Question

The double-helix model of DNA resembles a twisted ladder in which the rungs of the ladder are
a. Sugar-phosphate paired with sugar-phosphate
b. A paired with T and C paired with G
c. A purine pair with pyrimidine
d. Both (b) and (c)

Answer 1

Hint: This question can be easily answered by knowing and understanding the structure of DNA. The DNA structure is a characteristic twisted ladder with two strands that contain the chemical components. The chemical components form the backbone and also the steps or rungs of the ladder.

Complete answer:
Double-helix model of DNA was proposed by James Watson and Francis Crick based on the work of Rosalind Franklin. The predominant form of DNA is the B-DNA.

DNA has three kinds of components chemically: phosphate, a sugar called deoxyribose, and four nitrogenous bases—adenine, guanine, cytosine, and thymine. Two of the bases, adenine (A) and guanine (G), have a two-ring structure characteristic of a kind of substance called a purine. The other two bases, cytosine (C) and thymine (T), have a solitary ring structure of a sort called a pyrimidine. The chemical parts of DNA are organized into bunches or groups called nucleotides, each made out of a phosphate group, a deoxyribose sugar, and any of the four bases.

DNA is made up of two side by side chains called strands of nucleotides contorted into the state of a double helix. The two nucleotide strands are held together by weak bonding between the bases of each strand, framing a structure like a twisting flight of stairs. The backbone of each strand is a repeating phosphate–deoxyribose sugar polymer. The sugar-phosphate bonds in this backbone are called phosphodiester bonds.

Interactions between sets of bases, one from each strand, hold the two strands of the DNA atom together. The bases of DNA interact very straightforwardly, that there are just two kinds of base pairs: A-T and G-C. The bases in these two base sets are said to be complementary. This implies that at any "step" of the stair like double stranded DNA particle, the main base-to-base affiliations that can exist between the two strands without significantly distorting the DNA are adenine to thymine and guanine to cytosine. Purines always bind to pyrimidines. The bonding of A with T and G with C is through hydrogen bonds.

Hence, the correct answer is option (D).

Additional information:
According to Chargaff’s rule the number of G should always be equal to the number of C and the number of A should always be equal to the number of T since they can only bind to each other. Incorrectly paired nucleotides can cause conformational change in the DNA structure and cause mutations if the body is unable to repair the errors.

Note: There are types of DNA other than the B-DNA which is commonly found everywhere. In B-DNA there are ten base pairs in each turn whereas in A-DNA it has eleven base pairs in each turn and in Z-DNA it has twelve base pairs in each turn of the helix. It can also be noted that A-DNA and B-DNA are right-handed helices whereas Z-DNA is left-handed.