DNB CET Biochemistry Syllabus

What vitamin/cofactor is required by all transaminases (e.g., ALT, AST)?

Pyridoxal phosphate (vitamin B6).

What enzyme defect causes phenylketonuria (PKU), and what is the treatment?

Phenylalanine hydroxylase deficiency (or BH4 cofactor deficiency). Treatment: low-phenylalanine diet; avoid aspartame. Features: musty odor, intellectual disability, fair skin/hair.

What enzyme is deficient in classic alkaptonuria, and the clinical features?

Homogentisate oxidase deficiency. Features: dark urine on standing, ochronosis (dark connective tissue), and arthritis.

What is the defect in maple syrup urine disease and which amino acids accumulate?

Branched-chain alpha-ketoacid dehydrogenase deficiency. Accumulation of branched-chain amino acids: leucine, isoleucine, valine. Burnt-sugar/maple urine odor; requires thiamine.

What enzyme is deficient in classic homocystinuria, and the key features?

Cystathionine beta-synthase deficiency. Features: marfanoid habitus, downward lens dislocation, thrombosis, intellectual disability; treat with B6, B12, folate.

Which lysosomal storage disease is X-linked and shows angiokeratomas plus a deficiency of alpha-galactosidase A?

Fabry disease (X-linked). Accumulation of ceramide trihexoside; features include burning pain, angiokeratomas, renal/cardiac disease.

What enzyme deficiency causes Tay-Sachs disease, and the hallmark sign?

Hexosaminidase A deficiency causing GM2 ganglioside accumulation. Cherry-red macula, no hepatosplenomegaly, progressive neurodegeneration.

Which direction is DNA synthesized, and which enzyme proofreads in prokaryotes?

DNA is synthesized 5' to 3'. DNA polymerase III is the main replicative enzyme; its 3'-to-5' exonuclease activity proofreads.

What is the function of DNA primase, helicase, and ligase in replication?

Primase synthesizes RNA primers; helicase unwinds the double helix; ligase seals Okazaki fragment nicks (forms phosphodiester bonds).

Which DNA repair defect causes xeroderma pigmentosum, and what is impaired?

Defective nucleotide excision repair (cannot repair UV-induced pyrimidine dimers). Causes extreme photosensitivity and skin cancers.

Which repair system is defective in hereditary nonpolyposis colorectal cancer (Lynch syndrome)?

DNA mismatch repair (e.g., MSH2, MLH1), leading to microsatellite instability.

Differentiate transition, transversion, and frameshift mutations.

Transition: purine to purine or pyrimidine to pyrimidine. Transversion: purine to pyrimidine (or vice versa). Frameshift: insertion/deletion not a multiple of 3, shifting the reading frame.

What is a silent, missense, and nonsense mutation?

Silent: codon change with same amino acid. Missense: codon change to a different amino acid. Nonsense: codon change to a stop codon (premature termination).

Which RNA polymerase synthesizes mRNA in eukaryotes, and what inhibits it?

RNA polymerase II synthesizes mRNA; inhibited by alpha-amanitin (death cap mushroom).

What are the three main eukaryotic mRNA post-transcriptional modifications?

5' 7-methylguanosine cap, 3' poly-A tail, and splicing out of introns by the spliceosome (snRNPs).

During translation, what energy molecule is used and at which sites do tRNAs bind the ribosome?

GTP is used. Sites: A (aminoacyl/incoming), P (peptidyl/growing chain), E (exit). Peptidyl transferase (rRNA, a ribozyme) forms peptide bonds.

Explain the lac operon: what happens with lactose present and glucose absent?

Lactose (allolactose) inactivates the repressor, and low glucose raises cAMP which binds CAP to enhance transcription, maximizing lac gene expression.

What are the essential components and steps of PCR?

Template DNA, two primers, dNTPs, Taq (thermostable) polymerase, Mg2+. Steps: denaturation (~95C), annealing (~55C), extension (~72C); product doubles each cycle.

Differentiate Southern, Northern, and Western blots by target molecule.

Southern blot = DNA; Northern blot = RNA; Western blot = protein. (Southwestern = DNA-binding proteins.)

What is the role of restriction endonucleases and reverse transcriptase in recombinant DNA technology?

Restriction endonucleases cut DNA at specific palindromic sequences (leaving sticky/blunt ends); reverse transcriptase makes cDNA from mRNA.

What does ELISA detect and what is the typical signal mechanism?

ELISA detects antigens or antibodies using an enzyme-linked antibody that produces a color/chemiluminescent signal proportional to the analyte.

State the typical pattern of an autosomal dominant inheritance.

Affects both sexes, no skipping of generations (vertical transmission), each child of an affected heterozygote has 50% risk; often shows variable expressivity/incomplete penetrance.

Describe the inheritance pattern of mitochondrial (maternal) disorders.

Transmitted only from the mother to all offspring; affected fathers do not transmit. Heteroplasmy explains variable severity; examples: LHON, MELAS, MERRF.

Contrast autosomal recessive and X-linked recessive inheritance patterns.

AR: both sexes equally affected, parents are carriers, may skip generations. XLR: mainly affects males, no male-to-male transmission, carrier mothers pass to 50% of sons.

Write the Michaelis-Menten equation and define Km and Vmax.

v = (Vmax x [S]) / (Km + [S]). Vmax = maximal rate at saturating substrate; Km = substrate concentration at half Vmax (inversely reflects enzyme affinity).

How do competitive vs noncompetitive inhibitors affect Km and Vmax?

Competitive: increases Km, Vmax unchanged (reversed by more substrate). Noncompetitive: Vmax decreased, Km unchanged.

How does an uncompetitive inhibitor affect Km and Vmax?

Uncompetitive inhibitor decreases both Km and Vmax (binds only the enzyme-substrate complex); the Vmax/Km ratio stays roughly constant.

Match water-soluble vitamins B1, B2, B3, and B5 to their coenzyme forms.

B1 (thiamine) -> TPP; B2 (riboflavin) -> FAD/FMN; B3 (niacin) -> NAD+/NADP+; B5 (pantothenic acid) -> coenzyme A.

Match vitamins B6, B7, B9, and B12 to their coenzyme roles.

B6 (pyridoxal phosphate) -> transamination/decarboxylation; B7 (biotin) -> carboxylation; B9 (folate, THF) -> one-carbon transfer; B12 (cobalamin) -> methylmalonyl-CoA mutase and methionine synthase.

What deficiency causes Wernicke-Korsakoff syndrome, and the classic triad?

Thiamine (B1) deficiency. Wernicke triad: confusion, ophthalmoplegia, ataxia; Korsakoff adds confabulation and memory loss.

What are the 4 D's of pellagra and the causative deficiency?

Niacin (B3) deficiency: Diarrhea, Dermatitis, Dementia, and Death; can occur in Hartnup disease and carcinoid syndrome (tryptophan diversion).

Distinguish vitamin B12 vs folate deficiency findings.

Both cause megaloblastic anemia and high homocysteine. B12 deficiency also raises methylmalonic acid and causes neurologic deficits (subacute combined degeneration); folate deficiency does not.

List the fat-soluble vitamins and their primary deficiency syndromes.

A (night blindness, xerophthalmia), D (rickets/osteomalacia), E (hemolytic anemia, neuropathy), K (bleeding/coagulopathy).

Which clotting factors are vitamin K-dependent, and what is vitamin K's biochemical role?

Factors II, VII, IX, X and proteins C and S. Vitamin K is a cofactor for gamma-carboxylation of glutamate residues (warfarin inhibits this via vitamin K epoxide reductase).

Which cardiac biomarkers rise after MI, and which is the most specific/preferred?

Troponin I/T (most specific, rises 3-6 h, peaks ~24 h, stays ~7-10 days), CK-MB (useful for reinfarction), and myoglobin (earliest but nonspecific).

Which enzyme ratio suggests alcoholic liver disease, and what does a very high ALT indicate?

AST:ALT ratio greater than 2 suggests alcoholic hepatitis; very high ALT (more than AST) suggests viral hepatitis or toxic/ischemic injury.

Match tumor markers to their cancers: AFP, CA-125, CA 19-9, CEA, PSA, beta-hCG.

AFP -> hepatocellular carcinoma & nonseminomatous germ cell; CA-125 -> ovarian; CA 19-9 -> pancreatic; CEA -> colorectal; PSA -> prostate; beta-hCG -> choriocarcinoma/germ cell.

What enzyme is elevated in acute pancreatitis, and which is more specific?

Amylase and lipase are both elevated; lipase is more specific and stays elevated longer.

Which isoenzyme of alkaline phosphatase elevation, with GGT, points to hepatobiliary rather than bone origin?

If ALP is elevated together with GGT, the source is hepatobiliary; if GGT is normal, the elevated ALP is likely of bone origin.