All Free Biology MCQs with Answers

2971. "Two triangular structures with a common base" is the description of the structure of _ of the T4 phage.

• A. Tail
• B. Head
• C. Collar
• D. Base plate

Explanation: "Two triangular structures with a common base" is the description of the structure of the base plate of the T4 phage. The baseplate is a complex structure located at the end of the phage's tail and plays a crucial role during infection. It serves as the site of attachment to the bacterial cell surface and coordinates the contraction of the tail sheath during DNA injection. The description "two triangular structures with a common base" refers to the characteristic arrangement of the base plate's subunits, which form a hexagonal or star-like structure that appears as two triangular parts sharing a common base when viewed in certain orientations.

2972. Within the head of the T4 phage, the _ molecule is present.

• A. Single-Stranded DNA
• B. Double-Stranded DNA
• C. Single-Stranded RNA
• D. Double-stranded RNA

Explanation: The T4 bacteriophage contains a double-stranded DNA genome of about 169,000 base pairs. Like all dsDNA, the strands are antiparallel and held together by hydrogen bonds between complementary bases. Inside the phage head, this DNA is packed at very high density in a spool-like, liquid-crystalline arrangement. The DNA is enclosed within a rigid protein capsid, which is made of capsid proteins arranged in an icosahedral lattice with hexamers and pentamers. This structure both protects the genome from the environment and allows efficient injection into the host cell during infection.

2973. The bacteriophage is adsorbed to the host cell at _ of the bacterium.

• A. Receptor site
• B. Cell wall
• C. Cell membrane
• D. Capsule

Explanation: The answer is "Receptor site." Explanation: Being obligate parasites, viruses require a receptor site on the host surface for attachment. The bacteriophage is adsorbed to the receptor site of the bacterium. A receptor site is a specific molecule on the surface of the bacterium that the bacteriophage can recognize and bind to. Once the bacteriophage has attached to the receptor site, it injects its DNA into the host cell. The DNA then takes over the host cell's machinery and starts to replicate itself. The replicated DNA then produces new bacteriophage particles, which are released from the host cell.

2974. The transmissible nature of TMV was first observed by:

• A. W.M Stanley
• B. Ivanowsky
• C. Chamberland
• D. Twort

Explanation: The answer is "Ivanowsky."Explanation: Ivanovsky took bacteri-free extract from infected tobacco leaves and sprinkled it over healthy plants, which suffered from TMV later on.

2975. What gives definite shape to a virion?

• A. Tail
• B. Base plate
• C. Capsid
• D. Genome

Explanation: The capsid is the protein shell that surrounds and protects the viral genome, and it plays a crucial role in giving the virion its definite shape. It is composed of repeating protein subunits called capsomeres, which self-assemble in a precise and symmetrical manner. This assembly follows geometric rules, ensuring that each virus particle has a consistent and stable structure. Depending on the arrangement of capsomeres, the capsid can form three main types of shapes: helical, icosahedral, or complex. In helical viruses, the capsid proteins arrange around the nucleic acid in a spiral, producing rod-like or filamentous virions, as seen in the tobacco mosaic virus and rabies virus. In icosahedral viruses, capsomeres form a spherical structure with 20 triangular faces, providing a highly stable and efficient design, as in adenovirus and poliovirus. Some viruses, such as bacteriophages and poxviruses, exhibit complex structures that combine helical and icosahedral features. Thus, by self-assembling into specific symmetries, the capsid provides the virion with its distinct shape, stability, and ability to interact with host cells.

2976. Pick up the one that is absent in viruses:

• A. DNA
• B. RNA
• C. Enzymes
• D. Metabolic machinery

Explanation: The answer is "metabolic machinery."Explanation: Viruses are acellular entities and lack biosynthetic machinery for their replication. All others are structural parts of viruses.

2977. The capsid of the herpes virus and that of the adenovirus differ from each other concerning:

• A. Chemical nature
• B. Type of subunits
• C. Number of subunits
• D. Role in the life cycle

Explanation: The answer is "Number of subunits."Explanation: Herpes virus has 162 capsomeres, whereas adenovirus has 252 capsomeres. The net difference is that of 90 capsomeres.

2978. How many additional capsomeres are found in an adenovirus, as compared to those in a herpes virus?

• A. 70
• B. 80
• C. 90
• D. 100

Explanation: Adenoviruses and herpesviruses are both icosahedral viruses, but they differ in the number of capsomeres present in their capsids. An adenovirus has a total of 252 capsomeres, consisting of 240 hexons and 12 pentons. In contrast, a herpesvirus capsid is composed of 162 capsomeres, made up of 150 hexons and 12 pentons. This means that adenoviruses possess 90 additional capsomeres compared to herpesviruses, which contributes to their structural differences despite both sharing icosahedral symmetry.

2979. For the synthesis of proteins, a virus uses:

• A. Its own metabolism
• B. Metabolism of its host organism
• C. Metabolism of its host cell
• D. None of these

Explanation: The answer is "Metabolism of its host cell"Explanation: Viruses are acellular entities having no cellular/metabolic machinery. Thus, they have to rely upon the biosynthetic machinery of the host for this purpose.

2980. Rabies is due to:

• A. Fungi
• B. Bacteria
• C. Protozoan
• D. Virus

Explanation: Rabies is caused by a virus (Lyssavirus), which infects nerve cells and travels to the brain, leading to fatal neurological symptoms. It is not bacterial, since antibiotics don't work and bacteria cause different infections. It is not fungal, as fungi affect skin or lungs, not nerves. It is also not a protozoan, because protozoa cause diseases like malaria, not rabies, and respond to antiparasitic drugs, not vaccines.