Genes in your DNA have the information to make proteins, which do most of the things that happen in your cells (check out THE CORE). Due to the processes of transcription and translation, only some genes are expressed (i.e. turned into proteins) at certain times. This control is the reason we have different kinds of cells in our body, and the reason our cells can change over time e.g. when we are growing/are sick. Most of the time, the reason that a cell changes is because it is switching on and off different sets of genes.
E.g.
How does this change happen?
Genes are not the only thing in your DNA. In fact, genes that code for proteins only make up 1% of all of your DNA! Another roughly 25% of your DNA has the role of controlling whether these genes are on or off. The 'context' a gene is in decides when it is switched on and off.
How does this work?
First we need to know how transcription works.
In order to be expressed, the genes need to be transcribed. For this to happen, the 'transcribing machine' called an RNA polymerase need to 'read' the genes. RNA polymerase has to be right up close to a gene to read it (imagine its short-sighted). RNA polymerase also need to start reading the DNA a little before the gene (sort of like needing to read an introduction).
RNA polymerase (the yellow blob), need to first attach itself to the 'introduction' of the gene (1) and then read along the gene from start to finish (2) to transcribe the gene.
This is what happens when a gene is transcribed i.e. switched on. It's possible to switch off a gene by stopping the RNA polymerase attaching to the 'introduction' of the gene. This can be done in different ways, the simplest one is by actually putting something else in the introduction, so there is literally no space for the RNA polymerase to attach. This is what happens in one of in one of biologists' favourite model organisms, E. coli.
E. coli can metabolise (a cell's version of 'digest') lactose, a sugar found in milk. In order to do this it needs special lactose-metabolising proteins. These proteins are encoded in particular genes in the E. coli's DNA. It was discovered that these genes are only switched on when there is lactose in the E.coli, and they are switched off when there isn't lactose. Using what we know about genes when they are switched on from the picture above, we can see how this might look like when E. coli has lactose and when it hasn't:
So, how come RNA polymerase attaches when there is lactose but doesn't when there isn't? That's because we are missing a crucial bit of our diagram: there is another player in the mix, a protein called a repressor. A repressor 'represses' transcription, i.e. switches genes off. It does this here by sticking to the 'introduction' bit of DNA, literally taking up space so the RNA polymerase can't attach to where it needs to. When the E. coli swims into some milk there will be lots of lactose around. The lactose sticks to the repressor and stops it working, making the DNA free for the RNA polymerase to attach and transcribe the genes.
This 'control system' in E. coli might look complicated but is actually one of the simplest one scientists have found. Many genes that we have are controlled by loads of different proteins and repressors (as well as activators, that do the opposite thing), which all switch each other on and off. There are also many more ways to switch off a gene other than just sitting in the place of RNA polymerase (see coming Genes in Context posts for details).
Although we will probably never fully understand every single control system for each gene in the world (or even just for E. coli!), knowing that these kind of control systems exist is very important. From this, scientists now:
Have a whole area of biology called Systems Biology that study these control systems and think about them like electric circuits.
Have noticed that sometimes these systems have one or two proteins that control lots of other ones. These 'master regulators' are normally what we look to change when trying to change a cell e.g. to make it healthy if it’s a cancer cell.
Have realised that its not just the genes we have that’s important, but the regulator bits in our DNA too. When these control systems go wrong, we can get diseases and cancer.