Question

16-a) What are the characteris s of the Watson's and Craig's Structures for the DNA-Molecules? State five of these features: (iii) b)- Choose the correct answer. Chargaff's rule states that: (a) DNA has a double helix structure (b) DNA runs in anti-parallel manner (c) The numbers of A and T are equal (d) No U found in the RNA (e) None of the above Give the structure of the most important d-saccharides: Sucrose and Lactose.

Answer 1

Ans- 16 (a) The important features of Watson-Crick model or double helix model of DNA are as follows:

(i) The DNA molecule consists of two polynucleotide chains or strands that spirally twisted around each other and coiled around a common axis to form a right-handed double-helix.

(ii) The two strands are antiparallel i.e. they run in opposite directions so that the 3′ end of one chain facing the 5′ end of the other.

(iii) The sugar-phosphate backbones remain on the outside, while the core of the helix contains the purine and pyrimidine bases.

(iv) The two strands are held together by hydrogen bonds between the purine and pyrimidine bases of the opposite strands. Adenine (A) always pairs with thymine (T) by two hydrogen bonds and guanine (G) always pairs with cytosine (C) by three hydrogen bonds.

(v) The base sequence along a polynucleotide chain is variable and a specific sequence of bases carry the genetic information.

(b) The correct answer is (c)

Explanation- According to Chargaff’s rule, number of Adenine is equal to Thymine and that of Guanine is equal to Cytosine i.e. A=T and G=C.

Also (A + C) = (G + T).

Ans- The structures of most important d-saccharides Sucrose and Lactose are-

Ans. Sucrore Сн, он Сн, бҢ он IX он н/ од TCH₂OH I OH 07 н Calycaridic bond OH Calucare Fructare Lactare си,рн СИОН НО ДА ОН он TH ОН он он Calucore Celucare