Genes can be expressed as either RNA or protein. However, not every gene product is needed all the time, nor are they needed in the same amounts. In fact, it would be very energetically demanding for a cell to express every gene all the time. Further, some gene products are harmful to the cell and thus only expressed at the end of a cell’s life. Because of these reasons, we regulate the expression of our genes.
How do we know what gene product is needed, when it is needed, and how much is needed? Our cells actually have a very complicated system of mechanisms to monitor themselves and their environment. They take internal and external signals, analyze them, and then decide if a gene product is needed. Once it is decided that a gene product is needed, cells have a process to create a gene product.
The first step in making a gene product is to make a copy of the gene. Genes cannot leave the nucleus, so we need to make a copy, which is called messenger RNA (mRNA). The mRNA can leave the nucleus. So, how do we make mRNA? The first step is for a transcription factor to bind to the DNA at a point upstream to the target gene. This initiation point is a sequence of DNA called a promoter region.
Transcription is the process by which a transcript (mRNA) of the gene is created. Transcription factors can recruit the rest of the mRNA creating machinery to the target gene. The number and types of other factors varies with species, cell type, and even what the target gene is. An important component that is always present is RNA polymerase. This is the machine that actually reads the DNA code and makes the RNA. Transcription factors that promote gene expression are called activators. Figure 1 shows transcription factors and RNA polymerase (RNAP) making an mRNA copy of the DNA.
Interestingly, transcription factors can also prevent the transcription of a gene. So, once analyzed, the signals a cell is receiving may say to the cell that there is enough or even too much of a gene product being produced. In this event, the cell sends a different type of transcription factor to the over-expressed gene. These transcription factors still bind the promoter, but they block access to the gene. Thus, RNA polymerase cannot bind to the gene and mRNA is not made. Transcription factors that block transcription are called repressors.« Back to Glossary Index