All Free Biology MCQs with Answers

2941. Which virus that infects humans has no vector and is unable to survive outside the human body?

• A. Polio virus
• B. Mumps virus
• C. Chickenpox virus
• D. All of these

Explanation: All viruses cause infection in human beings and are transmitted by specific vectors. Viruses always depend on living hosts for their survival.

2942. Which of the following are the simplest forms of pathogens causing disease?

• A. Viruses
• B. Prions
• C. Fungus
• D. Amoeba

Explanation: Prions are the simplest form of pathogens because they are only proteins structurally. Viruses are nucleoproteins. Fungi are multicellular and eukaryotic organisms. An amoeba is a eukaryotic and unicellular organism.

2943. Attachment of HIV DNA with host DNA is done via the action of:

• A. lntegrase
• B. Reverse transcriptase
• C. Protease
• D. Nuclease

Explanation: HIV integration is the insertion of HIV genetic material into the genome of the infected cell. This process is completed with the help of integrase.

2944. Uncoating of the HIV virion occurs:

• A. Outside the cell
• B. In cytoplasm
• C. In nucleus
• D. At any place

Explanation: Uncoating of the HIV virion occurs in the cytoplasm of the host cell shortly after the viral envelope fuses with the host plasma membrane. Following attachment to the CD4 receptor and a co-receptor, the viral envelope merges with the cell membrane, allowing the viral core, or capsid, to enter the cytoplasm. The capsid then undergoes controlled disassembly, releasing the viral RNA genome along with essential enzymes such as reverse transcriptase and integrase. This uncoating process is closely linked to reverse transcription, as the viral RNA is converted into DNA while the disassembly proceeds gradually, often as the reverse transcription complex moves toward the nucleus. Thus, uncoating is an early cytoplasmic event that prepares the viral genome for integration into the host DNA.

2945. Receptor proteins present on T helper lymphocytes where an HIV particle binds:

• A. CD4
• B. CD8
• C. CD2
• D. CD1

Explanation: CD means cluster of differentiation. There are many types of CD proteins, such as CD4 and CD8, attached to the cell membranes of different types of T cells. HIV attaches to CD4, which is part of the T helper cell membrane.

2946. Which of the following cells does HIV mainly infect?

• A. T-killer lymphocytes
• B. B-plasma cells
• C. T-helper lymphocytes
• D. B-memory cells

Explanation: HIV primarily targets immune cells that play a central role in defending the body against infections. The virus mainly infects CD4+ T helper cells, which are a type of white blood cell essential for coordinating the immune response. By binding to the CD4 receptor on these cells, along with co-receptors such as CCR5 or CXCR4, HIV gains entry and begins replicating inside the cell. Over time, the depletion of CD4+ T cells weakens the immune system, making the body more susceptible to opportunistic infections and illnesses. In addition to CD4+ T cells, HIV can also infect other cells expressing the CD4 receptor, such as macrophages and dendritic cells, but the primary and most critical target remains the CD4+ T helper cells.

2947. The main target of HIV in AIDS is

• A. Liver cells
• B. B-lymphocytes
• C. Red blood cells
• D. Helper T-lymphocytes

Explanation: HIV, the virus responsible for AIDS, primarily targets the immune system, specifically a type of white blood cell called the CD4+ T lymphocyte, also known as helper T cells. These cells play a crucial role in coordinating the body's immune response by signaling other immune cells to fight infections. When HIV infects these cells, it integrates its genetic material into the host cell and gradually destroys them, reducing their numbers over time. This weakening of the immune system leaves the body vulnerable to opportunistic infections and certain cancers, which are hallmarks of AIDS. The progressive loss of CD4+ T cells is therefore central to the development and severity of the disease.

2948. Numerous opportunistic diseases can affect individuals with the following conditions.

• A. Measles
• B. Influenza
• C. Hepatitis A
• D. AIDS

Explanation: Among the given options, the correct answer is d. AIDS. Measles, influenza, and hepatitis A can temporarily weaken the immune system, increasing susceptibility to other infections, but they are not classified as opportunistic infections. In contrast, AIDS (Acquired Immunodeficiency Syndrome), caused by HIV, leads to a profound loss of CD4⁺ T cells and severe immunodeficiency. This immunosuppressed state is specifically characterized by the occurrence of opportunistic infections such as tuberculosis, pneumonia, candidiasis, and other fungal, bacterial, and viral diseases. Thus, unlike measles, influenza, and hepatitis A, AIDS is directly associated with opportunistic infections due to its defining immune system damage.

2949. Which of the following is not true about the human immunodeficiency virus?

• A. Retrovirus
• B. It is surrounded by an envelope
• C. It does not cause AIDS
• D. It causes a deficiency in the human immune system

Explanation: HIV is the virus that causes AIDS (Acquired Immunodeficiency Syndrome). It attacks and weakens the immune system over time, leading to the development of AIDS in untreated individuals. It is also an enveloped virus.

2950. In which step of the life cycle of a bacteriophage does the tail release the enzyme lysozyme?

• A. Attachment
• B. Adsorption
• C. Penetration
• D. Injection

Explanation: The correct answer is penetration. During the penetration phase of the bacteriophage life cycle, lysozyme is released to break down the bacterial cell wall, enabling the viral genetic material to enter the host cell. The other options are incorrect because:Attachment: The bacteriophage only attaches to the bacterial surface, with no lysozyme release.Adsorption: This step involves binding to the bacterial cell but not the release of lysozyme.Injection: Although DNA is injected into the host cell, lysozyme is released during penetration, not injection.