All Free Biology MCQs with Answers

1031. According to Lock and Key model, the active site is regarded as:

• A. Rigid and specific
• B. Flexible and specific
• C. Rigid and non specific
• D. Flexible and non specific

Explanation: Fischer's lock and key model treats the active site as a pre formed rigid shape that only the matching substrate fits, which explains specificity but not the conformational changes later observed. Koshland's induced fit model keeps the specificity but makes the site flexible, moulding itself around the substrate as it binds. Both models agree that the enzyme is specific.

1032. Peptide bonds are important in protein because they:

• A. Affect solubility
• B. Hold R-groups
• C. Link amino acid
• D. Help in releasing oxygen

Explanation: A peptide bond forms by condensation between the carboxyl group of one amino acid and the amino group of the next, so it is what creates the polypeptide chain and therefore the primary structure. R groups project from that backbone and determine folding, but the bond itself does not hold them. Oxygen release is a function of haemoglobin's haem group rather than of any peptide bond.

1033. Why are retroviruses placed in a separate class of RNA viruses?

• A. They always infect animal cells only
• B. Their RNA acts directly as mRNA
• C. They lack protein coats
• D. They use reverse transcriptase to make DNA from RNA

Explanation: A retrovirus reverses the usual flow of information by copying its RNA genome into DNA, which integrase then inserts into the host chromosome as a provirus, and that integration is why HIV infection is lifelong. In other RNA viruses the genome often does act directly as messenger RNA, which is the contrast the second option draws. Retroviruses have perfectly ordinary capsids and envelopes.

1034. Reversible inhibitors differ from irreversible inhibitors because they:

• A. Bind permanently and cannot be removed
• B. Bind temporarily and can be removed
• C. Permanently inactivate the enzyme
• D. Change enzyme structure permanently

Explanation: A reversible inhibitor binds through weak interactions and detaches again, so activity returns when it is removed or outcompeted, which is how competitive and non competitive inhibitors behave. An irreversible inhibitor forms a covalent bond, as heavy metal ions and nerve gases do, and the enzyme is destroyed for good. The other three options all describe the irreversible case.

1035. Which is an enzyme activator secreted by the intestinal glands:

• A. Amylase
• B. Pepsinogens
• C. Enterokinase
• D. Lipase

Explanation: Enterokinase from the duodenal lining converts trypsinogen into active trypsin, which then activates the other pancreatic proenzymes in turn. Pepsinogen is itself an inactive precursor rather than an activator, and it is activated by hydrochloric acid in the stomach, which makes it the tempting answer. Amylase and lipase are ordinary digestive enzymes.

1036. In which of the following combination, both components have hyaline cartilage?

• A. Epiglottis & intervertebral disc
• B. Trachea & intervertebral disc
• C. Nose & pinna
• D. Nose & trachea

Explanation: The C shaped rings of the trachea and the supporting plates of the nose are both hyaline cartilage, the smooth glassy type that also covers the ends of bones. The intervertebral discs are fibrocartilage, built for shock absorption, and the epiglottis and pinna are elastic cartilage, which springs back into shape. Each pairing in the wrong options mixes two different types.

1037. According to Lamarck, the evolution of long necks in giraffes is explained by:

• A. Natural selection
• B. Genetic mutation
• C. Survival of the fittest
• D. Stretching of necks over generations

Explanation: In Lamarck's account each giraffe stretched to reach higher leaves, its neck lengthened a little during its own life, and that gain was passed to its offspring. Darwin's explanation instead starts from neck length varying at birth and lets the environment select among those variations, which is what the first and third options describe. This single example is the standard way of contrasting the two theories.

1038. The process of spermatogenesis occurs in which part of the male reproductive system?

• A. Seminiferous tubules
• B. Vas deferens
• C. Scrotum
• D. Epididymis

Explanation: The germinal epithelium lining the seminiferous tubules divides by mitosis and then meiosis to produce spermatids, which mature into spermatozoa. The epididymis stores sperm and gives them motility after they leave the testis, so it is the closest distractor but hosts no division. The scrotum is the sac holding the testes, keeping them a few degrees below core temperature.

1039. Which layer of uterus nourishes the embryo after implantation?

• A. Cervical epithelium
• B. Myometrium
• C. Endometrium
• D. Perimetrium

Explanation: The endometrium is the inner glandular lining, thickened and richly supplied with blood by progesterone, and it is where the blastocyst implants and from which the maternal part of the placenta develops. The myometrium is the thick smooth muscle whose contractions expel the baby at birth, and the perimetrium is the outer covering. Shedding of the endometrium is what menstruation consists of.

1040. The main role of mRNA during protein synthesis is to:

• A. Carry genetic information for protein synthesis
• B. Stabilize ribosome
• C. Deliver amino acids
• D. Provide platform for proteins synthesis

Explanation: Messenger RNA is transcribed from a gene and carries that base sequence out of the nucleus to the ribosome, where it is read in triplets. Delivering amino acids is the job of transfer RNA and providing the platform is the job of ribosomal RNA, so each of the three RNAs appears once among the options. Using a working copy protects the original DNA from the risks of the cytoplasm.