Reading File
This is a biochemistry exam paper. Let me answer all the MCQs and short answer questions directly from my medical knowledge - these are well-established biochemistry facts.
Here are the answers to all questions visible on the paper:
Section 6 - MCQs (Choose the Correct Option)
i. Cell mediated immunity is mediated by:
Answer: a) T lymphocytes
- T lymphocytes (specifically cytotoxic T cells and helper T cells) mediate cell-mediated immunity. B lymphocytes handle humoral immunity.
ii. Which is a free radical?
Answer: c) NO (Nitric oxide)
- NO (nitric oxide) is a free radical - it has an unpaired electron. CO is a gas but not a free radical in the classical sense. CO₂ and N₂O are stable molecules.
iii. Which is an example of a Phase I reaction in xenobiotic metabolism?
Answer: d) Oxidation
- Phase I reactions include oxidation, reduction, and hydrolysis. Phase II reactions include glucuronidation, sulphation, and acetylation (options a, b, c).
iv. Which hormone has nuclear receptors?
Answer: c) TSH - Wait, actually the best answer here is a) Testosterone
- Steroid hormones (testosterone) bind to intracellular/nuclear receptors. TSH, ACTH, and Glucagon are peptide/protein hormones that act on cell surface receptors.
v. In essential pentosuria, considerable quantities of which is found in urine:
Answer: d) L-xylulose
- Essential (benign) pentosuria is caused by a deficiency of L-xylulose reductase, leading to accumulation and excretion of L-xylulose in urine.
vi. Which disease occurs due to defective DNA repair?
Answer: d) Xeroderma pigmentosum
- XP is caused by defective nucleotide excision repair (NER) of UV-induced DNA damage. Cystic fibrosis is a chloride channel disorder; Kuru is a prion disease; Dementia is not primarily a DNA repair disorder.
vii. Detection of a DNA sequence can be done by which blotting technique?
Answer: d) Southern blotting
- Southern blot detects DNA sequences. Northern blot detects RNA. Western blot detects proteins. Eastern blot detects post-translational modifications.
viii. Which is a tumour marker?
Answer: c) Alpha fetoprotein (AFP)
- AFP is a classic tumour marker for hepatocellular carcinoma and germ cell tumours. Fructosamine indicates glycemic control; ALT is a liver enzyme; reverse T3 is a thyroid metabolite.
ix. Which is a non-essential nutrient?
Answer: a) Sucrose
- Sucrose is a disaccharide (dietary carbohydrate) that can be synthesized from glucose - it is not an essential nutrient. Linoleic acid is an essential fatty acid; tryptophan is an essential amino acid; ascorbic acid (Vitamin C) is an essential vitamin (humans cannot synthesize it).
x. Which has the least iron content?
Answer: a) Milk
- Milk is notoriously poor in iron - this is why iron deficiency anemia is common in infants fed exclusively cow's milk. Meat (especially red meat) is the richest dietary iron source. Leafy vegetables and cereals contain moderate amounts.
xi. Which is an intermediate of HMP shunt (Hexose Monophosphate pathway)?
Answer: a) Erythrulose-4-P - Actually the correct answer is b) Ribulose-5-P ... but that's not listed. Looking at the options:
- a) erythrulose-4-P, b) galactose-6-P, c) glucuronic acid, d) oxaloacetate
Answer: a) Erythrulose-4-P (should be erythrose-4-phosphate - likely a typo on the paper). Erythrose-4-phosphate is a key intermediate of the HMP shunt/pentose phosphate pathway. Oxaloacetate is a TCA cycle intermediate; glucuronic acid is from uronic acid pathway; galactose-6-P is not a standard HMP intermediate.
xii. C-4 of the purine ring is donated by:
Answer: c) Glutamine
The purine ring atom donations (classic mnemonics):
| Atom | Donor |
|---|---|
| C2, C8 | Formate (via THF) |
| N1 | Aspartate |
| N3, N9 | Glutamine |
| C4, C5, N7 | Glycine (entire glycine contributes C4, C5, N7) |
| C6 | CO₂ |
Wait - C4, C5, and N7 come from Glycine. So C-4 is donated by a) Glycine.
Answer: a) Glycine
Section 5 - Short Notes (any two, 2×4=8 marks)
a. Okazaki fragments
- Short DNA fragments (100-200 nucleotides in eukaryotes, ~1000-2000 in prokaryotes) synthesized discontinuously on the lagging strand during DNA replication. DNA is synthesized only 5'→3', so the lagging strand is made in short pieces, each initiated by an RNA primer. After synthesis, primers are removed by RNase H / DNA Pol I, gaps are filled, and fragments are joined by DNA ligase.
b. Galactosemia
- An autosomal recessive disorder of galactose metabolism. Classic type: deficiency of galactose-1-phosphate uridylyltransferase (GALT). Galactose-1-phosphate accumulates, causing damage to liver, brain, kidneys, and eyes (cataracts). Presents in newborns with jaundice, vomiting, hepatomegaly after milk feeding. Galactose also accumulates in the lens where it is converted to galactitol by aldose reductase, causing cataracts. Treatment: galactose-free diet.
c. Base excision repair (BER)
- Repairs small, non-helix-distorting base lesions (oxidized, alkylated, or deaminated bases). Steps:
- DNA glycosylase recognizes and removes the damaged base, creating an AP (apurinic/apyrimidinic) site
- AP endonuclease cleaves the phosphodiester backbone at the AP site
- DNA polymerase β fills in the correct nucleotide
- DNA ligase seals the nick
- Repairs damage caused by reactive oxygen species, spontaneous deamination (e.g., cytosine → uracil), and alkylating agents.
Summary of MCQ Answers:
| Q | Answer |
|---|---|
| i | a) T lymphocytes |
| ii | c) NO |
| iii | d) Oxidation |
| iv | a) Testosterone |
| v | d) L-xylulose |
| vi | d) Xeroderma pigmentosum |
| vii | d) Southern blotting |
| viii | c) Alpha fetoprotein |
| ix | a) Sucrose |
| x | a) Milk |
| xi | a) Erythrose-4-P (erythrulose-4-P as written) |
| xii | a) Glycine |
This is a shared conversation. Sign in to Orris to start your own chat.