All Free Biology MCQs with Answers

3001. Bacteriophage is made up of:

• A. RNA
• B. Proteins
• C. Nucleoproteins
• D. DNA

Explanation: Bacteriophages, the viruses that infect bacteria, are composed of nucleic acids and proteins, which together form their structure. The nucleic acid can be either DNA or RNA, and it carries the genetic information necessary for the replication of the virus inside a bacterial host. Surrounding the nucleic acid is a protein coat called a capsid, which protects the viral genome and helps in attaching to and injecting it into the bacterial cell. Some bacteriophages also have additional structures, such as tails and tail fibers, which aid in recognizing and binding to specific bacterial surface receptors. Because bacteriophages lack metabolic machinery, they rely entirely on the host bacterium to reproduce, making them obligate intracellular parasites.

3002. AIDS is due to:

• A. Deficiency of T lymphocytes
• B. Bacterial infection
• C. Deficiency of riboflavin
• D. High blood pressure

Explanation: The deficiency of CD4+ T lymphocytes during HIV infection occurs through multiple mechanisms. HIV specifically targets CD4+ T cells by binding to the CD4 receptor and co-receptors on their surface, entering the cells, and replicating, which eventually causes the infected cells to lyse and die. Additionally, uninfected CD4+ T cells may undergo apoptosis due to chronic immune activation and viral protein toxicity. The virus also damages lymphoid organs like lymph nodes and the thymus, impairing T cell production and maturation, while some infected cells are destroyed by cytotoxic CD8+ T cells. Together, these processes lead to a progressive loss of CD4+ T cells, weakening the immune system and resulting in the immunodeficiency characteristic of AIDS.

3003. The virus was first discovered by:

• A. K. M. Smith
• B. Twart
• C. M. Stanley
• D. Ivanowsky

Explanation: The discovery of viruses dates back to the late 19th century, when scientists were investigating diseases in plants that could not be explained by known bacteria. In 1892, Russian scientist Dmitri Ivanovsky first identified the existence of a new type of infectious agent while studying tobacco mosaic disease. He found that the disease could be transmitted by sap from infected plants, even after the sap was passed through a porcelain filter fine enough to remove all known bacteria. Later, in 1898, Dutch microbiologist Martinus Beijerinck confirmed Ivanovsky's findings and described the agent as a "contagium vivum fluidum" (a contagious living fluid), coining the concept of a virus as something fundamentally different from bacteria. This discovery marked the beginning of virology, the study of viruses, and laid the foundation for understanding these unique infectious agents.

3004. Which of the following is true for viruses?

• A. Multiply only in host cells
• B. Behave as if they are plants
• C. They are made up of proteins only
• D. It occurs only inside bacteria

Explanation: Viruses are unique infectious agents with certain defining features, and among the given statements, the correct one is that they multiply only in host cells. The claim that viruses occur only inside bacteria is incorrect, because although some viruses specifically infect bacteria (bacteriophages), many others infect plants, animals, fungi, and humans. The idea that they are made up of proteins only is also wrong, as all viruses contain genetic material in the form of either DNA or RNA in addition to a protein coat, and some also have a lipid envelope. Similarly, the statement that they behave as if they are plants is misleading, since viruses do not carry out plant-like functions such as photosynthesis. The only accurate description is that viruses multiply only inside host cells, as they are obligate intracellular parasites and depend entirely on the machinery of living cells to reproduce.

3005. The genetic material of retroviruses (AIDS virus) is:

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

Explanation: The genetic material of retroviruses, including the AIDS-causing Human Immunodeficiency Virus (HIV), is single-stranded RNA (ssRNA). Unlike most viruses, retroviruses use a special enzyme called reverse transcriptase to convert their RNA into DNA once inside the host cell. This viral DNA then integrates into the host's genome, allowing the virus to replicate using the host's cellular machinery. This unique replication strategy is a key feature of retroviruses and is central to the persistence and progression of HIV infection.

3006. Interferons are produced in response to infection of:

• A. Fungi
• B. Tapeworms
• C. Viruses
• D. Bacteria

Explanation: Interferons are produced mainly in response to viral infections because host cells recognize viral nucleic acids, such as double-stranded or unusual single-stranded RNA, through specialized receptors. These signals trigger interferon production, which helps block viral replication, activate natural killer cells, and boost immune defenses. In contrast, fungi, tapeworms, and bacteria do not produce such viral nucleic acids; instead, they stimulate other immune responses like antibody production or phagocytosis. This is why interferons are specific to viral infections.

3007. Most RNA viruses carry a gene for an enzyme that uses viral RNA as a template in the synthesis of more viral RNA. This enzyme is

• A. RNA primase
• B. Reverse transcriptase
• C. RNA polymerase
• D. Viral nuclease

Explanation: The correct option is RNA polymerase. Most RNA viruses carry a gene for the enzyme RNA-dependent RNA polymerase (RdRp), which uses viral RNA as a template to synthesize new RNA molecules. This enzyme is essential because host cells cannot normally replicate RNA directly from an RNA template. RdRp allows RNA viruses to produce both complementary RNA strands and multiple copies of their genome, enabling them to replicate and spread within the host. Other enzymes listed, such as viral nuclease, reverse transcriptase, and RNA primase, serve different functions and are not responsible for RNA-to-RNA synthesis in RNA viruses.

3008. Which of the following statements is correct?

• A. RNA is the primary genetic material in bacteria
• B. RNA is the genetic material for all viruses
• C. DNA is the genetic material for the majority of living organisms
• D. Some viruses use RNA as their genetic material

Explanation: Some viruses use RNA as their genetic material. Viruses can contain either DNA or RNA, depending on the type. Examples of RNA viruses include the influenza virus, HIV, and SARS-CoV-2.Option A: Bacteria use RNA as their genetic material. This is incorrect because bacteria always use DNA, not RNA, as their hereditary material.Option B: All viruses use RNA as their genetic material. This is incorrect because many viruses, such as herpesvirus or adenovirus, have DNA genomes instead.Option C: DNA is the genetic material for most living organisms. While true in general, this statement does not specifically address viruses, so it does not answer the question in context.

3009. Which of the following types of virus is illustrated in the diagram?

• A. Animal virus
• B. Plant virus
• C. Bacteriophage
• D. Viroid

Explanation: The correct option is an animal virus. HIV is a retrovirus that infects human immune cells (CD4+ T cells) and causes AIDS. It has a spherical shape with an RNA genome enclosed in a protein capsid and surrounded by a lipid envelope with glycoprotein spikes, which help it attach to host cells.The options are incorrect: A plant virus, such as Tobacco Mosaic Virus, infects plants and usually has a rod-like or helical shape, not the spherical, enveloped structure seen in HIV. A viroid is even simpler, consisting only of circular RNA without a protein coat, and infects plants, not animals. A bacteriophage has a head-and-tail structure and specifically infects bacteria, which looks very different from HIV. Therefore, the diagram represents an animal virus (HIV).

3010. The enzyme reverse transcriptase, essentially found in all retroviruses, is

• A. DNA-dependent RNA polymerase
• B. DNA-dependent DNA polymerase
• C. RNA-dependent RNA polymerase
• D. RNA-dependent DNA polymerase

Explanation: Reverse transcriptase is an enzyme found in all retroviruses that enables them to replicate by converting their RNA genome into complementary DNA (cDNA). This DNA is then integrated into the host cell's genome, allowing the virus to use the cell's machinery for producing viral proteins and new virus particles. By reversing the normal flow of genetic information (RNA → DNA), reverse transcriptase is essential for viral survival and has also become an important tool in molecular biology research.