and free amino groups of collagen.
• Additionally, the enzyme lysyl oxidase catalyzes oxidative deamination of lysine and hydroxylysine residues, forming inter- and intra-chain covalent cross-links.
• These cross-links make collagen fibers rigid, insoluble, and resistant to enzymatic degradation.

• Elastin provides extensibility and elastic recoil to tissues.
• With aging, elastin fibers undergo fragmentation, calcification, and degeneration.
• There is reduced synthesis of new elastin and increased degradation by elastases.
• The elastic fiber network in arterial walls and skin gradually deteriorates.

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Q2. Brief account of CSF alterations in biochemical parameters in pathological state (2010)

• Protein: markedly increased (100-500 mg/dL)
• Glucose: markedly decreased (bacteria consume glucose) - CSF/serum glucose ratio <0.4
• Chloride: decreased
• Cells: markedly elevated (neutrophils predominant)
• Appearance: turbid/purulent

• Protein: slightly elevated
• Glucose: normal or slightly decreased
• Cells: elevated (lymphocytes predominant)
• Appearance: clear

• Protein: elevated (100-500 mg/dL)
• Glucose: decreased
• Cells: elevated (lymphocytes + occasional neutrophils)
• Characteristic: spider-web clot formation on standing

• Protein: elevated (due to blood contamination)
• Glucose: normal initially
• Appearance: xanthochromic (yellow) due to bilirubin from hemoglobin breakdown

• Protein: mildly elevated
• Oligoclonal bands of IgG on electrophoresis (diagnostic)
• Glucose: normal

• Protein: elevated
• Glucose: may be decreased

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NUCLEIC ACIDS AND NUCLEOTIDES

Q1. Importance of nucleosome in DNA organization (2021)

• The nucleosome is the fundamental repeating unit of chromatin - the "bead-on-a-string" structure.
• Each nucleosome consists of:
  • Core particle: ~147 base pairs of DNA wound ~1.75 times around an octamer of 8 histone proteins (2 copies each of H2A, H2B, H3, H4)
  • Linker DNA: 20-80 bp connecting adjacent nucleosomes, associated with histone H1
• The diameter of a nucleosome is ~10 nm.

1. Compaction: DNA in a human cell is ~2 meters long but must fit into a nucleus of ~6 micrometers. Nucleosomes provide the first level of ~7-fold compaction (from 2 nm double helix to 10 nm fiber).
2. Higher-order packing: The 10 nm fiber coils into a 30 nm solenoid fiber (~6 nucleosomes per turn) with the help of H1. This provides ~40-fold compaction.
3. Gene regulation: Histone modifications (acetylation, methylation, phosphorylation) alter the accessibility of DNA to transcription factors - loosely packed (euchromatin = active genes) vs. tightly packed (heterochromatin = silent genes).
4. Protection: Nucleosomes protect DNA from mechanical stress and enzymatic degradation.

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Q2. Diverse physiological functions of nucleotides (2021)

1. Building blocks of nucleic acids: ATP/GTP/CTP/UTP for RNA; dATP/dGTP/dCTP/dTTP for DNA synthesis.
2. Energy currency: ATP is the universal energy currency of the cell. GTP is used in protein synthesis and signal transduction. CTP is used in phospholipid synthesis. UTP is used in glycogen synthesis (UDP-glucose).
3. Coenzymes: NAD+ (contains AMP), FAD, CoA - all nucleotide-containing coenzymes essential for oxidative metabolism.
4. Second messengers: cAMP and cGMP are intracellular second messengers in hormone signaling (adenylyl cyclase pathway, NO-cGMP pathway).
5. Activation of metabolic intermediates:
   • UDP-glucose - intermediate in glycogen synthesis and glucuronidation
   • CDP-diacylglycerol - intermediate in phospholipid synthesis
   • SAM (S-adenosylmethionine) - methyl group donor in methylation reactions
6. Signal transduction: GDP/GTP exchange drives G-protein activity.
7. Allosteric regulation: ATP, ADP, AMP act as allosteric regulators of key enzymes in glycolysis, gluconeogenesis, and fatty acid metabolism.
8. Purinergic signaling: Extracellular ATP and adenosine act as signaling molecules via purinergic receptors (P1 and P2 receptors).

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Q3. Note on Z-DNA (2018, 2010)

• Z-DNA forms in GC-rich sequences under high salt concentrations or negative supercoiling.
• Sequences with alternating purine-pyrimidine (especially d(CG)n repeats) favor Z-DNA.

• Appears transiently during transcription (behind the moving RNA polymerase).
• May play a role in gene regulation.
• Z-DNA binding proteins (e.g., ADAR1) are involved in innate immunity.

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Q4. Types of DNA (2018)

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Q5. Note on ribozymes (2014)

1. Self-splicing introns (Group I and II introns): Catalyze their own excision from pre-mRNA without protein enzymes - the first ribozyme discovered.
2. RNase P: A ribonucleoprotein complex that cleaves the 5' leader sequence from pre-tRNA. The RNA component is the catalytic part.
3. Hammerhead ribozyme: Found in plant viroids; catalyzes self-cleavage in RNA replication.
4. Hairpin ribozyme: Similar to hammerhead; found in satellite RNA.
5. Ribosome (Peptidyl transferase): The 23S rRNA (in prokaryotes) or 28S rRNA (in eukaryotes) in the large ribosomal subunit catalyzes the formation of peptide bonds - arguably the most important ribozyme.
6. Spliceosomal snRNAs: Components of the spliceosome that catalyze pre-mRNA splicing.

• Support the "RNA World" hypothesis - life may have originally been based on RNA.
• Potential therapeutic applications (gene silencing).

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Q6. Short note on A and Z DNA (2012)

• A-DNA: Right-handed, 11 bp/turn, forms under dehydrated conditions and in RNA:DNA hybrid duplexes and double-stranded RNA.
• Z-DNA: Left-handed, 12 bp/turn, zig-zag backbone, forms in GC-rich regions under high salt or negative supercoiling. See full note in Q3.

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Q7. Sources of purine ring (2011)

• Glycine - provides C4, C5, N7 (the backbone of the imidazole ring)
• Glutamine - provides N3 and N9 (two nitrogen atoms)
• Aspartate - provides N1
• CO2 - provides C6
• N10-formyl-THF - provides C2
• N5,N10-methenyl-THF - provides C8

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Q8. Short note on tRNA (2011)

• 74-95 nucleotides in length
• Contains many modified/unusual bases (pseudouridine, dihydrouridine, inosine, ribothymidine)
• Comprises ~15% of total cellular RNA

1. Acceptor arm (3'-CCA end):
   • Stem of 7 bp with 3' unpaired overhang: 5'-CCA-3'-OH
   • Amino acid attaches here via aminoacyl-tRNA synthetase
2. D arm (DHU arm):
   • Contains dihydrouridine (D) residues
   • Helps in recognition by aminoacyl-tRNA synthetase
3. Anticodon arm:
   • Contains the anticodon triplet at the loop
   • Reads the mRNA codon by antiparallel complementary base pairing (3'→5' anticodon reads 5'→3' codon)
4. TΨC arm (T arm):
   • Contains the sequence ribothymidine-pseudouridine-cytosine (TΨC)
   • Involved in ribosome binding
5. Variable (extra) loop:
   • Between the anticodon and TΨC arms
   • Size varies between different tRNAs

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Q9. Watson-Crick model of DNA structure and features of Z-DNA (2008)

1. Double helix: Two antiparallel polynucleotide chains wound around a common axis.
2. Antiparallel orientation: One strand runs 5'→3', the other runs 3'→5'.
3. Base pairing (Chargaff's rules):
   • Adenine pairs with Thymine (A=T) - 2 hydrogen bonds
   • Guanine pairs with Cytosine (G≡C) - 3 hydrogen bonds
   • Therefore: A = T and G = C in any DNA
4. Right-handed helix: Coils in a clockwise direction.
5. Dimensions:
   • Diameter: 20 Å (2 nm)
   • Rise per base pair: 3.4 Å
   • Pitch: 34 Å (10 bp per turn)
6. Grooves: Major groove (wide, deep - transcription factor binding) and minor groove (narrow, deep).
7. Backbone: Deoxyribose-phosphate backbone on the outside; bases stacked in the interior.
8. Base stacking: Hydrophobic interactions between adjacent base pairs stabilize the helix.
9. Complementarity: The sequence of one strand dictates the sequence of the other (basis for replication and transcription).

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Q10. Short note on nucleosome (2008, 2007)

• Nucleosome = fundamental unit of chromatin
• Core: 147 bp DNA + histone octamer (2×H2A, 2×H2B, 2×H3, 2×H4)
• Linker DNA (~50 bp) + H1 histone connect adjacent nucleosomes
• "Bead on a string" = 10 nm fiber
• 30 nm solenoid (H1-dependent)
• Functions: DNA compaction, gene regulation, protection

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Q11. Arms of tRNA and their functions (2007)

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Q12 & Q13. Synthetic purine analogues and their clinical significance (2007, 2005)

• Analog of hypoxanthine (6-OH replaced by 6-SH)
• After conversion to its ribonucleotide, it inhibits PRPP amidotransferase (the first committed step of de novo purine synthesis) and also inhibits conversion of IMP to AMP and GMP
• Clinical use: Acute lymphoblastic leukemia (ALL), Crohn's disease (as azathioprine prodrug)

• Analog of guanine
• Inhibits purine de novo synthesis and interconversion
• Clinical use: Leukemias

• Prodrug of 6-MP
• Clinical use: Immunosuppression after organ transplantation, rheumatoid arthritis, inflammatory bowel disease

• Analog of hypoxanthine; inhibits xanthine oxidase
• Reduces uric acid production
• Clinical use: Gout, hyperuricemia, Lesch-Nyhan syndrome

• Guanosine analogs; terminate viral DNA chain after phosphorylation
• Clinical use: Herpes simplex, varicella-zoster, CMV infections

• Purine nucleoside reverse transcriptase inhibitors (NRTIs)
• Clinical use: HIV/AIDS treatment

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Q14. Structure of DNA (2005)

• DNA = polymer of deoxyribonucleotides joined by 3'→5' phosphodiester bonds
• Double helix with complementary antiparallel strands
• Base pairing: A=T (2H bonds), G≡C (3H bonds)
• Right-handed B-form under physiological conditions
• Stabilized by: hydrogen bonds (between base pairs) + base stacking (hydrophobic, van der Waals forces)
• Diameter 20Å, pitch 34Å, 10 bp/turn

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GENETICS

DNA REPLICATION, RECOMBINATION AND REPAIR

Q1. Base excision repair (2023)

• Deamination of cytosine → uracil (most common)
• Deamination of adenine → hypoxanthine
• Oxidative damage (8-oxoguanine)
• Alkylated bases
• ~10,000 purine bases are lost spontaneously per cell per day (creating AP sites)

1. Base recognition and removal:
   • A specific DNA glycosylase recognizes and hydrolyzes the N-glycosidic bond of the damaged base.
   • This creates an AP site (apurinic/apyrimidinic site) - a deoxyribose with no base.
   • Example: Uracil-DNA glycosylase removes uracil; 8-oxoguanine glycosylase removes oxidized guanine.
2. AP site cleavage:
   • AP endonuclease recognizes the AP site and cuts the DNA backbone on the 5' side of the AP site.
3. Sugar removal:
   • Deoxyribose phosphate lyase (associated with DNA pol β) removes the base-free deoxyribose phosphate residue.
4. Gap filling:
   • DNA polymerase β (pol β) fills in the 1-nucleotide gap using the undamaged strand as template.
5. Ligation:
   • DNA ligase seals the nick, completing the repair.

• Defects in BER enzymes are associated with increased cancer risk (colorectal cancer, others).
• BER is the pathway responsible for correcting the G→U mutations that would otherwise cause G:C to A:T transitions.

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Q2. Salient features of genetic code (2021)

1. Triplet code: Each codon consists of 3 consecutive nucleotides (codons). With 4 bases, 4³ = 64 codons are possible.
2. Degenerate (redundant): Most amino acids are coded by more than one codon. Only methionine (AUG) and tryptophan (UGG) have single codons. This degeneracy is mostly in the 3rd (wobble) position.
3. Non-overlapping: Each nucleotide belongs to only one codon; codons do not share nucleotides.
4. Comma-less (continuous): The code is read continuously without punctuation between codons (except start and stop).
5. Universal: The same codons specify the same amino acids in virtually all organisms (from bacteria to humans). Notable exceptions: mitochondria and some protozoa.
6. Non-ambiguous (unambiguous): Each codon specifies only one amino acid (though one amino acid may have multiple codons).
7. Start codon: AUG codes for methionine (formyl-methionine in prokaryotes) and serves as the initiation codon.
8. Stop (termination) codons: UAA ("ochre"), UAG ("amber"), UGA ("opal/umber") - three codons that do not code for any amino acid but signal chain termination.
9. Wobble hypothesis (Crick): The 3rd base of the anticodon can pair with multiple bases in the 3rd position of the codon - explains degeneracy.
10. Sense codons: 61 out of 64 code for amino acids. 3 are stop codons.

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Q3. Why do silent mutations have no effect on the phenotype? (2021)

• CUU, CUC, CUA, CUG all code for Leucine
• A mutation changing CUU → CUC still produces Leucine in the protein - no change in amino acid sequence.

1. The amino acid sequence of the protein remains identical.
2. The protein structure (primary, secondary, tertiary, quaternary) is unchanged.
3. The protein function is preserved.
4. Therefore, the phenotype (observable characteristics) is unaffected.

• They can affect mRNA splicing if they occur near splice sites.
• They can affect translation efficiency (codon usage bias).
• They can affect mRNA stability. These are exceptions rather than the rule.

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Q4. Missense mutations (2019, 2004)

• One base is substituted for another in the coding sequence.
• The altered codon specifies a different amino acid.
• The protein is produced but with one amino acid substituted at that position.

1. Conservative missense: The substituted amino acid has similar properties (e.g., Valine → Isoleucine, both nonpolar). Usually minimal effect on protein function.
2. Non-conservative missense: The substituted amino acid has very different properties. Usually severely affects protein function.

• Glu (glutamic acid) → Val (valine) at position 6 of the β-globin chain
• DNA: GAG → GTG (codon change)
• mRNA: GAG → GUG
• Result: HbS instead of HbA - causes sickling of red blood cells under low O2

• Many enzyme deficiency diseases are caused by missense mutations (e.g., Phenylketonuria - Phe hydroxylase mutations)

• Basis of many single-gene disorders
• Can be identified by DNA sequencing
• Some missense mutations cause partial loss of function (hypomorphic alleles)

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Q5. Frame shift mutation (2018, 2009)

• The ribosome reads mRNA in consecutive non-overlapping triplets (codons).
• Insertion or deletion of 1, 2, 4, or 5 bases (not a multiple of 3) shifts all downstream codons.
• Every codon after the mutation encodes a different amino acid → completely altered protein sequence.
• Often a premature stop codon is encountered → truncated, non-functional protein.

• Produces a drastically altered protein from the point of mutation onward
• Usually results in loss of function
• Insertion or deletion of 3 nucleotides (or multiples of 3) = in-frame mutation (adds/removes one amino acid; reading frame preserved)

• Duchenne Muscular Dystrophy (DMD): Frameshift deletions in the dystrophin gene → premature stop → absent dystrophin protein
• Tay-Sachs disease: Some forms due to frameshift insertion in HEXA gene
• Cystic fibrosis: ΔF508 (deletion of 3 bp = in-frame, not frameshift) - note this is actually an in-frame deletion

• Acridine dyes (proflavine, acridine orange) - classic frameshift mutagens
• Slippage during DNA replication in repetitive sequences

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Q6. Point mutations (2017, 2010)

1. Transitions: Purine ↔ Purine, or Pyrimidine ↔ Pyrimidine substitution (A↔G, C↔T). More common.
2. Transversions: Purine ↔ Pyrimidine substitution (A↔C, A↔T, G↔C, G↔T). Less common.

1. Silent (synonymous) mutation: Codon change but same amino acid due to code degeneracy. No phenotypic effect.
2. Missense mutation: Different amino acid incorporated. May affect function. (e.g., sickle cell disease)
3. Nonsense mutation: Amino acid codon → stop codon. Premature chain termination → truncated, non-functional protein. (e.g., β-thalassemia)

• Spontaneous (deamination, tautomeric shifts, oxidative damage)
• Chemical mutagens (nitrous acid, alkylating agents, base analogs like 5-bromouracil)
• Radiation (UV, X-rays)

• Genetic disease, cancer predisposition, evolutionary change

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Q7. Cell cycle (2016)

• Cell grows in size and synthesizes proteins and organelles
• Preparation for DNA synthesis
• Restriction point (R point): After this, cell is committed to division
• Duration: variable (hours to days)

• DNA replication occurs - genome duplicated from 2N to 4N DNA content
• Histone synthesis occurs
• Duration: ~6-8 hours

• Continued cell growth, protein synthesis
• Preparation for mitosis, centrosome duplication
• DNA repair occurs here too
• Duration: ~3-4 hours

• Prophase, Prometaphase, Metaphase, Anaphase, Telophase (PMAT)
• Cytokinesis
• Duration: ~1 hour

• Cyclins and CDKs (Cyclin-Dependent Kinases): Cyclin-CDK complexes phosphorylate target proteins to drive cell cycle progression.
  • G1: Cyclin D-CDK4/6; Cyclin E-CDK2
  • S phase: Cyclin A-CDK2
  • G2/M: Cyclin A-CDK1; Cyclin B-CDK1 (MPF = Maturation Promoting Factor)
• Checkpoints: G1/S checkpoint (monitors DNA integrity), G2/M checkpoint, Spindle assembly checkpoint
• Tumor suppressors (p53, Rb) apply brakes; mutations lead to cancer

• Retinoblastoma protein (Rb) is hyperphosphorylated by Cyclin D-CDK4/6 → releases E2F transcription factor → S-phase genes activated

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Q8. DNA polymerases (2015)

• Require a template (copy-directed)
• Require a primer (free 3'-OH)
• Synthesize only in 5'→3' direction
• Cannot initiate a new chain de novo

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Q9. Nucleosome assembly (2015)

1. Histone synthesis: New histones (H3, H4, H2A, H2B) are synthesized during S phase, coupled with DNA replication.
2. Histone chaperones: Newly synthesized histones are bound by chaperone proteins:
   • CAF-1 (Chromatin Assembly Factor 1): Main chaperone for H3-H4 deposition, coupled to the replication fork
   • ASF1 (Anti-Silencing Function 1): Assists CAF-1
   • NAP1: Chaperone for H2A-H2B
3. Deposition sequence:
   • Step 1: Two H3-H4 dimers are deposited to form a (H3-H4)₂ tetramer on DNA
   • Step 2: Two H2A-H2B dimers are added (one on each side of the tetramer) to form the complete histone octamer
   • 147 bp of DNA wraps 1.75 turns around this octamer = nucleosome core particle
4. H1 addition: Histone H1 binds to the linker DNA (~50 bp) between nucleosomes and helps compact the 10 nm fiber into the 30 nm fiber.
5. Maturation: Histone tails undergo post-translational modifications (acetylation, methylation, phosphorylation) that influence chromatin function.

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Note: All answers are consistent with standard 1st year MBBS biochemistry curriculum and referenced to Satyanarayana 6th edition and Vasudevan 10th edition as indicated in the question paper. Good luck on your exam!