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To really understand molecular genetics, it's helpful to look at the parts of a gene. Each of these parts have their own special jobs that play a role in the expression of genes. Cells do not use DNA directly to carry out their functions. Instead, they follow what is known as the central dogma of molecular biology. This concept describes the general flow of genetic information in a cell: DNA is transcribed into RNA, and RNA is then translated into a protein. These proteins carry out structural roles, regulate processes, or act as enzymes to help reactions happen. They are ultimately responsible for the traits we observe.
We'll start by talking about enhancers. These are non-coding sequences of DNA that help regulate when and how strongly a gene is expressed. Essentially, they turn a gene on or off. They can be located upstream or downstream of the gene they influence (and can be close or very far away), and sometimes even within the gene itself. Proteins called transcription factors bind to the enhancer and can activate transcription (if the TF is an "activator") or prevent transcription (if the TF is a "repressor"). If you think of a gene being expressed as a light turning on - the enhancer is like the light switch, and the TF is like your finger.
Next is the promoter, which is found just upstream of the coding portion of the gene. This is like the starting line - it's where an enzyme called RNA Polymerase first binds and initiates the process of transcription. If the promoter is blocked or missing, the gene won't be transcribed, regardless of the coding information that follows.
The coding region comes after the promoter and contains the actual instructions for building a protein. In keeping with my "DNA is like a recipe" analogy - this is the actual steps for what protein to make. But it's not a protein that is made from this - not yet at least. The molecule made using this coding region is mRNA. In eukaryotic cells, the coding region includes both exons, which are expressed and kept in the final mRNA, and introns, which are removed during RNA processing. This is the part of the gene that determines the order of amino acids in the resulting protein.
Finally, the terminator marks the end of the gene. This region signals RNA polymerase to stop transcription.Â
Some prokaryotes have special structures known as operons that help regulate gene expression.
Some prokaryotes have special structures known as operons that help regulate gene expression.
In prokaryotes, genes that work together are often grouped into units called operons. An operon is a section of DNA that includes several genes under the control of a single regulatory system. These genes are transcribed together as one long mRNA, which allows the cell to produce all the proteins needed for a specific task at the same time.
Each operon includes a single promoter, where RNA polymerase binds to begin transcription, and an operator, which is a DNA sequence that acts as a control switch. Whether or not the genes in the operon are transcribed depends on the interaction between the operator and a repressor protein.
A repressor is a regulatory protein that binds to the operator and physically blocks RNA polymerase from transcribing the genes. When the repressor is bound, the operon is "off." In some operons, this repression can be lifted by an inducer, which is a small molecule that binds to the repressor and changes its shape. This prevents the repressor from binding to the operator, allowing transcription to occur.
The lac operon in E. coli is an example of an inducible operon. It is normally turned off, but when lactose is present, lactose (or a derivative of it - allolactose) acts as the inducer. It binds to the repressor, inactivates it, and allows the genes for lactose digestion to be transcribed. The bacteria won't want to start producing one of the three genes needed for lactose production without the others - that's where the operon comes in handy. (The lac operon is also regulated by catabolite activator protein - CAP - which promotes RNA Polymerase binding when glucose levels are low.)
In contrast, the trp operon is an example of a repressible operon, which is usually on. When tryptophan levels are high, tryptophan acts as a corepressor by binding to the repressor protein. This activates the repressor, which then binds to the operator and shuts down transcription.
Operons are an efficient way for prokaryotes to regulate gene expression, especially in changing environments, by turning entire groups of genes on or off together.
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