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UK Blood Test Predicts Life Expectancy

Posted 5/17/2011 12:05 AM by Nicole Garneau | Comments

How does a blood test tell you how long you might live? It's all in the DNA- but not the sequence of the DNA- in the replication of the DNA.

cell division

Background: When cells want to divide and make daughter cells, they need to first make copies of their DNA (shown as smiley faces in the cells above) so that both the original cell and the daughter call both have the same DNA (instructions) for making the cell work properly. Many organisms, like humans, have double stranded DNA in the form of the double helix (like a twisted ladder). The sides of the ladder are the complimentary strands of DNA twisted together.

To copy the DNA, the DNA ladder is unzipped to access sequence of nucleotides (A, T, G, and C) the letters that make up the DNA alphabet. The order of these nucleotides, just like in a book, spells out exactly how to make different things the cell needs to live. When the order gets messed up, or parts of the sequence are deleted, the cell has problems making the things it needs and can die depending on how big or important the change is. So when the cell copies its DNA, it has to do it very carefully.

This is no problem for one strand of the DNA, called the leading strand. It get read in a specific way by using a dock (small piece of RNA, kind of like DNA) that matches the beginning in order to start making a copy, almost like putting your finger at the beginning of the sentence and reading left to right in the English language (5' to 3' in the DNA language). The other strand though, called the lagging strand is the compliment of the leading strand, and so it is also read left to right, but the order of the letters is backwards. So the cell has to read by jumping forward in short stints to read the order correctly. It would be like having a book be written in reverse (down would be written "nwod"), and you put your finger a few words into the first chapter (covering a word or two) and then read in reverse move forward a few words, make a copy and then have to move your finger to the next few backwards words, continuing on until the end of the chapter and then connecting all the fragments by filling in the words that your finger had covered initially.

End Replication Problem

But there's a problem, the very last section of the chapter you read you have to put your finger down knowing that whatever your finger covers on this last bit, you won't be able to fill in because there is nothing after it (this is the last Okazaki fragment). Scientists know this to be the "End Replication Problem" because towards the end of the lagging strand there's not enough room to place your finger and still read the last words to copy them.

telomeres

The blood test just released in the UK doesn't look at the part of the DNA that contains the instructions for your cells; it looks at the end part of the DNA, called telomeres. Telomeres are how the cell solves the "End Replication Problem" by giving a little bit of extra sequence at the end to fill in gap in instructions. So the ends of the DNA, which do not give instructions to the cell, do undergo shortening over time in order to prevent the instructions in the center part of the DNA from being subject to that shortening. But all good things must come to an end, and eventually the telomeres are exhausted, cycles of copying leaving them diminished, and leaving the DNA open to progressive shortening in the instructions section.

As mentioned above, any time you have significant changes or deletions in the DNA, it can affect survival. The "Hayflick Limit" is the theory that when the telomeres run out, the cell dies. Although a hypothesis, there is mounting evidence that this might be true. The scientists behind the UK blood test are cashing in on this hypothesis by correlating the length of a person's telomeres to how much longer their cells and therefore they will live. These questions remain: what is the science behind this correlation, how accurate is the prediction, and what are the repercussions of this knowledge to the person seeking it and to others if it were to get in the wrong hands?

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