All Free Biology MCQs with Answers

4711. The pentose of DNA is:

• A. Ribose
• B. Ribulose
• C. Deoxyribulose
• D. Deoxyribose

Explanation: The correct answer is Deoxyribose. Deoxyribose is a five-carbon sugar and a key component of DNA, making it a deoxyribonucleotide. It lacks an oxygen atom on the 2' carbon compared to ribose, which is found in RNA. Ribose is not part of DNA, and ribulose plays a role in carbohydrate metabolism, not in nucleic acids. Deoxyribulose is not a real sugar and does not exist in this context.

4712. The pentose of RNA is:

• A. Ribose
• B. Ribulose
• C. Deoxyribulose
• D. Deoxyribose

Explanation: The correct answer is Ribose. RNA, or ribonucleic acid, contains ribose as its pentose sugar. Ribose is essential for the structure of RNA, linking with phosphate groups and nitrogenous bases to form the nucleotide structure. In contrast, Deoxyribose is the sugar found in DNA, not RNA. Ribulose is involved in other metabolic pathways and not nucleic acid formation. There is no sugar named 'deoxyribulose', which makes that option invalid.

4713. Single ringed nitrogenous bases are:

• A. Purine
• B. Pyrimidines
• C. Adenine
• D. Guanine

Explanation: The correct answer is pyrimidines. Pyrimidines are one of the two types of nitrogenous bases found in nucleic acids and are distinguished by their single-ring structure. This group includes cytosine, thymine, and uracil. In contrast, purines, which include adenine and guanine, have a two-ring structure and are not single-ringed. Thus, options A, C, and D are incorrect because they refer to purines, which do not match the single-ring criteria.

4714. Pick up the smaller nitrogenous base:

• A. Purines
• B. Adenine
• C. Cytosine
• D. Guanine

Explanation: The correct answer is Cytosine. Pyrimidines, such as cytosine, thymine, and uracil, have a single-ring structure, making them smaller nitrogenous bases compared to purines, which have a two-ring structure. Cytosine is a pyrimidine, hence smaller than purines like adenine and guanine. The other options, adenine and guanine, are purines and are therefore larger in size.

4715. Purines include:

• A. Adenine and cytosine
• B. Adenine and thymine
• C. Adenine and guanine
• D. Adenine and uracil

Explanation: The correct answer is Adenine and guanine. Both adenine and guanine are purines, which are characterized by a two-ring structure. Purines are larger nitrogenous bases compared to pyrimidines, which have a single-ring structure. The other options include one purine and one pyrimidine, which makes them incorrect in identifying purines exclusively.

4716. Pyrimidines found in RNA are:

• A. Cytosine and Thymine
• B. Cytosine and Uracil
• C. Cytosine and Adenine
• D. Cytosine and Guanine

Explanation: RNA contains the pyrimidine bases cytosine and uracil. In RNA, uracil replaces thymine, which is found in DNA. Cytosine is common to both DNA and RNA as a pyrimidine. Options A, C, and D include bases either not found in RNA or are purines rather than pyrimidines.

4717. In a typical nucleotide the nitrogenous base is attached to carbon at:

• A. Position 01 of pentose sugar
• B. Position 05 of pentose sugar
• C. Position 03 of pentose sugar
• D. Position 02 of pentose sugar

Explanation: In nucleotides, the nitrogenous base is always attached to the 1' carbon of the pentose sugar. This is a fundamental aspect of nucleotide structure. The 5' carbon is where the phosphate group binds, forming the backbone of the nucleic acid chain. The 3' carbon is crucial for the formation of phosphodiester bonds in the nucleic acid strand. The 2' carbon differentiates between ribose and deoxyribose sugars, affecting RNA and DNA respectively.

4718. The compound formed by combination of a base and a pentose sugar is called:

• A. Nucleoside
• B. Nucleotide
• C. Nucleic acid
• D. Nuclein

Explanation: The correct answer is nucleoside. A nucleoside is formed when a nitrogenous base combines with a pentose sugar. It is a crucial component in the structure of nucleic acids, serving as a precursor for nucleotides. Nucleotides, on the other hand, also include phosphate groups in addition to the base and sugar. Therefore, the term 'nucleotide' is incorrect for this question because it requires the presence of a phosphate group. 'Nucleic acid' refers to the entire macromolecule composed of nucleotide chains, and 'nuclein' is an outdated term that is not applicable here.

4719. It is an important nucleotide used as an energy currency by the cell:

• A. FAD
• B. NAD
• C. ATP
• D. AMP

Explanation: Adenosine triphosphate (ATP) is the correct answer because it is universally recognized as the primary energy currency of the cell. It stores energy in its high-energy phosphate bonds, which can be readily used to fuel cellular processes. In contrast, FAD and NAD are coenzymes involved in energy metabolism but do not serve as the direct energy currency. AMP is a component of ATP and participates in energy transfer but lacks the high-energy bonds that characterize ATP.

4720. It controls the properties and potential activities of the cell:

• A. DNA
• B. RNA
• C. ATP
• D. AMP

Explanation: DNA is the correct answer because it contains the genetic blueprint that determines the properties and potential activities of a cell. DNA's sequences of nucleotides encode the information necessary for building and maintaining an organism, thus controlling all cellular functions. RNA, ATP, and AMP are important molecules within the cell, with RNA involved in protein synthesis and ATP and AMP in energy transfer, but they do not hold the primary role of controlling cell properties and activities.