Question

1. In the DNA from any given organism, which one of the following is true?

A. The molecular ratio of A:T is always 1

B. The molecular ratio of T:C is always 1

C. The molecular ratio of A:G is always 1

D. The molecular ratio of purine to pyrimidines is never 1

2. Regarding polysaccharides, which of the following is incorrect?

		Cellulose is the polysaccharide that provides structure to plants
		Glycogen is the polysaccharide that stores energy in plants
		Sugars are joined together to form polysaccharides by glycosidic linkages
		A molecule of water is produced when a glycosidic linkage is formed

Answer 1

Answer 1- Correct answer is option A i.e. "The molecular ratio of A:T is always 1"

DNA is composed of nucleotides and  each nucleotide comprises of a nitrogen-containing base, a five-carbon deoxyribose sugar and a phosphate group. There are four possible nitrogen bases involved in DNA i.e. adenine (A), guanine (G)[PURINE] and cytosine (C), or thymine (T)[PYRIMIDINE]. In dsDNA, two purines will not fit between the strands due to bulkier ring while two pyrimidines will be too far to bond. Thus according to Chargaff's base pairing rules, Adenine forms double bond with thymine and guanine forms triple bond with cytosine.

Chargaff's rule states that in cell, DNA contain pyrimidine and purine bases in the ratio of 1:1. Thus option D which says that "The molecular ratio of purine to pyrimidines is never 1" is not correct. In any double-stranded DNA, the amount of guanine (a purine base), is equal to cytosine(a pyrimidine base) and the amount of adenine(a purine base) is equal to thymine(a pyrimidine base). Thus option B and C is not true and dont follow Chargaff's rule.

Answer 2- Glycogen is the polysaccharide that stores energy in plants

Cellulose is a polysaccharide with the formula (C6H10O5)n  made up of a linear chain of several hundred to many thousands of glucose linked with β(1→4) glycosidic bonds. Plant cell walls are primarily made of cellulose, It provides a structural framework to support plant cells. Thus option A is correct and can be opt out.

Polysaccharides are complex biomacromolecules which is made up of long chains of monosaccharides. In polysaccharides, these monosaccharides are linked with each other with glycosidic linkages. For eg in cellulose, thousands of glucose units is linked with β(1→4) glycosidic bonds. Thus option C is correct and can be opt out.

Glycosidic bonds are covalent chemical bonds which forms by a condensation reaction, which means that one water molecule is produced during formation of a glycosidic linkage. Thus option D is correct and can be opt out.

Starch is the storage form of carbohydrate in plants. Glucose synthesized during photosynthesis combines to form starch which is stored in the plants cells for future use. Glycogen is the storage form of glucose in animals and humans. It is analogous to the starch found in plants.Glycogen is synthesized and stored mainly in the liver and the muscles. Thus option B i.e the statement "Glycogen is the polysaccharide that stores energy in plants" is wrong and this is the correct answer.