Life as we know it is specified by genomes. Every organism posseses a genome that contains the biological information needed to construct and maintain a living example of that organism. Most genomes, including those for all cellular lifeforms, are made of DNA (deoxyribonucleic acid) but a few viruses have RNA (ribonucleic acid) genomes. DNA and RNA are polymeric molecules made up of linear, unbranched chains of monomeric substances called nucleotides. Each nucleotide has three parts: a sugar, a phosphate group, and a base (fig 1.1). In DNA, the sugar is 2’- dedeoxyribose and the bases are adenine (A), cytosine (C), guanine (G) and thymine (T). Nucleotides are linked to one another by phosphodiester bonds to form a DNA polymer, or polynucleotide, which might be several million nucleotides in length. DNA in living cells is double-stranded, two polynucleotides being wound around one another to form the double heix. The double helix is held together by hydrogen bonds between the base components of the nucleotides in the two strands. The base-pairing rules are that A base-pairs with T, and G base-pairs with C. The two DNA molecules in a double helix therefore have comlementary sequences. In an RNA nucleotide sequence, the sugar is ribose rather than 2’- deoxyribose, and thymine is replaced by the related base uracil (U). RNA polymers are rarely more than a few thousand nucleotides in length, and RNA in the cell is usually single-stranded, although base pairs might form between different parts of a single molecule.The biological information contained in a genome is encoded in the nucleotide sequence of it’s DNA or RNA molecules and is divided into discrete units called genes. The information contained ina gene is read by proteins that attach to the genome at the appropriate positions and initiate a series of biochemical reactions referred to as gene expression. For organisms with DNA genomes, on the simplist level, gene ex...
