Science Questions

How are genes switched on and off?

Sun, 1st Nov 2009

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Question

Catherine, Bromley asked:

It has been discussed on your show about genes being turned on and off through things like genetic engineering and Iíve 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?

Answer

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 Iíve 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 Ė 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 proteinís 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 whatís 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, thereís 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, isnít 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.

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catherine wells asked the Naked Scientists: Hi, † You often talk about genes being switched on and off. I have in my head a picture of a double helix with lots of old fashioned light switches sticking out of it - but seriously doubt this is correct.† What actually happens when genes are switched on and off. †† Many thanks, Cath What do you think? catherine wells , Wed, 14th Oct 2009

hi cath..
u ask "What actually happens when genes are switched on and off."
actually genes are turned of and on during the embryonic level to start or cease the specific funtion of that gene.gene of and on is related to its expression.U start to think like that in every cell there is complte information of each and every thing in the form of genome but some cell converted to eye cell,other into liver cell and so on...this all is controlled by the gene on and of.When some genes turned on in specfic cell,that cell will be concverted into its specfic kind......
hplfuly u got my point
khurrum khurrum, Sun, 18th Oct 2009

Hi Cath

Commonly, genes are controlled by chemicals called "transcription factors", which are proteins, usually phosphate-bearing proteins, which lock onto specific regions of the DNA and alter its shape or chemistry in such a way as to make an adjacent section of the DNA more or less accessible to the miniature machines (polymerase enzymes) that read the DNA code and turn it into gene products. Other modifications can also turn genes on and off including hormones - thyroxine from the thyroid gland binds directly to DNA to activate certain genes - whilst other enzymes can add or remove chemical groups (usually methyl or acetyl groups) from the proteins around which DNA is wound. This alters the shape of that section of the genetic code, making it more or less readable by the cell.

Using these mechanisms cells can achieve both short and long-term control of gene expression in a highly dynamic way.

Chris chris, Sun, 18th Oct 2009

Gene expression is controlled by regulatory factors. For example, to turn on Ras gene, it requires GTPase, meaning that GDP is phosphorylated to GTP, thus activating Ras pathway. It can be turned off by sensor molecules and other regulator factors. Jonathan Madriaga, Wed, 25th Nov 2009



This is not right.  The Ras protein is activated/deactivated by exchange factors GAP and GEF.  However, the Ras gene itself is not directly turned on or off.  One should make a distinction between the _gene_ and the _gene product_ (or protein), as it pertains to the question of OP. kmiller755, Mon, 30th Nov 2009

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