All Free Biology MCQs with Answers

2351. Reverse transcriptase is present in____________?

• A. All RNA viruses
• B. All DNA viruses
• C. All retroviruses
• D. All viruses

Explanation: Retroviruses contain reverse transcriptase, which copies their RNA genome into DNA so it can integrate into the host-cell genome. Students may choose all RNA viruses because the enzyme is associated with an RNA genome, but most RNA viruses replicate without reverse transcription.

2352. Viruses are considered living organisms because__________?

• A. They can be crystallized
• B. They occur in plants and animals
• C. They are microscopic
• D. They contain DNA and can be reproduced

Explanation: Viruses contain genetic material, either DNA or RNA, and can make new virus particles inside a suitable living host cell. Crystallization, microscopic size, and occurrence in plants and animals do not prove that something is living. Students most often choose option a because they confuse the ability to crystallize with a characteristic of life, although crystallization is evidence of viruses' non-cellular nature.

2353. Sex chromosomes present in female are _________?

• A. XY
• B. XX
• C. XXY
• D. XXX

Explanation: In the usual human chromosome pattern, females have two X sex chromosomes, written XX, while males have XY. Students may choose XY because it is the other common human sex-chromosome combination, but XY describes a typical male rather than a female.

2354. Which bacteriophage resembles a tadpole?

• A. T2
• B. T4
• C. Both A and B
• D. None

Explanation: The bacteriophage that resembles a tadpole is the T-even phage, such as the the T2, T4, and T6 phages. These phages have a distinct structure consisting of a large polyhedral head attached to a long, slender tail. The head contains the viral DNA, while the tail serves as a tubular structure used to inject this genetic material into the bacterial host. When viewed under an electron microscope, this head-and-tail arrangement closely resembles the body and tail of a tadpole, which is why they are described in this way. This unique morphology not only gives them their characteristic appearance but also plays a crucial role in their infection cycle, as the head stores genetic information and the tail enables attachment and DNA transfer into the host bacterium.

2355. Which type of viruses infect E. coli bacteria?

• A. T phages
• B. P phages
• C. Both A and B
• D. None

Explanation: T phages carry out a lytic life cycle, which means they directly infect the bacteria and immediately kill it; however, P phages carry out a lysogenic life cycle, due to which they do not cause immediate cell lysis.

2356. What is the phage that causes the lytic cycle called?

• A. Virulent phage
• B. Lytic phage
• C. Temperate phage
• D. Both a and b

Explanation: A lytic (virulent) phage infects a bacterium, injects its DNA, uses the host machinery to produce new phages, and causes the cell to burst (lysis), releasing many new particles. In contrast, a temperate phage can follow either the lytic cycle or the lysogenic cycle, where its DNA integrates into the host chromosome as a prophage without immediately killing the cell.

2357. Where does the bacteriophage replicate?

• A. Human
• B. Horse
• C. Bacteria
• D. Animal

Explanation: Bacteriophages replicate inside bacterial cells.They attach to the bacterium, inject their genetic material, use the host's machinery to make new phage DNA and proteins, assemble new viruses, and then release them (often by lysing the bacterial cell).

2358. Which of the following is the simplest form of pathogens causing diseases?

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

Explanation: Prions are the simplest disease-causing pathogens because they are merely misfolded proteins with with no nucleic acid (no DNA or RNA), making them the most minimal infectious agents compared to viruses, fungi, or protozoa like amoeba.

2359. In which step is lysozyme released by the bacteriophage?

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

Explanation: Lysozyme is a digestive enzyme, so it will have to be released when the bacterial cell wall has to be digested, which occurs during the process of 'penetration.'

2360. How many bacteriophages are formed after 25 minutes of initial infection?

• A. 50
• B. 200
• C. 10
• D. 1000

Explanation: After 25 minutes of initial infection, about 200 bacteriophages are formed, which corresponds to the burst size of bacteriophage T4. Options with much smaller or much larger numbers (10, 50, 1000) are incorrect because they do not match the typical replication output of bacteriophages.