Biochemistry - MCQ Practice Questions

Cholesterol molecules insert between phospholipids in the bilayer, regulating membrane fluidity. At high temperatures, it reduces fluidity; at low temperatures, it increases it.

Lipoproteins are organized with a hydrophobic lipid core (cholesterol esters and triglycerides) surrounded by a hydrophilic shell of proteins, free cholesterol, and phospholipids.

Familial hypercholesterolemia results from mutations in the LDL receptor gene, leading to impaired LDL uptake and elevated plasma LDL cholesterol levels.

During fasting, the liver increases synthesis and secretion of VLDL to transport endogenous triglycerides produced from gluconeogenesis and fatty acid oxidation.

LCAT esterifies free cholesterol on lipoprotein surfaces, forming cholesterol esters that move to the hydrophobic core. This process is essential for lipoprotein maturation.

ApoE acts as a ligand recognized by hepatic receptors (LDL receptor and LDL receptor-related protein), mediating the uptake of chylomicron remnants and VLDL remnants.

Niemann-Pick disease results from deficiency of sphingomyelinase, leading to accumulation of sphingomyelin in lysosomes, particularly affecting the nervous system and liver.

Saturated fatty acids have higher melting points and are typically solid at room temperature due to tight packing. They are more stable and less prone to oxidation than unsaturated fatty acids.

Approximately 70-80% of body cholesterol is synthesized de novo, primarily in the liver. The remaining 20-30% comes from dietary sources.

Tangier disease results from mutations in the ABCA1 gene, which encodes a cholesterol transporter. This leads to severe HDL deficiency and accumulation of cholesterol esters in tissues.

LDL carries cholesterol into arterial walls and can be oxidized, promoting atherosclerosis. HDL mediates reverse cholesterol transport, removing cholesterol from tissues and delivering it to the liver.

Myelin contains approximately 80% lipid by weight, with cholesterol being the most abundant lipid (27% of total myelin mass), followed by galactocerebroside.

Deficiency of lipoprotein lipase or its cofactor apoC-II results in severe hypertriglyceridemia with elevated chylomicrons and VLDL but relatively normal LDL levels.

Acetoacetate is the first ketone body formed from two molecules of Acetyl-CoA via HMG-CoA synthase. It can be reduced to beta-hydroxybutyrate or decarboxylated to acetone.

Carnitine, via carnitine palmitoyltransferase (CPT), transfers long-chain fatty acyl groups across the inner mitochondrial membrane, enabling their oxidation in the matrix.

Severe hypertriglyceridemia (>1000 mg/dL) significantly increases the risk of acute pancreatitis due to pancreatic inflammation caused by triglyceride-rich lipoproteins.

Prostaglandins are eicosanoids derived from arachidonic acid (20-carbon polyunsaturated fatty acid). They act as paracrine and autocrine signaling molecules.

Understanding:

We are asked to identify the correct statement of Chargaff's base equivalence rule for double-stranded DNA.

Step 1: Recall Chargaff's observations

Chargaff analysed the base composition of DNA from multiple species and made two key observations:

Step 2: Evaluate each option

The statement "total purines equal total pyrimidines" is the direct consequence of Watson-Crick base pairing and is universally true for all double-stranded DNA, regardless of species.

The ratio A/G is NOT fixed; it varies between species — this is actually what Chargaff used to distinguish species. A = C is incorrect because adenine pairs with thymine, not cytosine. The A+T to G+C ratio (also called the AT/GC ratio) varies widely between species and is NOT constant.

Answer:

Chargaff's rule states that in double-stranded DNA, total purines always equal total pyrimidines, i.e., A + G = T + C.

Quick Tip:

Chargaff's rules: A = T and G = C (first parity rule) for dsDNA. The A+T/G+C ratio differs between species — that is how Chargaff showed DNA composition is species-specific.

Understanding:

We need to identify which sequence has the highest melting temperature (Tm) based on its GC content.

Step 1: Recall the relationship between Tm and GC content

G-C base pairs are held together by three hydrogen bonds, whereas A-T base pairs are held by only two hydrogen bonds. A higher proportion of G-C pairs means more energy is needed to separate the strands, resulting in a higher Tm.

Step 2: Calculate GC content of each option

Step 3: Determine the highest Tm

The sequence with 100% GC content (5'-GCGCGCGC-3') will have the highest melting temperature because every base pair contributes three hydrogen bonds.

Answer:

The sequence 5'-GCGCGCGC-3' has 100% GC content and therefore the highest melting temperature.

Quick Tip:

A rough formula for short oligonucleotides is $T_m=2(A+T)+4(G+C)$ °C. For the GCGCGCGC 8-mer: $T_m=4\times 8=32°C$ (approximate). For the ATATATAT 8-mer: $T_m=2\times 8=16°C$. The GC-rich sequence always wins.

Understanding:

We are asked to identify the correct chemical differences between RNA and DNA.

Step 1: Identify the sugar component

DNA contains 2'-deoxyribose (lacks a hydroxyl group at the 2' carbon), while RNA contains ribose (has a 2'-OH group). This is a fundamental structural difference.

Step 2: Identify the pyrimidine base difference

DNA contains the pyrimidine thymine (5-methyluracil), while RNA contains uracil (lacks the methyl group at position 5). Both pair with adenine, but their structures differ by a methyl group.

Step 3: Combine both distinguishing features

RNA = ribose sugar + uracil (replacing thymine).

DNA = deoxyribose sugar + thymine (replacing uracil).

Answer:

RNA differs from DNA in that it contains ribose sugar and uracil in place of thymine.

Quick Tip:

Remember: RNA has an extra -OH at the 2' position of ribose — this makes RNA more susceptible to alkaline hydrolysis than DNA, which is why DNA is a more stable genetic storage molecule.