What is a gene? How are genes expressed? What does the term gene expression mean? Can we analyze gene expression? These questions will be addressed in this lesson.
Definition of Gene Expression
A gene is a small piece of genetic material written in a code and called DNA. Each gene has within it a set of instructions for making molecules that organisms need to survive.
Genes themselves cannot be used by an organism. Instead they must be turned into a gene product. Gene expression is the process by which the information contained within a gene becomes a useful product.
Regulation of Gene Expression
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. Figure 2 shows a repressor (green) binding to DNA to block transcription.
Analysis of Gene Expression
Scientists can use many techniques to analyze gene expression, i.e.
to find out how high or low the expression of a gene is. One of the simplest methods is to quantify the amount of protein product of the gene in question. There are a few problems with this technique. For example, the protein may be being exported out of the cell or it may be present in the cell even though the gene is not actively being expressed (transcribed into mRNA). Another problem is that you have to lyse a cell to get the proteins.
Thus, observing an intact cell or organism is not possible.An alternative method for analysis is to look at how much mRNA is present in the cell. This technique can also be used to determine where in an organism a particular gene is being expressed. Figure 3 shows the mRNA for Hunchback, a protein that regulates fruit fly embryo development. The changing pattern of Hunchback mRNA expression shows that the gene for Hunchback is being turned on and off in different cells as the fly develops.
Gene expression is a process by which the information contained within a gene becomes a useful product.
Gene expression isn’t needed all the time, but when it is the gene makes a copy called messenger RNA(mRNA). The creation of the mRNA is an intricate process that involves transcription. Transcription factors that promote gene expression are called activators while those that block transcription are called repressors. Scientists can analyze gene expression using several techniques like quantifying the amount of protein produced in a gene and looking at how much mRNA is present in a cell.