MLT GlobeChemistry: Biochemistry basics & molecular biology – page 3
58 Chemistry MCQs on Biochemistry basics & molecular biology with answers and explanations.
Which glycosaminoglycan is most characteristic of the corneal stroma, helping keep the cornea transparent?
Keratan sulfate proteoglycans regulate collagen fibril spacing in the cornea. Heparin is in mast cells, heparan sulfate on cell surfaces and basement membranes, and hyaluronate in vitreous and synovial fluid.
Surface tension of a liquid is expressed as:
Surface tension is the force acting along a line in the surface, measured in N/m (equivalently J/m²). Force per area is pressure; per volume is not a surface property.
In a DNA strand, each phosphodiester bond connects:
A phosphate bridges the 3′-OH of one deoxyribose and the 5′-OH of the next, giving the strand 5′→3′ polarity. DNA lacks a 2′-hydroxyl.
Using current P/O ratios, oxidation of one mitochondrial NADH through the electron transport chain yields about:
Current estimates give about 2.5 ATP per NADH and 1.5 per FADH2, because proton pumping and ATP synthase stoichiometry are non-integral. The older whole-number value was 3.
The conservation of gene order along chromosomes of different species is termed:
Synteny means genes kept in the same relative order. Orthologs are the same gene in different species after speciation; paralogs arise by duplication within a genome.
Vasopressin acting on V1 receptors of vascular smooth muscle raises intracellular calcium through:
V1 receptors are Gq-coupled and activate phospholipase C; IP3 releases stored calcium. The renal V2 receptor, by contrast, uses Gs and cAMP to insert aquaporin-2.
In bioenergetics, a reaction with a negative ΔG is:
Negative ΔG means free energy is released, so the reaction is thermodynamically favourable. ΔG says nothing about rate, which depends on activation energy and enzymes.
In the pyruvate dehydrogenase complex, which cofactor accepts the acetyl group and passes it to coenzyme A?
Lipoamide on E2 forms an acetyl-thioester and transfers the acetyl group to CoA. TPP carries a hydroxyethyl group earlier, and FAD and NAD+ reoxidise lipoamide.
Complex I (NADH:ubiquinone oxidoreductase) contains which flavin prosthetic group?
Complex I takes electrons from NADH on FMN and passes them through iron–sulfur clusters to ubiquinone. Complex II contains FAD.
Human mitochondrial DNA encodes only 13 proteins. Where are nearly all other mitochondrial proteins made?
Over 1000 mitochondrial proteins are nuclear-encoded and imported through TOM/TIM translocases. mtDNA encodes 13 respiratory chain subunits, 2 rRNAs and 22 tRNAs.
Which pyrimidine base is catabolised through β-aminoisobutyrate to methylmalonyl semialdehyde?
Thymine breakdown yields β-aminoisobutyrate, which is converted to methylmalonyl semialdehyde and then succinyl-CoA. Uracil yields β-alanine instead.
Supercoiled DNA free in solution forms extended, interwound right-handed coils. This type of supercoiling is described as:
Plectonemic supercoils twist DNA around itself in extended coils. Solenoidal (toroidal) supercoiling is tighter wrapping, e.g., around histone cores.
Which histone is among the most highly conserved in sequence across eukaryotes?
H3 and H4 form the core tetramer and are almost identical from plants to mammals. H1, the linker histone, varies most.
Which DNA-binding proteins use dimerisation motifs to pair with a partner protein?
Leucine zipper and bHLH motifs form coiled-coil or helical contacts that link two proteins into dimers. Zinc fingers and helix-turn-helix mainly contact DNA.
Which E. coli operon is regulated by AraC rather than by transcriptional attenuation?
Attenuation via a leader peptide controls amino acid biosynthesis operons such as trp, his and leu. The ara operon uses the AraC activator/repressor.
In trp operon attenuation with low tryptophan, the ribosome stalls in the leader, and which leader RNA pairing lets transcription continue?
Stalling at the Trp codons in region 1 lets region 2 pair with 3, preventing the 3:4 terminator hairpin, so RNA polymerase continues.
Which feature is characteristic of Z-DNA?
Z-DNA is left-handed, zig-zag and has about 12 bp per turn, i.e. more than B-DNA. It forms transiently in alternating purine–pyrimidine sequences under torsional stress.
Which coenzyme works without being phosphorylated, attached covalently to a lysine of its enzyme?
Lipoic acid is amide-linked to lysine in dehydrogenase complexes and needs no phosphate. TPP, PLP and NADP+ all carry phosphate groups essential to their active forms.