Some Sections of DNA Do Not Determine Traits, but Affect the Process of Transcription: Gene Regulation

The genetic code is universal and contains the instructions for all life on Earth. But the diversity of life relies on more than just the genetic code itself — it also relies on the variety of ways in which this code is used in different organisms. In much the same way that an orchestra depends upon a conductor to direct the individual musicians, all cells depend upon regulatory mechanisms to determine which of their genes are "turned on" and which are "turned off" at any given time. In other words, these regulatory mechanisms control gene expression.

But why is this control necessary? To better understand the answer to this question, consider the example of a skin cell and a brain cell located within the same organism. Both of these cells contain the same set of genetic information, but each has a unique function within the organism. Both cells, for instance, carry the gene associated with skin pigmentation, but only the skin cell actually expresses this particular gene and produces the pigment. In order for this gene to be expressed by the skin cell, it must be transcribed into mRNA and then translated into protein — and regulatory mechanisms are what trigger the transcription of this particular gene to occur (or not occur, in the case of the brain cell). In fact, regulatory mechanisms are the reason why some genes are expressed in every cell in an organism regardless of type, but other genes are expressed by only certain types of cells under specific sets of circumstances.

In order to understand how regulatory mechanisms work, it's first necessary to understand that not all nucleotide sequences in a strand of DNA code for the production of proteins. Rather, some of these noncoding sequences serve as binding sites for the various protein molecules required to start or regulate the transcription process. For example, a group of nucleotides known as a promoter sequence lies near the beginning of most genes and provides a binding site for RNA polymerase to begin transcription. Similarly, other noncoding sequences near the promoter sequence function as protein binding sites that can either induce or block transcription. This basic system affects gene expression in both prokaryotes and eukaryotes, albeit in different ways.

Gene expression is much more complicated in eukaryotic cells than it is in prokaryotic cells. This is due, in large part, to the fact that eukaryotic cells must differentiate into different cell types, and they also contain a greater number of genes than prokaryotic cells. Furthermore, the transcription and translation sites of eukaryotic DNA are separated from one another by the nuclear membrane. Given these complicating factors, eukaryotic cells employ a greater variety of control strategies than prokaryotic cells, and they do so at various steps in both transcription and translation. Nonetheless, each of these strategies begins at the level of DNA.