Formation of DNA and RNA Strands
Each nucleotide consists of a phosphate, a 5-carbon sugar, and a nitrogen-containing base (pyrimidine or purine). As demonstrated in the picture below, phosphate is represented by an orange P in a circle, the sugar is represented by "HO 3'", and the nitrogen base is represented by the beginning letter of the pyrimidine or purine.
DNA strands run anti-parallel to each other with one strand having a 5'-3' direction and one strand having a 3'-5' direction. The direction of a strand is determined by what is found at the end. A strand ending with a phosphate is the 5' end and a strand ending with a hydroxyl (HO) group is the 3' end (shown in the picture below). Bases bind to each other in a complementary fashion with adenine only binding to thymine or uracil and guanine only binding to cytosine. Complementary bases link by hydrogen bonds to create the DNA double helix. The process of complementary single strands of DNA forming double stranded DNA is called hybridization or annealing. Hybridization can also be performed between a strand of DNA and a strand of RNA.
DNA strands are always replicated in the same direction: 5' to 3'. The 5' phosphate ends attach to the 3' HO ends creating a phosphodiester bond. These bonds are what create the sugar-phosphate backbone to both DNA and RNA. By looking at the picture one can see the solid backbone created by the attachment of the phosphates (indicated by the orange circles).