The Number of Genes on an mRNA Varies

THE NUMBER OF GENES ON AN MRNA VARIES

Bacterial and eukaryotic chromosomes are organized very differently. In prokaryotes, the distance between genes is much smaller, and genes associated with one metabolic pathway are often found next to each other. For example, the lactose operon contains several clustered genes for lactose metabolism. Operons are clusters of genes that share the same promoter and are transcribed as a single large mRNA that contains multiple structural genes or cistrons. Thus these transcripts are called polycistronic mRNA (Fig. 2.4). The multiple cistrons are translated individually to give separate proteins. In eukaryotes, genes are often separated by large stretches of DNA that do not encode any protein. In eukaryotes, each mRNA only has one cistron and is therefore called monocistronic mRNA. If a polycistronic transcript is expressed in eukaryotes, the ribosome only translates the first cistron, and the other encoded proteins are not made.