lactose operon
In this section, we will discuss the differences in gene expression and regulation between eukaryotes (the cell that we are made of) and prokaryotes such as the bacteria Escherichia coli. To understand the differences we must first understand the chief characteristics of each, which make them unique. In eukaryotic cells the DNA or genetic information is stored in the nucleus of the cell, whereas in prokaryotic cells there is no nucleus and thus the genetic material (DNA) is stored in a sort of compressed coil called the nucleoid. Interestingly, the reason that the DNA of a prokaryotic cell is compressed into that coil/spherical shape is to maximize surface area to volume ratio, so that the DNA takes up the least space whilst holding information efficiently. Next, we will compare the mechanisms that regulate gene expression in the first place. In prokaryotic cells, if a gene is going to be expressed it will go through a process called transcription (which is covered more in depth on the previous page). Transcription begins with a step called initiation where a RNA Polymerase binds to the promoter on an area known as the TATA box (which signals the beginning of a gene) as to begin reading the 3’ template. However, this first step of transcription does not occur at all in prokaryotic cells because prokaryotic cells do not have TATA boxes at the beginning of their genes. Instead, prokaryotic cells utilize operons (named after the operator on them) at the beginning of interconnected gene units, in order to regulate gene expression. To further explore this idea, we will use the famous lac operon (lactose operon) as an example (see figure 1). The lac operon is a series of genes related to the transport and breakdown of lactose. Itself, works through a series of factors, including RNA polymerase, a repressor, a promoter, an operator, lactose, and the respective genes/codes (lacZ, lacY, and lacA). With these factors the lac operon exists in two states: off (not being expressed) and on (being expressed). The first state works in the absence of lactose, where the repressor is not being inhibited (due to the lack of lactose) and will therefore continue to be bound to the operator. Now, with the repressor bound to the operator, the RNA polymerase will not be able to bind to the promoter and make lactase (the genes cannot be expressed). In the second state, the on state, in the presence of lactose, the repressor will bind to the lactose releasing itself from the operator, allowing the RNA Polymerase to bind to the promoter. Thus with the RNAP bound to the promoter: the RNAP can begin to read and write (transcribe) the DNA creating an mRNA, which would then go to be translated into the enzyme lactase. Now, lactase is not just any enzyme, it is the enzyme that is specific to the break down of the disaccharide lactose into the monosaccharaides galactose and glucose (see figure 2); glucose being the bodies most preferred source of energy. The beauty of this system is that once all of the lactose has been broken down by the lactase, the repressor will re-bind to the operator due to the absence of lactose, preventing the production of more lactase, thus ending the process and stopping/regulating the genes’ expression.