How are genes switched on and off?

Catherine - I just got a quick question for clarification really. It has been discussed on your show about genes being turned on and off through things like genetic engineering and I've got a picture in my mind a sort of a big double helix with old-fashioned light switches sticking out the sides of it, which I seriously doubt is correct. And I was just wondering what is actually meant by genes being turned on off.

Kat - That is a great question and actually, your mental image is a fairly good analogy for it. So, if you think about - imagine a long string of DNA.

Catherine - Yes.

Kat - Now, a bit of that will be the actual gene and genes are basically instructions that tell a cell to make a particular protein. So you have kind of the recipe bit. And then around that, you have sort of instruction bits.

So, these are regions of DNA that attract proteins that come and sit on them and tell the gene to be on or off. So these are called transcription factors and they attract the kind of molecular machinery that actually churns through the instructions and tells a cell to make a particular protein.

So, you have all these different proteins sitting on different bits of the DNA and some protein's transcription factors tell a gene to be switched off and some transcription factors tell a gene to be switched on. So really, you do have these molecular switches. You also have another aspect of that ,and Chris mentioned this a little bit earlier on the show, is that you have kind of things called epigenetic switches as well. And these are things that are over and above what's in the DNA.

You get little molecular, almost like post-it notes or tags stuck to the DNA and stuck to the proteins that are wrapped around the DNA that have more information about when a gene should be used, for example during development. You know, you should turn on this gene for a bit while you're making hands and then turn it off again or should this gene be permanently switched off or permanently switched on. So really, there's this whole array of little molecular switches that are telling the DNA to be on or off in a particular cell at a particular time.

Catherine - Very clever, isn't it?

Kat - Yes, it is.

Dave - Can't you also get bits of DNA which are folded up so the chemistry can't get at it.

Kat - Yes. So, a lot of these sort of epigenetic factors. When a cell has decided that this gene should be permanently off, that gene gets all compressed up and squished up so that the molecular machines can't actually get to it. So we know that genes that are off are really compressed and wrapped up really tightly. Whereas genes that are very actively used are much more open so all the machines can get into there and read the genes.