All Free Biology MCQs with Answers

2991. CD4 receptors are present on:

• A. T-lymphocytes
• B. Helper T-lymphocytes
• C. B-lymphocytes
• D. HIV

Explanation: CD4 receptors are primarily present on the surface of CD4+ T lymphocytes (helper T cells), which play a central role in coordinating the immune response. These receptors are also found on monocytes, macrophages, and dendritic cells, which are important immune cells involved in antigen presentation and immune regulation. The CD4 receptor serves as the primary attachment site for HIV, allowing the virus to recognize, bind, and enter host cells. By interacting with CD4 and a co-receptor (CCR5 or CXCR4), HIV gains access to the host cell's machinery for replication. The presence of CD4 receptors on these immune cells explains why HIV selectively targets them, leading to a progressive decline in immune function and the development of AIDS.

2992. All are present in HIV except:

• A. Lipids
• B. DNA
• C. RNA
• D. Protein

Explanation: HIV (Human Immunodeficiency Virus) is an enveloped retrovirus with a complex structure that reflects its unique characteristics. The virus has a lipid envelope derived from the host cell membrane, which contains viral glycoproteins (gp120 and gp41) that are essential for attaching to and entering CD4+ T cells. Inside the envelope, HIV contains two identical single-stranded RNA molecules, which serve as its genetic material. The virus also carries enzymes like reverse transcriptase, integrase, and protease necessary for replication, which are protein components. Additionally, structural proteins form the capsid (p24) and matrix (p17), protecting the RNA and assisting in viral assembly. HIV does not contain DNA initially; rather, the RNA is converted into DNA by reverse transcriptase after entering the host cell. This combination of lipids, RNA, and proteins allows HIV to infect host cells efficiently and replicate within them.

2993. What is the shape of the TMV?

• A. Rod
• B. Conical
• C. Tadpole
• D. Spherical

Explanation: The tobacco mosaic virus (TMV) is a rod-shaped virus that is about 300 nanometers (nm) long and 18 nm in diameter.

2994. All are characteristics of the polio virus except:

• A. Small
• B. Spherical
• C. Acellular
• D. DNA

Explanation: Poliovirus is a small, non-enveloped RNA virus belonging to the family Picornaviridae. Its size is approximately 22-30 nanometers in diameter, making it one of the smallest animal viruses. Poliovirus has a single-stranded positive-sense RNA genome, about 7,500 nucleotides long, which encodes a single large polyprotein that is subsequently cleaved into structural and non-structural proteins necessary for viral replication. The virus's small size and simple RNA genome allow it to efficiently infect host cells, replicate rapidly, and cause diseases such as poliomyelitis, which can lead to paralysis in severe cases.

2995. The agent responsible for mad cow infection is

• A. Virus
• B. Vision
• C. Bacteria
• D. Prion

Explanation: Mad cow disease, also known as bovine spongiform encephalopathy (BSE), is a fatal neurodegenerative disorder that affects cattle. It is caused by prions, which are misfolded proteins capable of inducing abnormal folding of normal proteins in the brain. This leads to the formation of sponge-like lesions, resulting in progressive neurological symptoms such as abnormal behavior, lack of coordination, tremors, and difficulty walking. BSE can be transmitted to humans through the consumption of contaminated beef, causing a variant form of Creutzfeldt-Jakob disease (vCJD), which similarly affects the brain and is ultimately fatal. The disease has no cure, and prevention relies on strict control of animal feed and surveillance of livestock.

2996. The first crystallized virus is

• A. TMV
• B. Bacteriophage
• C. Herpes
• D. HBV

Explanation: The first virus to be crystallized was the Tobacco Mosaic Virus (TMV), discovered by Wendell Stanley in 1935. TMV is a plant virus that infects tobacco and other members of the Solanaceae family, causing characteristic mosaic-like patterns on the leaves. Stanley was able to isolate and crystallize the virus, demonstrating that it was a distinct chemical entity composed of protein and RNA, rather than a simple toxin or living organism. This groundbreaking work provided the first clear evidence that viruses could be studied as biochemical particles, laying the foundation for modern virology and molecular biology.

2997. A viroid is infectious._.

• A. Protein
• B. DNA
• C. Nucleoprotein
• D. RNA

Explanation: Viroids are the smallest known infectious agents, consisting solely of a short, circular strand of single-stranded RNA without any protein coat or envelope. Unlike viruses, viroids do not encode proteins and rely entirely on the host plant's cellular machinery for replication. They primarily infect plants, causing diseases by interfering with normal gene regulation and cellular processes. Viroids are capable of autonomous replication within the host but lack the structural components of viruses, such as capsids or envelopes, making them unique among infectious agents. Their simplicity and high replication rate can lead to significant agricultural losses in infected crops.

2998. Reverse transcriptase is an enzyme contained in:

• A. All viruses
• B. Bacteriophages
• C. Retroviruses
• D. Plant viruses

Explanation: Reverse transcriptase is an enzyme contained in retroviruses, such as HIV (Human Immunodeficiency Virus). This enzyme allows the virus to convert its RNA genome into DNA after entering a host cell, a process called reverse transcription. The newly formed viral DNA is then integrated into the host cell's genome, enabling the virus to use the host's machinery to produce new viral particles. Reverse transcriptase is essential for the replication of retroviruses and is a key target for antiviral drugs that aim to inhibit HIV replication.

2999. AIDS will lead to:

• A. Liver cirrhosis
• B. Mental retardation
• C. Opportunistic infections
• D. Angina

Explanation: The consequences of AIDS arise from the severe weakening of the immune system due to the loss of CD4+ T cells. This leads to increased susceptibility to opportunistic infections such as tuberculosis, pneumonia, and fungal infections, as well as a higher risk of certain cancers like Kaposi's sarcoma and lymphoma. Other effects include chronic weight loss, persistent diarrhea, neurological complications, and general debility, which together can become life-threatening if the disease is untreated.

3000. In a newly discovered virus you might find:

• A. DNA
• B. RNA
• C. DNA or RNA
• D. DNA and RNA

Explanation: In viruses, the type of genetic material-either RNA or DNA-is a key feature that determines their classification. A newly discovered virus will carry only one type of nucleic acid, never both, which serves as the template for producing viral proteins and replicating the viral genome. RNA viruses use RNA as their genetic material, while DNA viruses use DNA. This distinction is fundamental because it influences the virus's replication strategy, mutation rate, and interaction with the host cell. For example, RNA viruses often replicate in the cytoplasm and may have higher mutation rates, whereas DNA viruses usually replicate in the nucleus and are more stable. Therefore, identifying whether a virus contains RNA or DNA is one of the first steps in understanding its biology and classifying it.