A gene is a segment of DNA consisting of codons specifying the amino acids for a protein and of control sequences which regulate gene expression. When a cell divides, the entire human genome, that is, all the DNA stored within the nucleus, must be copied. In Figure 2.1 the replication of DNA is demonstrated. The regulation of a cell’s function is controlled by altering gene expression. The steps in gene expression are (1) transcription of the gene’s DNA to RNA, (2) RNA processing to produce a spliced mRNA, (3) translation of mRNA on a polyribosome to a polypeptide chain, and (4) final protein processing to the functional tertiary form. Figures 2.2 through 2.5 illustrate the steps in this process, which will be covered in more detail later in this chapter.