I suppose I really should start with DNA replication, but since I'm anxious to move on to microarrays and complain, I choose to cover transcription first.
Most people should know by now that every cell in an organism has the same DNA. Numerous tv shows have helped popularize the science that allows a villain to be identified by his/her DNA left on a cup, or on a victim's body. But if all the cells have the exact same genetic information, how does a liver cell become so different than a skin cell, or a brain cell? The answer is RNA.
As I mentioned before, each gene has a start and an end, and takes up a relatively small part of a DNA molecule. Within each cell a certain number of genes are transcribed into RNA, while other are not. The genes that are transcribed can be translated into protein, giving each cell a complement of proteins that determine what kind of cell it is. So a root cell in the plant will have a different set of genes turned on than stem or leaf cells. Genes that are transcribed are said to be turned on, while genes that are not, are said to be turned off. There are genes that are expressed in every cell, no matter what kind of cell it is, and no matter what the developmental stage of the organism is. This is called constitutive expression. At the other end of the spectrum there are genes that are only turned on in one particular cell-type. Most genes lie somewhere in between and are turned on in some, but not in other cell types.
In addition to spatial regulation (some cells but not others), there is also developmental regulation, so that cells change the genes that are turned on or off as a result of development from a young cell/organism to an older one. Genes can be turned on or off in response to environmental cues, like wounding, or a response to pathogens. An important point to make here is that expression levels of genes are not as simple as "on" or "off." For any gene a little more or a little less transcription can have dramatic effects on the cell. Changes in the expression levels of genes are called "differential regulation."
As a result of differential regulation, the RNA in a particular cell can say a lot about the activities that are going on in that particular cell at that particular moment in time. Scientists therefore often want to know which genes are being expressed and what that level of expression is.
In a later post I intend to write a little more about how cells turn genes on and off, and regulate expression levels. For the description of microarrays, it's enough to know that genes are differentially expressed.