Free Biological Molecules MCQs with Answers

11. Which statement about RNA is correct?

• A. It contains thymine in place of uracil
• B. It contains ribose and is usually single stranded
• C. It is always double stranded like DNA
• D. It contains deoxyribose and uracil

Explanation: RNA carries ribose, which has a hydroxyl group at carbon 2, and it is normally a single strand that can fold back on itself, as tRNA does into its clover leaf shape. Uracil replaces thymine in RNA, not the other way round, and deoxyribose belongs to DNA. The extra hydroxyl is also why RNA is chemically less stable than DNA.

12. In a double stranded DNA molecule, adenine pairs with thymine through

• A. one hydrogen bond
• B. two hydrogen bonds
• C. three hydrogen bonds
• D. a covalent bond

Explanation: Adenine and thymine are held together by two hydrogen bonds, while guanine and cytosine are held by three. That difference is why a DNA region rich in G and C needs a higher temperature to separate the strands, a fact used directly when designing PCR conditions. The covalent bonds in DNA run along each backbone, not across the strands.

13. If one strand of a DNA molecule contains 22% adenine, what percentage of the whole molecule is guanine?

• A. 22%
• B. 28%
• C. 44%
• D. 56%

Explanation: Chargaff's rule applies to the double stranded molecule, so A equals T and G equals C. With A at 22%, T is also 22%, leaving 100 minus 44 which is 56% shared equally between G and C, giving 28% guanine. The common error is to stop at 56% and forget that it is split between two bases.

14. The two strands of a DNA double helix are described as antiparallel, which means that

• A. they carry identical base sequences
• B. one runs 5' to 3' while the other runs 3' to 5'
• C. they are held together by covalent bonds
• D. they coil in opposite directions around each other

Explanation: Antiparallel refers to the direction of the sugar-phosphate backbones: one strand runs 5' to 3' while its partner runs 3' to 5'. This is why replication is continuous on the leading strand and discontinuous, in Okazaki fragments, on the lagging strand. The strands are complementary rather than identical, and they are held together by hydrogen bonds.

15. A conjugated molecule such as a glycoprotein consists of

• A. two identical monomers joined together
• B. a protein combined with a non-protein group
• C. a lipid combined with another lipid
• D. a polysaccharide broken into monosaccharides

Explanation: Conjugated molecules are built from two different classes of biological molecule joined together, so a glycoprotein is protein plus carbohydrate and a lipoprotein is protein plus lipid. The cell surface is coated with such molecules, and they are what allow cells to recognise one another. Two identical monomers joined together would simply be a dimer of a single class.

16. Which class of organic molecule forms the greatest proportion of the dry mass of a typical cell?

• A. Carbohydrates
• B. Proteins
• C. Lipids
• D. Nucleic acids

Explanation: Proteins account for roughly half the dry mass because they serve as enzymes, transporters, receptors, antibodies and structural fibres all at once, so a cell needs thousands of different ones. Carbohydrates and lipids are present in large amounts too but fill a narrower range of roles, mainly energy and structure. Nucleic acids are a small fraction by mass despite controlling everything else.

17. Which of the following is NOT a macromolecule?

• A. Starch
• B. Glycogen
• C. Glucose
• D. Cellulose

Explanation: Glucose is a monosaccharide, a single small unit and therefore a micromolecule, while the other three are polysaccharides built from thousands of glucose units joined by glycosidic bonds. Macromolecules are polymers assembled from repeating monomers by condensation, so the test is whether the molecule can be hydrolysed into smaller repeating units. Glucose cannot.

18. The four elements that together make up about 99 per cent of the mass of living matter are

• A. carbon, hydrogen, oxygen and nitrogen
• B. carbon, hydrogen, oxygen and sulphur
• C. sodium, potassium, calcium and chlorine
• D. carbon, nitrogen, phosphorus and sulphur

Explanation: Carbon, hydrogen, oxygen and nitrogen are the backbone of carbohydrates, lipids, proteins and nucleic acids, with phosphorus and sulphur adding roughly another one per cent. Sodium, potassium and calcium are essential but are needed only in small quantities as ions. Carbon is central because it forms four stable covalent bonds and so builds long chains and rings.

19. The monomer unit from which a nucleic acid is built is

• A. an amino acid
• B. a monosaccharide
• C. a nucleotide
• D. a fatty acid

Explanation: Each nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base, and successive nucleotides are linked by phosphodiester bonds between the sugar of one and the phosphate of the next. Amino acids are the monomers of proteins and monosaccharides those of polysaccharides. Fatty acids are components of lipids, which are not true polymers at all.

20. Water makes up approximately what proportion of the fresh mass of a typical living cell?

• A. about 10 per cent
• B. about 30 per cent
• C. about 50 per cent
• D. 70 to 90 per cent

Explanation: Most cells are between seventy and ninety per cent water, which is why it is treated as the medium of life rather than as one nutrient among others. Every metabolic reaction occurs in aqueous solution, water is itself a reactant in hydrolysis and a product of condensation, and it transports dissolved substances. Seeds and spores are the low water exceptions, and their metabolism is correspondingly almost stopped.