All Free Biology MCQs with Answers

• A. capsid
• B. capsomere
• C. envelope
• D. nucleoid

Explanation: The capsid is the complete protein shell around the viral nucleic acid, and it protects the genome and helps the virus attach to the host…

• A. double stranded DNA
• B. single stranded DNA
• C. single stranded RNA
• D. double stranded RNA

Explanation: TMV is a rod shaped plant virus whose genome is a single strand of RNA wound in a helix inside about 2,130 identical capsomeres.

• A. head
• B. tail fibres
• C. collar
• D. sheath

Explanation: The tail fibres carry the binding proteins that fit specific receptor molecules on the bacterial cell wall, which is why a phage infects…

• A. Synthesis of viral coat proteins
• B. Injection of viral nucleic acid into the host
• C. Attachment of the virus to the host cell
• D. Integration of viral DNA into the host chromosome

Explanation: In lysogeny the phage DNA becomes part of the bacterial chromosome as a prophage and is copied silently every time the bacterium divides…

• A. prophage
• B. provirus
• C. prion
• D. viroid

Explanation: Prophage is the term reserved for phage DNA sitting integrated and inactive in a bacterial chromosome.

• A. They can be crystallised like a chemical substance
• B. They are destroyed by antibiotics such as penicillin
• C. They contain either DNA or RNA but never both
• D. They multiply only inside a living host cell

Explanation: Antibiotics act on structures and pathways that only cells have, such as peptidoglycan wall synthesis or bacterial ribosomes, and a virus…

• A. prions
• B. bacteriophages
• C. viroids
• D. capsomeres

Explanation: Viroids are short circular RNA molecules with no capsid at all, and they cause plant diseases such as potato spindle tuber disease.

• A. RNA only
• B. DNA only
• C. an envelope of host lipid
• D. no nucleic acid at all

Explanation: A prion is a misfolded form of a normal cellular protein that converts healthy copies of that protein into the same abnormal shape, so…

• A. 20 to 300 nm
• B. 20 to 300 um
• C. 1 to 10 mm
• D. 0.5 to 5 um

Explanation: Viruses are measured in nanometres, from about 20 nm for the smallest to roughly 300 nm for the largest such as poxviruses, which is why…

• A. protein synthesised by the viral genome alone
• B. the membrane of the host cell it budded from
• C. the nucleic acid core of the virus
• D. capsomeres rearranged after assembly

Explanation: As the assembled particle buds out of an infected cell it wraps itself in a patch of the host's plasma membrane or nuclear membrane, so…

• A. antibodies secreted by plasma cells that neutralise virus particles
• B. antibiotics that block viral protein synthesis
• C. glycoproteins released by infected cells that make neighbouring cells resistant
• D. enzymes that digest the viral capsid in the bloodstream

Explanation: An infected cell secretes interferon, which binds receptors on nearby uninfected cells and switches on genes whose products block viral…

• A. Poliomyelitis
• B. Measles
• C. Hepatitis B
• D. Smallpox

Explanation: Smallpox was declared eradicated by the WHO in 1980 after a global vaccination campaign, helped by the fact that the virus had no animal…

• A. helper T lymphocytes carrying the CD4 receptor
• B. red blood cells
• C. neutrophils in the bone marrow
• D. platelets in circulating blood

Explanation: The gp120 spike of HIV binds the CD4 receptor, so helper T cells are the main target, and these are the cells that activate both B…

• A. carries double stranded DNA as its genome
• B. uses reverse transcriptase to make DNA from its RNA genome
• C. replicates inside the nucleolus of the host cell
• D. lacks an envelope around its capsid

Explanation: A retrovirus reverses the usual flow of information: its enzyme reverse transcriptase copies the single stranded RNA genome into DNA…

• A. Sharing a contaminated hypodermic needle
• B. From an infected mother to her child during breastfeeding
• C. Shaking hands with or sharing utensils with an infected person
• D. Transfusion of blood from an infected donor

Explanation: HIV is fragile outside the body and spreads only through blood, semen, vaginal secretions and breast milk, so ordinary social contact…

• A. antibodies against HIV first appear in the blood
• B. the provirus enters the latent phase
• C. reverse transcriptase is detected in the plasma
• D. the CD4 count falls below about 200 cells per microlitre and opportunistic infections appear

Explanation: AIDS is the late clinical stage, defined by the collapse of helper T cell numbers to roughly a fifth of normal together with opportunistic…

• A. ELISA
• B. Widal test
• C. ABO blood grouping
• D. Routine urine analysis

Explanation: ELISA detects antibodies against HIV and is cheap and sensitive enough for mass screening, with a positive result confirmed afterwards by…

• A. Typhoid fever
• B. Pneumocystis pneumonia
• C. Malaria
• D. Cholera

Explanation: Pneumocystis jirovecii causes pneumonia almost only in people whose cell mediated immunity has failed, so it is one of the defining…

• A. dissolving the lipid envelope of the virus
• B. stimulating plasma cells to make more antibody
• C. inhibiting the enzyme reverse transcriptase
• D. killing the infected helper T cells before the virus matures

Explanation: AZT is a nucleoside analogue that reverse transcriptase inserts into the growing DNA chain, after which the chain cannot be extended, so…

• A. matrix of the mitochondrion
• B. inner mitochondrial membrane
• C. cytoplasm
• D. nucleus

Explanation: Every enzyme of glycolysis is dissolved in the cytosol, which is why the pathway runs in prokaryotes and in red blood cells that have no…