DNA is a double stranded macromolecule. Two polynucleotide chains, held together by weak thermodynamic forces, form a DNA molecule.
Two DNA strands form a helical spiral, winding around a helix axis in a right-handed spiral. The two polynucleotide chains run in opposite directions. The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a sprial staircase. The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.
The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3'-hydroxyl and 5'-hydroxyl groups to phosphate groups in ester links, also known as phosphodiester bonds.
Within the DNA double helix, A forms 2 hydrogen bonds with T on the opposite strand, and G forms 3 hyrdorgen bonds with C on the opposite strand.
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