All Free Biology MCQs with Answers

3021. Viruses are classified into many groups based on:

• A. Nucleic acid
• B. Capsid symmetry
• C. Host cell infectivity
• D. All of these

Explanation: Viruses are extremely diverse and do not fit into the typical classification systems used for cellular organisms. To properly study and identify them, scientists classify viruses based on several important characteristics:Nucleic acid type (DNA or RNA, single-stranded or double-stranded) helps determine their replication strategies and genetic properties.Capsid symmetry (helical, icosahedral, or complex) gives insight into their structure and stability.Host cell infectivity distinguishes viruses based on the organisms they infect, such as bacteria (bacteriophages), plants (plant viruses), or animals/humans (animal viruses).

3022. Viruses use which of the following enzymes for the breakdown of the bacterial cell wall?

• A. Lysozyme
• B. Lipases
• C. Proteases
• D. Nucleases

Explanation: Bacteriophages use lysozyme to destroy the bacterial cell wall because peptidoglycan is made of alternating NAG-NAM sugars linked by β-1,4 glycosidic bonds, and lysozyme specifically hydrolyzes these bonds, causing lysis. Lipases, proteases, and nucleases are ineffective since they act on lipids, proteins, and nucleic acids, respectively, which are not the main structural components of the bacterial wall. For example, just as a key is designed to open a specific lock, lysozyme is the precise "key" for breaking peptidoglycan, while the other enzymes do not fit this purpose.

3023. Viral proteins and the genome of enveloped viruses in the host cell are assembled at:

• A. Plasma membrane
• B. Endoplasmic Reticulum Lumen
• C. Host cell ribosomes
• D. Host cell mitochondria

Explanation: Among the given options, the correct site of viral assembly is the plasma membrane or the site of viral replication within the host cell. Ribosomes are important for synthesizing viral proteins but do not carry out assembly. The endoplasmic reticulum lumen plays a role in processing and glycosylating viral membrane proteins but is not the final assembly site. Mitochondria provide energy to the cell but are not directly involved in viral replication or assembly. Therefore, only Option A correctly identifies the site where viral proteins and the genome come together to form complete viral particles.

3024. Viruses can bud from:

• A. Mitochondrial membrane
• B. Ribosomes
• C. Lysosomal membrane
• D. Plasma Membrane

Explanation: Viruses acquire their envelopes by budding through host cell membranes, with the plasma membrane being the primary site for many enveloped viruses like HIV and influenza. This allows the virus to incorporate host-derived lipids along with its own viral proteins into the envelope. In contrast, mitochondria are metabolic organelles and not used for budding, ribosomes lack membranes and only make proteins, and lysosomes are degradative organelles rather than exit sites. Therefore, the correct answer is that viruses form buds from the plasma membrane, while the other options are incorrect.

3025. _ refers to the breakdown or removal of the capsid.

• A. Assembly
• B. Uncoating
• C. Integration
• D. Maturation

Explanation: Option A: Assembly-The stage where viral genomes and structural proteins come together inside the host cell to form new virions.Option B: Uncoating-The step where the viral capsid is disassembled after entry, releasing the viral genome into the host cell. Triggered by receptor signals, enzymes, or pH changes.Option C: Integration-The process in which certain viruses (e.g., retroviruses) insert their genome into the host's DNA, allowing long-term persistence.Option D: Maturation-Final modifications of virions to become infectious, often involving proteolytic cleavage or envelope acquisition.

3026. Which viruses enter the host cell as a whole?

• A. Plant virus
• B. Bacteriophages
• C. Animal virus
• D. None of these

Explanation: Animal viruses can enter host cells as whole particles because animal cells lack rigid walls, using processes like membrane fusion or endocytosis before uncoating their genome. In contrast, plant viruses cannot enter as complete virions due to the tough cell wall; they depend on wounds or insect vectors to introduce only their genome. Bacteriophages also do not enter entirely-they attach to bacteria and inject their genetic material while the capsid remains outside.

3027. The branch that deals with the study of viruses is known as what?

• A. Entomology
• B. Virology
• C. Epidemiology
• D. Mycology

Explanation: Virology is the scientific discipline concerned with the study of the biology of viruses and viral diseases, including the distribution, biochemistry, physiology, molecular biology, ecology, evolution, and clinical aspects of viruses. While all other options are incorrect.

3028. According to lwanowsky what are soluble living germs?

• A. Bacteria
• B. Viruses
• C. Fungi
• D. Both A and B

Explanation: According to Dmitri Ivanovsky (1892), the discoverer of the first virus (tobacco mosaic virus), the infectious agent that passed through porcelain filters (which normally trapped bacteria) was described as "soluble living germs."By this, he meant that the disease-causing agent was so small it could pass through bacterial filters, appearing to be a contagious, soluble substance rather than a typical microorganism. Later, this "soluble living germ" was recognized as a virus.

3029. When was the bacteriophage phenomenon rediscovered by D'Herelle?

• A. 1918
• B. 1917
• C. 1920
• D. 1990

Explanation: Félix d'Hérelle, a French-Canadian microbiologist, is credited with the discovery of bacteriophages and the recognition of the phenomenon known as phage therapy. He first observed the lytic activity against bacteria in 1917, during his work at the Pasteur Institute in Paris. D'Hérelle's groundbreaking discovery of bacteriophages and their ability to infect and kill bacteria laid the foundation for the field of virology.

3030. Virus that was discovered in 1901:

• A. Yellow fever
• B. Tobacco mosaic
• C. Bacteriophages
• D. Corona

Explanation: The yellow fever virus originated in Africa and was brought to the Western Hemisphere during the slave trade era, with the first epidemic reported in 1901 in the Yucatan.