All Free Biology MCQs with Answers

2981. Viruses are the particles of _.

• A. Nucleic acid only
• B. Proteins only
• C. Nucleic acid & Protein
• D. None of these

Explanation: Viruses are acellular entities composed of genetic material, which can be either DNA or RNA but never both. This genetic material is enclosed within a protein coat called a capsid, which protects the nucleic acid and aids in infection. Together, the nucleic acid and the capsid form the nucleocapsid. In many viruses, an additional outer envelope derived from the host cell membrane surrounds the capsid; this envelope contains lipids as well as viral proteins (such as glycoprotein spikes) that help the virus attach to and enter host cells.

2982. The major cells infected by HIV are

• A. B-lymphocyte
• B. T-Lymphocyte
• C. Helper T-Lymphocyte
• D. Suppressor T Lymphocyte

Explanation: The major cells infected by HIV are CD4+ T lymphocytes (helper T cells), which play a central role in coordinating the immune response. HIV targets these cells by binding to the CD4 receptor and coreceptors (CCR5 or CXCR4) on their surface, leading to viral entry and replication. In addition to CD4+ T cells, HIV can also infect other CD4-bearing cells such as macrophages and dendritic cells, which serve as reservoirs and help in the spread of the virus throughout the body. The progressive loss of CD4+ T cells is the hallmark of HIV infection and ultimately results in immune system failure, leading to AIDS.

2983. Which one of the following is a retrovirus?

• A. HIV
• B. Influenza
• C. Mumps
• D. All of These

Explanation: HIV is a retrovirus because it contains RNA as its genome and uses the enzyme reverse transcriptase to convert this RNA into DNA, which then integrates into the host cell's genome. This unique replication strategy (RNA → DNA → RNA → protein) is the defining feature of retroviruses. In contrast, both the mumps virus (Paramyxoviridae) and the influenza virus (Orthomyxoviridae) are RNA viruses that do not use reverse transcription. They carry negative-sense RNA and rely on RNA-dependent RNA polymerase to directly produce positive-sense RNA for protein synthesis. Since they never make a DNA intermediate or integrate into the host genome, they are not classified as retroviruses.

2984. Viruses cannot multiply on their own because they:

• A. Do not have sex organs.
• B. Lack of genetic material.
• C. Lack of cellular machinery to use its genetic material.
• D. None of these

Explanation: Viruses cannot multiply on their own because they lack the cellular machinery required for independent life processes. Unlike living cells, viruses do not have ribosomes, enzymes for energy production, or the ability to synthesize proteins. Their genetic material, whether DNA or RNA, cannot be expressed or replicated without the help of a host cell. Therefore, viruses must infect living cells and hijack the host's transcription and translation machinery to produce viral proteins and replicate their genome. This absolute dependence on a host cell for reproduction is why viruses are considered obligate intracellular parasites.

2985. The viral genome, incorporated and integrated with the bacterial genome, is referred to as

• A. Prophages
• B. DNA
• C. RNA
• D. Both A and B

Explanation: The viral genome, when incorporated and integrated into the bacterial genome, is referred to as a prophage. This occurs during the lysogenic cycle of certain bacteriophages, where the phage DNA becomes stably integrated into the host bacterium's chromosome instead of immediately directing viral replication. In this dormant state, the viral genome is replicated along with the bacterial DNA and passed on to daughter cells during cell division. The prophage may remain inactive for many generations, but under certain stress conditions, it can be induced to enter the lytic cycle, leading to the production of new phage particles and eventual lysis of the bacterial cell.

2986. The branch which deals with the study of viruses is called

• A. Biology
• B. Virology
• C. Cytology
• D. Taxonomy

Explanation: The branch of science that deals with the study of viruses is called virology. Virology is a specialized field of microbiology that focuses on understanding the structure, classification, genetics, and replication of viruses, as well as their interactions with host cells and the diseases they cause. This field also studies how viruses spread, how the immune system responds to them, and how antiviral drugs and vaccines can be developed to prevent or treat viral infections. Since viruses occupy a unique position at the border of living and non-living entities, virology plays a crucial role in medicine, biotechnology, and public health.

2987. Which of the following diseases is not caused by a virus?

• A. Cholera
• B. Influenza
• C. Hepatitis
• D. Polio

Explanation: Cholera is not caused by a virus but by a bacterium called Vibrio cholerae. It is a bacterial disease that spreads mainly through contaminated water and food. The bacterium produces a toxin, known as cholera toxin, which affects the lining of the small intestine and leads to severe watery diarrhea and dehydration. Since cholera is bacterial in origin, it can be treated effectively with antibiotics and rehydration therapy. In contrast, influenza, hepatitis, and polio are viral diseases caused by different viruses: influenza by the influenza virus, hepatitis (various forms) by hepatitis viruses, and polio by poliovirus. These viruses rely on host cells for replication and cause illness by damaging host tissues or triggering immune responses. Unlike cholera, viral diseases cannot be treated with antibiotics but are managed with antiviral drugs (in some cases), supportive care, and prevention through vaccines. Thus, the difference lies in the nature of the causative agents-cholera is bacterial, while influenza, hepatitis, and polio are viral.

2988. Twort in 1915 and 'D'Herelle in 1917 discovered:

• A. Pox virus
• B. Bacteriophages
• C. Adenovirus
• D. Herpes Virus

Explanation: Bacteriophages, the viruses that infect bacteria, were discovered independently by Frederick W. Twort in 1915 and later by Félix d'Herelle in 1917. Twort, a British bacteriologist, observed a mysterious phenomenon where bacterial cultures were being destroyed by an unknown agent, but he was unable to study it further. Two years later, d'Herelle, a French-Canadian microbiologist, clearly demonstrated that this agent was a virus capable of infecting and lysing bacteria. He named these viruses bacteriophages, meaning "bacteria eaters." D'Herelle's detailed experiments and descriptions provided solid evidence for their existence, earning him recognition as the co-discoverer and main pioneer in bacteriophage research.

2989. A bacteriophage replicates only in a_ cell

• A. Animal
• B. Bacterial
• C. Plant
• D. Fungi

Explanation: A bacteriophage replicates only in a bacterial cell. This is because bacteriophages are viruses that specifically infect bacteria, and like all viruses, they lack the cellular machinery required for independent reproduction. When a bacteriophage attaches to a susceptible bacterial cell, it injects its genetic material into the host. The bacterial cell's machinery is then hijacked to synthesize viral proteins and replicate the viral genome, leading to the assembly of new phage particles. Eventually, the bacterial cell may burst (lyse) to release the newly formed phages, which can then infect other bacteria. Thus, bacteriophages are obligate intracellular parasites that can only multiply inside bacterial cells.

2990. Smallpox is a _.

• A. DNA non-enveloped virus
• B. RNA non-enveloped virus
• C. DNA Enveloped Virus
• D. RNA-Enveloped Virus

Explanation: The smallpox virus (Variola virus) has a double-stranded DNA (dsDNA) genome, which is large and linear, encoding all the proteins needed for its replication and assembly. Unlike most DNA viruses, it replicates entirely in the cytoplasm of the host cell, carrying its own enzymes for transcription and DNA replication. Its genome is surrounded by a complex protein capsid and a lipid envelope, which helps the virus enter host cells and evade immune defences. This DNA genome allows the virus to produce all the components necessary for infection and spread, making it a highly efficient and pathogenic virus.