Richard Thornley asked:
What is the very first and last piece of DNA on the chain responsible for - or is it a marker solely to indicate the ends?
Rich Thornley UN International School, NYC
Kat - Now itís time to look at your genetics questions. Richard Thornley asks ďWhat is the very first and last piece of DNA on the chain responsible for? Is it a marker, or solely to indicate the ends?Ē To answer, hereís Dr Jessica Greenwood from the Telomere Biology lab at the Cancer Research UK London Research Institute.
Jessica - I really like this question because it highlights the fact that the ends of the DNA called telomeres actually have a number of important responsibilities. One of which is to indicate the end. So first, what is a telomere? A telomere is a structure found at the end of the DNA in all organisms with linear chromosomes. In most organisms, the underlying foundation for the telomere is the DNA sequence, which is a unique stretch of short repeats. This DNA does not contain genes that code for protein, but it does create a unique region in the DNA upon which telomere-specific proteins bind and form the telomere.
Itís the very fact that the cell knows that the telomere is the end that protects the end itself and therefore, the entire genome. You see, the cell is always on the lookout for breaks in the DNA chain and when the break is discovered, the cell acts quickly to repair it by sealing broken ends back together. If it werenít for the telomeres, indicating to the cell that the end was an end, the cell might be under the mistaken impression that the end was in fact a break. And then this would be extremely dangerous because if the cell attempted to repair the end, it could seal it to another chromosome end and the chromosomes would get stuck to each other in this manner. It would lead to serious problems when the cell try to divide and separate this DNA into daughter cells. So, end protection or capping as itís known is an absolutely critical role for the telomere.
But there's another equally important job for the telomere and that is to help guide the enzyme that adds the unique DNA sequence to the ends, called telomerase. Because your cells are constantly making more of themselves, they prepare for this by copying their DNA and then dividing it so that one copy ends up in each daughter cell. The copying of the DNA is not completely perfect. At each time a copy is made, a little bit on the end is missed. While this may sound like a bad thing, itís actually another built-in safety device to impose a life span on our cells because we donít want our cells to just be able to divide without end.
So, most of our healthy cells actually have turned off telomerase because we donít want them to be able to keep dividing. Cancer cells have found a way to re-activate telomerase. This unfortunately allows the cells to continue to make copies of itself. So, you can see that actually, the end, the telomere does have and number of important roles in protecting the cell, both in survival and making sure that it doesnít get out of control.
Kat - Thanks to Dr Jessica Greenwood, from the Cancer Research UK Telomere Biology lab. And if youíve got any questions about genes, DNA and genetics youíd like us to answer, just email them to me at firstname.lastname@example.org.