Biology Question Bank – 35 MCQs on “Biomolecules” – Answered!

35 Questions with Answers and Explanations on “Biomolecules” for Biology Students.

1. In double helix of DNA, the two DNA strands are

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(a) coiled around a common axis

(b) coiled around each other

(c) coiled differently

(d) coiled over protein sheath.


Answer and Explanation:

1. (a): According to Watson-Crick model, the DNA molecule consists of two long, parallel chains which are joined together by short crossbars at regular intervals. The two chains are spirally coiled around a common axis in a regular manner to form a right handed double helix.

2. RNA does not possess

(a) uracil


(b) thymine

(c) adenine

(d) cytosine.

Answer and Explanation:

2. (b): DNA and RNA both consist of nitrogenous bases-purines and pyrimidines. The purines (adenine and guanine) in DNA and RNA are the same. The pyrimidine, cytosine is same in both while the other pyrimidine thymine of DNA is replaced by uracil in RNA.

3. Which is not consistent with double helical structure of DNA?

(a) A = T, C = G

(b) density of DNA decreases on heating

(c) A + T/C + G is not constant

(d) both a and b.

Answer and Explanation:

3. (c): The density of DNA decreases on heating as hydrogen bonds breakdown. According to Chargaff’s rules, the amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine i.e., A = T and G = C. The base ratio A + T/ G + C may vary from species to species, but is constant for a given species

4. A nucleotide is formed of

(a) purine, pyrimidine and phosphate

(b) purine, sugar and phosphate

(c) nitrogen base, sugar and phosphate

(d) pyrimidine, sugar and phosphate.

Answer and Explanation:

4. (b): Nucleotide is an organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose) and a phosphate group.

5. Mineral associated with cytochrome is

(a) Cu

(b) Mg

(c) Cu and Mg

(d) Fe.

Answer and Explanation:

5. (d): Cytochromes are generally membrane-bound hemoproteins that contain heme groups and carry out electron transport. The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding a metal ion, which readily interconverts between the oxidation states.

For many cytochromes the metal ion present is that of iron, which interconverts between Fe2+ (reduced) and Fe3+ (oxidized) states (electron-transfer processes) or between Fe2+ (reduced) and Fe3+ (formal, oxidized) states (oxidative processes). Cytochromes are thus capable of performing oxidation and reduction.

6. In RNA, thymine is replaced by

(a) adenine

(b) guanine

(c) cytosine

(d) uracil.

Answer and Explanation:

6. (d): Refer answer 2.

7. The basic unit of nucleic acid is

(a) pentose sugar

(b) nucleoid

(c) nucleoside

(d) nucleotide.

Answer and Explanation:

7. (d): The nucleic acids (DNA and RNA) are the molecules having complex structure and very high molecular weights. The nucleic acid is composed of a large number of nucleotide molecules joined into a linear, unbranched chain. Nucleotide is an organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose) and a phosphate group.

8. A segment of DNA has 120 adenine and 120 cytosine bases. The total number of nucleotides present in the segment is

(a) 120

(b) 240

(d) 480.

Answer and Explanation:

8. (d): According to Chargaff’s rules, the amount of adenine is always equal to that of thymine, and the amount of guanine is always equal to that of cytosine i.e A = T (120) and G = C (120), therefore, the total no. of nucleotides would be 120 x 4 = 480.

9. DNA is composed of repeating units of

(a) ribonucleosides

(b) deoxyribonucleosides

(c) ribonucleotides

(d) deoxyribonucleotides.

Answer and Explanation:

9. (d): DNA is the largest macromolecule in the organisms. It is a long, double” chain of deoxyribonucleotide, or deoxyribotide, units. The two deoxyribonucleotide chains are twisted around a common axis to form a right-handed double helix (spiral) that encloses a cylindrical space in it. Each deoxyribonucleotide unit, in turn, consists of three different molecules: phosphate, (P034), a 5-carbon deoxyribose sugar (C3Hi0O4) and a nitrogenous base.

10. A nucleotide is formed of 95% water. Water present

(a) Purine, pyrimidine and phosphate

(b) Purine, sugar and phosphate

(c) Nitrogen base, sugar and phosphate

(d) Pyrimidine, sugar and phosphate.

Answer and Explanation:

10. (c): Nucleotide is an organic compound consisting of a nitrogen-containing purine or pyrimidine base linked to a sugar (ribose or deoxyribose) and a phosphate group.

11. Adenine is

(a) purine

(b) pyrimidine

(c) nucleoside

(d) nucleotide.

Answer and Explanation:

11. (a): The nitrogenous bases are the constituents of nucleic acids. The nitrogenous bases may be purines or pyrimidines. Purines are the 9-membered double rings with nitrogen at 1, 3, 7 and 9 positions. The two purines are adenine and guanine.

12. Living cell contains 60 – in human body is

(a) 60 – 65%

(b) 50 – 55%

(c) 75 – 80%

(d) 65 – 70%.

Answer and Explanation:

12. (d): Water is the most abundant substance of living beings. The water content of actively living cells varies between 60-95%. In human beings maximum water content is found in the embryo 90 – 95%. Water content decreases thereafter in adult and the aged where it is 65 – 70%.

13. Amino acids are produced from

(a) proteins

(b) fatty acids

(c) essential oils

(d) a-keto acids.

Answer and Explanation:

13. (a): Proteins are large-sized polymeric macromolecules having one or more polypeptides (chains of amino acids). Amino acids are formed after digestion of proteins. Amino acids are organic acids (with carboxylic group – COOH) having amino group (-NH2) generally attached to a-carbon or carbon next to the carboxylic group. Carboxylic group provides an acidic property to the amino acid while amino group gives it a basic reaction.

14. Which is distributed more widely in a cell?

(a) DNA

(b) RNA

(c) chloroplasts

(d) sphaerosomes.

Answer and Explanation:

14. (b): RNA occurs in the nucleus as well as in the cytoplasm of the eukaryotic cells and in prokaryotic cell, it is found in the cytoplasm. DNA is found in the nucleus, mitochondria and chloroplast. Chloroplast and sphaerosomes are found only in cytoplasm.

15. Amino acids are mostly synthesised from

(a) mineral salts

(b) fatty acids

(c) volatile acids

(d) a-ketoglutaric acid.

Answer and Explanation:

15. (d): Amino acids are mostly synthesized from a-ketogluaric acid. These are the precursors of amino acids. A five carbon compound formed during kreb’s cycle is a a-ketoglutaric acid which is the first diearboxylic acid formed. Pyruvic acid converted into alanine, a-ketoglutaric acid into glutamic acid, OA’A into aspartic acid. Polymerization of such amino acids results into formation of proteins.

16. In RNA, thymine is replaced by

(a) adenine

(b) guanine

(c) cytosine

(d) uracil.

Answer and Explanation:

16. (d): In RNA and DNA, purines are the same. The two purines are adenine and guanine. The two pyrimidines are cytosine and thymine in DNA but in RNA thymine is replaced by uracil.

17. Glycogen is a polymer of

(a) galactose

(b) glucose

(c) fructose

(d) sucrose.

Answer and Explanation:

17. (b): Glycogen (animal starch) is a polysaccharide consisting of a highly branched polymer of glucose occurring in animal tissues, especially in liver and muscle cells. It is the major store of carbohydrate energy in animal cells.

18. Which is wrong about nucleic acids?

(a) DNA is single stranded in some viruses

(b) RNA is double stranded occasionally

(c) length of one helix is 45 A in B-DNA

(d) one turn of Z-DNA has 12 bases.

Answer and Explanation:

18. (c): The DNA duplex model proposed by Watson and Crick is right handed spiral and is called B-DNA. Its one complete turn is 34A° long and has 10 base pairs.

19. The four elements that make up 99% of all elements found in a living system are

(a) C, H, O and P

(b) C, N, O and P

(c) H, O, C and N

(d) C, H, O and S.

Answer and Explanation:

19. (c): Carbon, hydrogen, oxygen and nitrogen are called four big elements of living body they make up about 99% of the mass of most cells. These are lightest elements that are capable of forming covalent bonds. It is known that the strength of covalent bond is inversely related to the atomic weights of the bonded atoms.

As C, H, O and N are lightest elements so the bonds they form are the strongest covalent bonds. So that the compounds formed are stable, varied in size and shapes. Carbon constitutes more than 50% of the dry matter has been observed that human body contains 0.5% hydrogen, 18.5% carbon, 65% oxygen and 3.3% nitrogen. Other elements are present in very lesser amount.

20. Which of the following nucleotide sequences contains 4 pyrimidine bases?




(d) both (b) and (c).

Answer and Explanation:

20. (a): The bases are of two types-purines and pyrimidines. The purine derivatives adenine (A) and guanine (G) are double ring structures whereas pyrimidine derivatives thymine, cytosine and uracil are single ring structures. Thymine (T) and cytosine (C) are found in DNA and cytosine (C) and uracil (U) is found in RNA. In the given question there are 4 pyrimidines as 2 cytosine and 2 thymine in optional.

21. Which purine base is found in RNA?

(a) thymine

(b) uracil

(c) cytosine

(d) guanine.

Answer and Explanation:

21. (d): The bases are of two types-purines and pyrimidines. The purine derivatives are adenine (A) and guanine (G). They are common to both DNA and RNA.

Thymine, uracil and cytosine are pyrimidine bases. Thymine and cytosine are found in DNA and cytosine and uracil are found in RNA.

22. What are the most diverse molecules in the cell?

(a) lipids

(b) mineral salts

(c) proteins

(d) carbohydrates.

Answer and Explanation:

22. (c): Proteins show enormous diversity because of different proportions and sequences of twenty amino acid within the protein molecule. A large number of permutations and combinations of this amino acid are responsible for the unlimited variety of proteins. Proteins are the most abundant and most varied of the macromolecules having one or more polypeptides (chains of amino acids). The proteins constitute almost 50% of the total dry weight of the cell. Proteins may be simple or conjugated.

Among conjugated, proteins may be phosphoprotein, glycoprotein, nucleoprotein, chromoproiviti, lipoprotein, flavoprotein, metallo protein etc. Functionally proteins may be structural protein, enzymes, hormones, respiratory pigment etc.

23. In which of the following groups are all polysaccharides?

(a) sucrose, glucose and fructose

(b) maltose, lactose and fructose

(c) glycogen, sucrose and maltose

(d) glycogen, cellulose and starch.

Answer and Explanation:

23. (d): Polysaccharides are complex long chain carbohydrates which are formed by dehydrate synthesis or polymerisation of more than 10 but generally very large number of units called monosacccharides. Starch, glycogen and cellulose are all polysaccharides.

Starch is a glucosan homopolysaccharide which is the main reserve food of plants. Glycogen is also a glucosan homopolysaccharidt which is the major reserve food of fungi, animals and son-e bacteria. It is also called animal starch. Cellulose is the sructural polysaccharide of plant cell walls, some fungi, protists. It is a fibrous glucosan homopolysaccharide of high tensile strength.

24. Lactose is composed of

(a) glucose + galactose

(b) fructose + galactose

(c) glucose + fructose

(d) glucose + glucose.

Answer and Explanation:

24. (a): Lactose is popularly known as milk sugar. It is a disaccharide composed of one molecule of glucose and one molecule of galactose. The covalent bond that joins these two monosaccharide units is called glycosidic bond or glycosidic linkage. It is a reducing sugar.

25. Cellulose, the most important constituent of plant cell wall is made up of

(a) branched chain of glucose molecules linked by (3-1, 4 glycosidic bond in straight chain and a-1, 6 glycosidic bond at the site of branching

(b) unbranched chain of glucose molecules linked by (3-1, 4 glycosidic bond

(c) branched chain of glucose molecules linked by a-1, 6 glycosidic bond at the site of branching

(d) unbranched chain of glucose molecules linked by a-1, 4 glycosidic bond.

Answer and Explanation:

25. (b): Cellulose is the most abundant carbohydrate. Cellulose molecule is composed of 1600 to 6000 glucose molecules joined together. Those polymers form long twisting macromolecules of cellulose. The chains are unbranched and linear. The successive glucose residues are joined together by P-I-4, linkages.

26. Which of the following have carbohydrate as prosthetic group?

(a) glycoprotein

(b) chromoprotein

(c) lipoprotein

(d) nucleoprotein.

Answer and Explanation:

26. (a): Glycoproteins are proteins that contain sugars like carbohydrates as prosthetic group. In most glycoproteins, the linkage is between asparagine and N- acetyl-D-glucosamine. Some glycoproteins are immunoglobulins, membrane proteins and muscle
proteins. These are formed in Golgi apparatus in the process of glycosylation and are important components of plasma membrane in which they extend throughout the lipid bilayer. Lipoproteins are protein complexed with lipids like triglycerides, phospholipids etc. Nucleoproteins are proteins associated with nucleic acids and chromoproteins are proteins associated with pigments eg. Cytochrome, phytochrome.

27. ATP is

(a) nucleotide

(b) nucleoside

(c) nucleic acid

(d) vitamin.

Answer and Explanation:

27. (a): Nucleosides are condensation product of a pentose sugar and nitrogen base. Nucleotides are phosphorylated nucleosides with are formed by condensation of pentose, sugar, nitrogen base and at least one phosphoric residue. When nucleotides possess more than one phosphate radical, it is called higher nucleotides e.g. ATP.

ATP is a nucleotide as it is composed of adenine, ribose sugar and phosphoric acid. There are two additional phosphate groups attached to the phosphate group of AMP. The last two phosphate molecules are connected by high energy bonds.

28. Which of the following is a reducing sugar?

(a) galactose

(b) gluconic acid

(c) (3-methyl galactoside

(d) sucrose.

Answer and Explanation:

28. (a): All those sugars which have free aldehyde or ketone group are called reducing sugars. These are able to reduce cupric ions (Cu+2) into cuprous ions (Cu+). The two common tests are Benedicts test and Fehling’s test. Both tests employ alkaline solution of copper sulphate which is blue in colour with reducing sugar it gives orange to brick red precipitation. Galactose is a reducing sugar.

It gives brownish red precipitate with Fehling reagent and with Benedict’s reagent it gives yellow, red or green precipitate confirming the presence of sugar. Sucrose, starch are non-reducing sugars.

29. Lipids are insoluble in water because lipid molecules are

(a) hydrophilic

(b) hydrophobic

(c) neutral

(d) zwitter ions.

Answer and Explanation:

29. (b): Lipid molecules are insoluble or sparingly soluble in water but are freely soluble in organic solvents like ether, alcohol and benzene. Insolubility of lipids in water is due to the fact that the polar groups they contain are much smaller than their nonpolar portions.

The nonpolar chains are long complex hydrophobic hydrocarbon chains. If shaken in water lipids often form small droplets or micelles. The complex formed is called emulsions. These non polar proteins give them water repellent or hydrophobic property.

30. The major role of minor elements inside living organisms is to act as

(a) co-factors of enzymes

(b) building blocks of important amino acids

(c) constituent of hormones

(d) binder of cell structure

Answer and Explanation:

30. (a): Minor element are those which are required in quantity of less than milligram/gram of dry matter but they are essential for proper growth and development of an organism e.g., CI, Mn, B, Zn, Cu, Mo etc. These elements work as non-protein cofactor in enzymes eg. Zn, Cu etc. They also take part in oxidation reduction reactions eg. Cu, with variable valency. Chloride ion enhances activity of salivary amylase. Zinc is required for activity of carbonic anhydrase and alcohol dehydrogenase, etc.

32. Nucleotides are building blocks of nucleic acids. Each nucleotide is a composite molecule formed by

(a) base-sugar-phosphate

(b) base-sugar-OH

(c) (base-sugar-phosphate)

(d) sugar-phosphate.


(a) base-sugar-phosphate

33. Enzymes, vitamins and hormones can be classified into a single category of biological chemicals; because all of these

(a) help in regulating metabolism

(b) are exclusively synthesized in the body of a living organism as at present

(c) are conjugated proteins

(d) enhance oxidative metabolism.


(c) are conjugated proteins

35. About 98 percent of the mass of every living organism is composed of just six elements including carbon, hydrogen, nitrogen, oxygen and

(a) sulphur and magnesium

(b) magnesium and sodium

(c) calcium and phosphorus

(d) phosphorus and sulphur.

Answer and Explanation:

35. (d): Living organisms requires 6 elements in relatively large amounts. C, H, O, N, P, S. These elements contribute to the structural organization of living organisms. Phosphorus is important constituent of nucleic acids, sulphur and nitrogen constituent of amino acids.

Carbon, hydrogen, oxygen are frame work element. Nitrogen forms DNA base pairs. Smaller quantities of the elements required are Na, K, Mn, Ca, Fe, Mg, CI, Mg, CI, I, Co, Bo. Plants obtained their supplies of material from air, water, and soil. Animals obtained these by eating plants and drinking water.

All are present in rocks and are released by erosion and weathering into soils, rivers, lakes and oceans.

Some such as N, O, and C are also present in the atmosphere. The movement of the nutrient element through the biosphere is called the biogeochemical cycle. Eg, Carbon, nitrogen, sulphur, phosphorus, oxygen and water cycles.


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