Nucleotides are linked to each other by phosphodiester linkages between one nucleotide's phosphate group to a hydroxyl group on the sugar of an adjacent nucleotide. A and G are Purines and have a double ring structure, while C, T, and U are Pyrimidines and have a single ring structure. DNA is wound in a dobule helix with C's bonding to G's and A's bonding to T's and U's. The two strands are complimentary . Strands separate and are replicated.
Codons are sets of three adjacent nucleotides which code for an amino acid.
Ther are 20 amino acids and 64 possible combinations of the 4 bases into codons. Besides the 20 aa, there are start and stop codons. 61 code for amino acids. UAA, UAG, and UGA are the three stop codons or nonsense codons. Synthesis always occurs in a 5' to 3' direction.