Question

How are the complementary strands of DNA held together?

Answer 1

Hint: There are two long polynucleotide chains consisting of four types of nucleotide subunits in a DNA molecule. Each of these chains is referred to as a DNA line, or a strand of DNA.

Complete answer:
The two chains are held together by hydrogen bonds between the basic portions of the nucleotides. The nucleotides are composed of a five-carbon sugar to which one or more groups of phosphates and a base containing nitrogen are connected. In the case of nucleotides in DNA, a single phosphate group (hence the term deoxyribonucleic acid) is bound to deoxyribose sugar, and the base may be either adenine (A), cytosine (C), guanine (G), or thymine (T).

Via sugars and phosphates, the nucleotides are covalently connected together in a chain, forming a backbone of alternating sugar-phosphate-sugar-phosphate . Since only the base varies in each of the four subunit groups, each DNA polynucleotide chain is similar to a necklace (the backbone) strung with four bead types (the four bases A, C, G, and T).The four distinct nucleotides, that is, the bases with their attached sugar and phosphate groups, are often widely used to denote these same symbols (A, C, G, and T).

From the chemical and structural features of its two polynucleotide chains, the three-dimensional structure of DNA, the double helix, emerges. Since hydrogen bonding between the bases on the various strands binds these two chains together all the bases are on the inside of the double helix, and on the outside are the sugar-phosphate backbones.

Adenine is often combined with T-thymine, together with two bonds of hydrogen.Cytosine is often paired with Guanine, which is combined with three bonds of hydrogen.

This complementary base-pairing allows the base pairs in the interior of the double helix to be packed in the most energetically favourable configuration. Each base pair is of similar width in this arrangement, thus keeping the sugar-phosphate backbones an equal distance apart along the DNA molecule. The two sugar-phosphate backbones wind around each other to form a double helix, with one complete turn per 10 base pairs, to optimise the efficiency of base pair packing.

Note: DNA encodes data along each strand through the order or sequence of the nucleotides. In a four-letter alphabet, each base, A, C, T, or G, can be regarded as a letter that spells out biological messages in the DNA chemical structure. Organisms vary because their respective DNA molecules have distinct nucleotide sequences and therefore bear distinct biological messages.