11/11/2010 § 1 Comment
In both prokaryotes and eukaryotes, not all of the genes on DNA are transcribed and translated (expressed). If all of the genes were expressed and all the amino acids gathered and all of the proteins coded in the DNA were made, then obviously a lot of energy would be used but in the end, not every single one of the proteins would be used (because typically, only the proteins needed at the time would be used). Therefore, energy, materials and time into making the proteins would be wasted.
Prokaryotes’ way of managing which genes are expressed and which are not at certain times involves using enzymes. In one bacterium called Escherichia coli, the genes that make proteins that digest lactose are not apparent when lactose nutrients are not and a protein called a repressor is involved with the regulation of the genes. Usually, the repressor is bound to a site on the DNA called an operator, the section of genes that serves as an on-off area.
With the repressor bound to the operator, the RNA polymerase that does transcription and forms mRNA is blocked from continuing the process and not all of the coded proteins are made yet. When lactose is present, however, they attach themselves to the repressor and change its shape so that it is no longer in the RNA polymerase’s way. Once the genes have been transcribed and later translated into proteins or enzymes, in this case, the enzymes eat away the lactose—even the one attached to the repressor. This restores the repressor’s original shape.
This process of regulating genetic information controls the lactose in the bacterium because if there is too much lactose in it, it’ll probably die. If there is lactose, the bacteria makes lactase to take it away. If there isn’t any, then there’s no point of making lactase (the enzyme).
We were already beginning to run out of time in class but eukaryotes’ ways of controlling protein synthesis in eukaryotes is much more complex than those of prokaryotic cells. An RNA polymerase needs other proteins to help it transcribe the DNA molecule. These are called transcription factors. Activators activate transcription between DNA and RNA molecules and attach to regions of DNA called enhancers. The DNA curls up a little and only then can the RNA polymerase continue transcribing the genetic information. Therefore, transcription only begins when the complete group of transcription factors, including activators and enhancers together, are present for the gene that will be used for protein synthesis.