All Free Biology MCQs with Answers

2721. Which statement about bacteria is true:

• A. Gram-positive bacteria have more lipids in their cell wall
• B. Gram-negative bacteria have more lipids in their cell wall
• C. Lipids are absent in the cell wall of both gram-positive and negative bacteria
• D. Both have an equal amount of lipids

Explanation: The key difference in the cell walls of gram-positive and gram-negative bacteria is the presence of an outer membrane in gram-negative bacteria, which contains lipids and contributes to their overall structure and properties.

2722. RNA viruses appear to have higher rates of mutation because:

• A. RNA nucleotides are more unstable than DNA nucleotides
• B. Replication of their nucleic acid does not involve the proofreading steps of DNA replication
• C. RNA viruses replicate faster
• D. RNA viruses respond more to mutagens

Explanation: The primary reason for the higher mutation rate in RNA viruses is that their replication process lacks the proofreading mechanisms that are present in DNA replication. DNA replication includes proofreading and error-correcting mechanisms that help maintain the accuracy of the genetic code. RNA viruses, on the other hand, use RNA-dependent RNA polymerases (enzymes responsible for copying RNA) that lack the ability to correct errors. As a result, mistakes made during replication are more likely to become permanent mutations in the viral genome.

2723. Reverse transcriptase is:

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

Explanation: The term "reverse transcriptase" refers to an enzyme found in retroviruses (a type of RNA virus) that can synthesize a complementary DNA strand (cDNA) from an RNA template. This is a unique process because it involves the synthesis of DNA from an RNA template, which is the reverse of the typical flow of genetic information.

2724. Which one of the following statements about viruses is correct?

• A. Viruses possess their own metabolic system
• B. Viruses are obligate parasite
• C. All viruses contain both RNA and DNA
• D. The nucleic acid of viruses is known as capsid

Explanation: Viruses are obligate parasites, meaning they require a host cell to replicate and complete their life cycle.

2725. Bacteria that convert nitrates into free nitrogen are:

• A. Ammonifying
• B. Denitrifying
• C. Nitrifying
• D. Nitrogen-fixing bacteria

Explanation: Denitrifying bacteria are responsible for converting nitrates (NO3-) into free nitrogen gas (N2) through a process known as denitrification. This process helps to return nitrogen in the environment back to its atmospheric form, completing the nitrogen cycle. Denitrification occurs in anaerobic conditions (environments with little to no oxygen), where these bacteria use nitrates as an alternative source of oxygen, leading to the production of nitrogen gas.

2726. All are the characteristics of Gram-positive bacteria except:

• A. High content of peptidoglycan
• B. High permeability
• C. Teichoic acid is present
• D. Stain pink with primary dye

Explanation: Gram-positive bacteria stain purple with the primary dye in the Gram staining procedure. This is due to their thick peptidoglycan layer that retains the crystal violet dye during the staining process. Gram-negative bacteria, on the other hand, stain pink due to the thinner peptidoglycan layer and the presence of an outer membrane that can be penetrated by the counterstain safranin.

2727. Bacteria differ from viruses in:

• A. Pathogenic nature
• B. Having well-defined cytoplasm
• C. Genetic material
• D. Lacking proper nucleus

Explanation: Bacteria are unicellular organisms that have well-defined cellular structures, including a well-defined cytoplasm. They have cell membranes, cytoplasm, ribosomes, and other cellular components that are essential for carrying out various life processes. In contrast, viruses are not cells and lack cellular structures like cytoplasm. Instead, viruses consist of genetic material (DNA or RNA) surrounded by a protein coat.

2728. A bacterium divides every 36 minutes. If a culture containing 105 cells/ml is grown for 180 minutes, what will be the cell concentration after that period?

• A. 175 x 10^5
• B. 5 x 10^5
• C. 35 x 10^5
• D. 32 x 10^5

Explanation: To calculate the cell concentration after a certain period of growth, you can use the formula: Final cell concentration = Initial cell concentration × 2(number of divisions) In this case, the bacterium divides every 36 minutes, so it doubles its population every 36 minutes. The initial cell concentration is 105 cells/ml, and the growth period is 180 minutes. Number of divisions = Growth period / Division time = 180 minutes / 36 minutes/division = 5 divisions. Now, plug the values into the formula: Final cell concentration = 105 cells/ml × 25 = 105 cells/ml × 32 = 32 x 105 cells/ml. So, the correct answer is 32 x 105 cells/ml.

2729. Which vitamin is synthesized by bacteria in the human gut?

• A. A
• B. D
• C. C
• D. K

Explanation: Vitamin K is synthesized by bacteria in the human gut. Specifically, the bacteria found in the colon are responsible for producing vitamin K. This vitamin plays a crucial role in blood clotting and bone health.

2730. In case of HIV, only_ can infect host cells.

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

Explanation: While the original genetic material of HIV is single-stranded RNA, the virus undergoes a process called reverse transcription. During this process, the single-stranded RNA is converted into a complementary single-stranded DNA (cDNA) molecule. This cDNA is then integrated into the host cell's genome, forming a double-stranded DNA structure that can infect host cells.