How does evolution produce new genes?

17 May 2009


How does evolution produce new genes?


Chris Smith - So in other words you've got huge panoply of life on earth, all of it descended from some ancestor that must have got started something like 3.9 billion years ago, that's when we think life started on earth, how do we have this massive and dramatic genetic diversity we have on earth today?

Well the answer is that we use as our genetic material DNA and some organisms use RNA, they are two related molecules and the basis of evolution and inheritance is that that genetic material has to be copied and passed on from one generation to the next in gametes, in other words, sperms and eggs.

Now in copying the DNA, a job which is done by enzymes - miniature machines in cells that read a DNA chain and then make an identical (hopefully) copy of it; then you end up with faithful transmission of the genetic message from one cell to another. But occasionally mistakes occur and there are a variety of reasons why mistakes can occur. One of them is that those enzymes that do the copying make a mistake, it's like me copying a book - I've got a book opened in front of me and I'm reading off the book and making a new copy in a second book, I might miss-copy and make some mistakes. Secondly, there are things coming in from the environment that can damage DNA. There are drugs, there are chemicals in the environment; there's also radiation in the environment that could be ultraviolet radiation, it could be chemicals you take into your body, you could be living in Aberdeen and breathing the radon gas that comes out of the granite there. Anything radioactive that gets into your body can also damage DNA. If that happens to the cells that are making your gametes, your sperms or eggs, that can lead to mutations - changes in DNA. This can cause DNA to rearrange itself, bits of DNA can get copied or duplicated, and this means that there's the opportunity for new genetic combinations to emerge; and once they emerge they can then get co-opted or changed or manipulated in order to do other jobs; and we know this happens because if you look in human DNA you can find the ghosts of genes long dead hidden in our genetic closet. You can find for example, lots of old genes that used to make our haemoglobin which are now no longer effective, and no longer functioning. They're called pseudogenes. But this is where we have copied the genes somehow and then its becomes deactivated but it's still in the genome. Now some other process could come along and reactivate that gene and use it for something else so it's a way of making genetic diversity.

Another way this happens, is something called transposons, you have bits of genetic material that can literally jump and they take themselves out of one bit of your genetic material and put themselves somewhere else carrying bits of DNA with them, and so this is another way of rearranging your genome and producing new forms of genetic sequences which can then become other important genes. A final way is viruses, because viruses - certain kinds of viruses actually physically insert their genetic material into the DNA of their hosts. HIV does that, other viruses could - retro viruses do that and a really elegant example of this is that there is a certain sea slug, it's called a Sacoglossan sea slug which eats algae and when it eats that algae on the sea floor it actually gets the chlorophyll containing chloroplast - these are tiny bodies inside the algae that contain the green pigment that enables the algae to trap sunlight, and the slug gets hold of those chloroplasts and puts them into its own skin and keeps them alive so the slug can also use the energy of sunlight to get energy. The interesting thing is that the slug has had to steal some genes from the algae in order to power those chloroplasts in its own cells, and the only way researchers think that could happen is if a virus added the genes to the slug in the first place.

So the answer is, it's very complicated but it seems that with nature and evolution, almost anything is possible.

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