MLT GlobeChemistry: Biochemistry basics & molecular biology – page 2
58 Chemistry MCQs on Biochemistry basics & molecular biology with answers and explanations.
Why is Taq DNA polymerase used in the polymerase chain reaction?
Taq, from Thermus aquaticus, is heat-stable, so it is not destroyed during each denaturation cycle. Restriction enzymes such as EcoRI cut DNA; reverse transcriptase copies RNA; HRP is an ELISA label.
When building a recombinant plasmid, which enzyme seals the vector and insert DNA after both are cut with the same restriction enzyme?
Restriction enzymes create matching ends; DNA ligase forms the phosphodiester bonds joining them. Polymerase synthesises DNA but does not join fragments.
Which compound is the principal methyl donor in most biological methylation reactions?
SAM donates methyl groups, e.g. to DNA, creatine and epinephrine synthesis. Thiamine transfers aldehyde units, biotin carries CO2, and PLP handles amino groups.
Which molecule mainly couples energy-releasing catabolism to energy-requiring anabolism?
Catabolism regenerates ATP and anabolic reactions spend it. ADP is the discharged form, and cAMP is a signalling messenger, not an energy currency.
Cyanide stops respiration by binding which respiratory chain complex?
Complex IV passes electrons to O2. Cyanide, CO and azide bind its haem a3–CuB centre. Rotenone acts on Complex I and antimycin A on Complex III.
As the respiratory chain pumps protons into the intermembrane space, what happens as they flow back through ATP synthase?
The proton-motive force drives protons back through F0F1 ATP synthase, which phosphorylates ADP. This is Mitchell's chemiosmotic coupling.
The heat-stable DNA polymerase that made automated PCR thermal cycling practical was isolated from:
Taq polymerase comes from the hot-spring bacterium Thermus aquaticus and survives repeated 94-95 °C denaturation steps, so it need not be replaced after each cycle.
Making mRNA from a DNA template is called:
Transcription is DNA-directed RNA synthesis. Translation makes protein from mRNA, and reverse transcription makes DNA from RNA.
Which base is found in RNA and replaces thymine in DNA?
RNA uses uracil in place of thymine to pair with adenine. Adenine, guanine and cytosine are found in both nucleic acids.
Evaporating sweat cools the body efficiently because water has a high latent heat of vaporisation. Its approximate value is:
About 2.26 kJ is needed to vaporise 1 g of water at 100 °C, reflecting the energy required to break its extensive hydrogen bonding.
Just after DNA replication, which protein complex holds the two sister chromatids together until anaphase?
Cohesin rings encircle sister chromatids until separase cleaves them at anaphase. Condensins compact chromosomes but do not join sisters.
Cytosine deamination leaves uracil in DNA. Which repair pathway removes it via uracil-DNA glycosylase?
A glycosylase removes the single wrong base, an AP endonuclease cuts the backbone, and polymerase and ligase fill it in. NER removes bulky lesions such as thymine dimers.
The degree of ionisation of a weak electrolyte does NOT depend on:
Dissociation depends on solvent dielectric properties, temperature and dilution. Passing current does not change the equilibrium extent of ionisation.
Which recombination process requires extensive sequence similarity between the two participating DNA molecules?
Homologous recombination pairs long matching sequences, e.g., in meiotic crossing over. Site-specific recombination uses short defined sites; transposition and NHEJ need little or no homology.
Which step is part of mRNA processing but NOT of tRNA processing?
Pre-tRNAs are trimmed, get a CCA end, have bases modified (e.g., pseudouridine) and some have introns spliced; polyadenylation is specific to mRNA.
A castor-bean toxin depurinates 28S rRNA and so inactivates the large (60S) ribosomal subunit. The toxin is:
Ricin's A chain is an N-glycosidase acting on 28S rRNA. Diphtheria toxin ADP-ribosylates EF-2; chloramphenicol and tetracycline act on bacterial ribosomes.
Succinate dehydrogenase passes electrons via FADH2 to the respiratory chain. Using current P/O ratios, about how many ATP result?
FADH2 electrons enter at coenzyme Q, bypassing complex I, giving about 1.5 ATP; NADH gives about 2.5. Older texts quoted 2 and 3.
The pyruvate dehydrogenase complex requires TPP, lipoate, FAD, NAD⁺ and which other cofactor?
Coenzyme A accepts the acetyl group, producing acetyl-CoA. Pyridoxal phosphate serves transaminases; biotin serves carboxylases.
Which biological membrane has the highest ratio of protein to lipid?
The inner mitochondrial membrane is packed with electron-transport complexes and ATP synthase (about 75% protein). Myelin is the opposite extreme, mostly lipid for insulation.
After nuclei are removed, which cell fraction pellets next when the supernatant is spun at about 10,000–20,000 g?
Mitochondria (with lysosomes and peroxisomes) are the largest remaining particles and sediment at intermediate speed. Microsomes and ribosomes need ~100,000 g; cytosolic proteins stay in the supernatant.