3.1  DNA TranscriptionCitation

The genetic code is frequently referred to as a "blueprint" because it contains the instructions a cell requires in order to sustain itself. We now know that there is more to these instructions than simply the sequence of letters in the nucleotide code, however. For example, vast amounts of evidence demonstrate that this code is the basis for the production of various molecules, including RNA and protein. Research has also shown that the instructions stored within DNA are "read" in two steps: transcription and translation. In transcription, a portion of the double-stranded DNA template gives rise to a single-stranded RNA molecule. In some cases, the RNA molecule itself is a "finished product" that serves some important function within the cell. Often, however, transcription of an RNA molecule is followed by a translation step, which ultimately results in the production of a protein molecule.

The process of transcription can be visualized by electron microscopy (Figure 1); in fact, it was first observed using this method in 1970. In these early electron micrographs, the DNA molecules appear as "trunks," with many RNA "branches" extending out from them. When DNAse and RNAse (enzymes that degrade DNA and RNA, respectively) were added to the molecules, the application of DNAse eliminated the trunk structures, while the use of RNAse wiped out the branches.

DNA is double-stranded, but only one strand serves as a template for transcription at any given time. This template strand is called the noncoding strand. The nontemplate strand is referred to as the coding strand because its sequence will be the same as that of the new RNA molecule. In most organisms, the strand of DNA that serves as the template for one gene may be the nontemplate strand for other genes within the same chromosome.