All Free Biology MCQs with Answers

611. In feedback inhibition, the end product of a metabolic pathway usually binds to

• A. the substrate of the first reaction
• B. an allosteric site on the first enzyme of the pathway
• C. the active site of every enzyme in the cell
• D. the DNA that codes for the pathway

Explanation: The end product docks at a regulatory site on the pathway's first committed enzyme, changing its shape and shutting production until the product is used up. Binding the substrate or the DNA would be too slow and too blunt for minute-to-minute control.

612. Penicillin kills susceptible bacteria by inhibiting the enzyme that builds the

• A. bacterial DNA
• B. bacterial ribosomes
• C. bacterial cell wall
• D. bacterial flagellum

Explanation: Penicillin blocks the transpeptidase that crosslinks the peptidoglycan wall, so a growing bacterium bursts under its own osmotic pressure. Human cells have no wall at all, which is why the drug is selectively toxic.

613. Which change would best restore the rate of an enzyme controlled reaction in the presence of a competitive inhibitor?

• A. Increasing the concentration of substrate
• B. Increasing the concentration of inhibitor
• C. Raising the temperature above the optimum
• D. Lowering the pH well below the optimum

Explanation: Because substrate and inhibitor compete for the same active site, flooding the mixture with substrate outcompetes the inhibitor and the normal maximum rate returns. This is the classic test that distinguishes competitive from non competitive inhibition.

614. Aspirin relieves pain and fever by irreversibly blocking the enzyme cyclooxygenase. In enzyme terminology, aspirin acts as

• A. a coenzyme
• B. an activator
• C. a substrate
• D. an enzyme inhibitor

Explanation: By disabling cyclooxygenase, aspirin stops the making of prostaglandins, the chemicals that promote pain, fever and inflammation. It is not consumed as a substrate nor does it assist the enzyme, so it can only be classed as an inhibitor.

615. Evolution is best defined as

• A. gradual genetic change in populations over successive generations
• B. the growth of an individual from birth to adulthood
• C. the seasonal migration of animals
• D. the extinction of a single species

Explanation: Evolution acts on populations across generations, changing allele frequencies over time. An individual's own growth or a single extinction event is not evolution, since neither involves inherited change passed through a population.

616. The theory of evolution by natural selection was proposed independently by Darwin and

• A. Lamarck
• B. Alfred Russel Wallace
• C. Gregor Mendel
• D. Thomas Huxley

Explanation: Wallace reached the same conclusion while collecting in the Malay Archipelago, and his letter prompted the joint reading of both men's work in 1858. Mendel discovered inheritance, and Huxley defended Darwin, but neither proposed natural selection.

617. The study of the geographical distribution of organisms, such as the concentration of marsupials in Australia, is called

• A. palaeontology
• B. embryology
• C. biogeography
• D. taxonomy

Explanation: Biogeography explains why isolated landmasses evolve their own distinctive faunas, a pattern Darwin and Wallace both used as evidence. Palaeontology is the study of fossils and embryology of development, different branches of the same supporting case.

618. The smallest unit in which evolution occurs is the

• A. individual
• B. cell
• C. organelle
• D. population

Explanation: An individual's genes are fixed for life, so it cannot evolve; what changes is the genetic makeup of populations as generations pass. This is why evolutionary biology always speaks of populations, never of single organisms.

619. The fossil sequence of which animal is one of the most completely documented examples of gradual evolution?

• A. The horse
• B. The pigeon
• C. The frog
• D. The earthworm

Explanation: From the small multi toed Hyracotherium to the single hooved Equus, horse fossils record increasing size, fewer toes and taller teeth as grasslands spread. No other lineage offers so complete a series from North America and Europe.

620. The fact that almost all organisms use the same genetic code is evidence for evolution because it suggests that

• A. all species were created separately
• B. all life shares a common ancestor
• C. mutation is impossible
• D. DNA is chemically unstable

Explanation: A universal code is exactly what common descent predicts: one original system inherited by every branch of life. If organisms had separate origins there would be no reason for bacteria and humans to translate codons identically.