All Free Biology MCQs with Answers

981. Enzymes activity decreases at very low or high pH because:

• A. Substrate concentration increases
• B. Enzymes become denatured
• C. Product formation increases
• D. Temperature becomes constant

Explanation: Excess hydrogen or hydroxide ions disturb the ionic and hydrogen bonds between the side chains that hold the tertiary structure, so the active site loses its precise shape and the substrate no longer fits. A small shift is reversible, but an extreme one denatures the enzyme permanently. This is why pepsin works at about pH 2 and trypsin at about pH 8, each being inactive in the other's conditions.

982. The complementary base pairing in DNA is important because it:

• A. Maintains the tertiary structure of enzymes
• B. Provides energy for all cell metabolism
• C. Enables protein to fold properly
• D. Allows DNA act as a genetic blueprint during replication

Explanation: Because adenine always pairs with thymine and guanine with cytosine, each strand specifies the sequence of the other, so separating the two gives two templates that produce identical copies. This is precisely what Watson and Crick noted on proposing the structure. Protein folding and enzyme structure depend on interactions between amino acid side chains, not on base pairing.

983. A mature duplicated chromosome consists of:

• A. Two identical double helical DNA molecules
• B. Two different double helical DNA molecules
• C. A double helical DNA molecule distributed in both chromatids
• D. Histone chains wrapped around DNA core

Explanation: Replication is semi conservative and highly accurate, so the two sister chromatids each hold a complete double helix and the two are identical copies joined at the centromere. They are not different molecules, which is what distinguishes sister chromatids from homologous chromosomes. Note also that DNA wraps around the histones rather than the other way round, which makes the last option backwards.

984. The most primitive respiratory process occurring in a living cell is:

• A. Lactic acid fermentation
• B. Alcoholic fermentation
• C. Glycolysis
• D. Krebs's cycle

Explanation: Glycolysis occurs in the cytoplasm of every living cell, needs no oxygen and no organelle, and is common to bacteria, plants and animals alike, which marks it as the oldest of these pathways, dating from before the atmosphere contained oxygen. The two fermentations are add ons that merely regenerate NAD so glycolysis can continue. The Krebs cycle is later still, since it requires a mitochondrion and oxygen further down the chain.

985. Receive, retain and nourish a fertilized ovum is the main function of:

• A. Cervix
• B. Uterus
• C. Ovary
• D. Oviduct

Explanation: The blastocyst implants in the thickened endometrium of the uterus, which then supports the pregnancy through the placenta, so receiving, retaining and nourishing are all uterine functions. The oviduct is where fertilisation itself occurs, which is why it is the closest distractor, but the embryo only passes through it. The ovary releases the egg and the cervix is the muscular opening into the vagina.

986. The primary function of larynx is:

• A. Humidifying air
• B. Filtering dust
• C. Gaseous exchange
• D. Voice production

Explanation: The larynx houses the vocal cords, and air forced between them makes them vibrate to produce sound, which is why it is called the voice box. Humidifying and filtering the air are done earlier, in the nasal cavity and the ciliated mucous lining of the airways. Gaseous exchange happens only in the alveoli, since no other part of the tract has walls thin enough.

987. During menstrual cycle, luteinizing hormone (LH) is secreted due to:

• A. Decrease in Estrogen
• B. Decrease in FSH
• C. Increase in FSH
• D. Environmental effect

Explanation: This is the option marked correct in the official UHS answer key. The physiology worth carrying into the exam is that the LH surge is driven by rising oestrogen from the ripening follicle, which flips the pituitary from negative to positive feedback, and that FSH does fall in the mid follicular phase as inhibin from the growing follicle suppresses it. So the fall in FSH accompanies the surge rather than causing it, and no option here states the oestrogen rise directly.

988. Left and right cerebral hemispheres are connected with each other by:

• A. Corpus luteum
• B. Corpus callosum
• C. A band of dendrites
• D. Dorsal and ventral nerve roots

Explanation: The corpus callosum is a thick band of myelinated axons carrying traffic between the two hemispheres so that they work as one. The corpus luteum is the very similar sounding structure in the ovary that secretes progesterone after ovulation, and it is the deliberate trap here. Dorsal and ventral roots belong to the spinal nerves, not the brain.

989. Which is the common feature between cardiac and smooth muscles?

• A. Voluntary
• B. Involuntary
• C. Branched
• D. Unbranched

Explanation: Both contract without conscious control, cardiac muscle beating rhythmically on its own and smooth muscle working the gut, blood vessels and other hollow organs. They differ in almost everything else: cardiac fibres are branched and striated while smooth fibres are unbranched and non striated, which is why the third and fourth options each apply to one but not both. Only skeletal muscle is voluntary.

990. The joints which cause rotational movements are

• A. Hinge
• B. Ball and Socket
• C. Cartilage
• D. Sutures

Explanation: A ball and socket joint such as the shoulder or hip allows movement in every plane including rotation, because the rounded head can turn freely within its cup. A hinge joint like the elbow is restricted to one plane and permits no rotation at all. Sutures between the skull bones are immovable and cartilaginous joints allow only slight movement.