GENES

All living cells contain DNA in the form of chromosomes, which are linear polymers containing millions of nucleotide bases. The DNA is the genetic material of the cell, and the information encoded in a sequence of nucleotides is interpreted by the cellular machinery to produce proteins, which serve a wide range of functions in all cells. The DNA is functionally organized in discrete segments strung out along the length of the chromosome, and these segments are called genes. These genes not only encode proteins, but also control whether the DNA is active, that is whether the encoded information will be used or not. Genes are therefore are the functional units of chromosomes, and can be isolated and studied individually.

It should be remembered that a gene is not just a piece of DNA, a protein is encoded by a region of the DNA sequence called the "open reading frame", and in eukaryotic genomes this open reading frame is rarely a contiguous stretch of DNA on the chromosome. This is due to the presence of introns and exons in the coding sequence, and in eukaryotic cells, the cell machinery recognizes the exons in a given transcripts and forms a continuous RNA which encodes the protein. In prokaryotic cell theere are no exons or introns, and so the complexity of the gene in question will determine which strategy is employed in cloning. It is generally the open reading frame, rather than the entire gene, which is the target DNA sequence in directed evolution experiments.