MadSci Network: Genetics
Query:

Re: Can 'Junk' DNA be a mechanism to enhance the probability of gene survival?

Date: Thu Apr 30 09:38:22 2009
Posted By: Brian Foley, Molecular Genetics Staff Scientist
Area of science: Genetics
ID: 1234097925.Ge
Message:

Excellent question! And one that has been widely discussed and researched over the years.

Although it is often stated that "mutations occur at random", this is actually not true. At every level, there are biases, or non-random influences, on changes in DNA over time. There are thousands of different types of biases, so I will discuss just a few, in order to have some time to get to the question you ask. There are 4 bases; 2 purines C (Cytidine) and T (Thymidine) and 2 pyrimidines G (guanosine) and A (adenosine). There are 12 possible point (single base change) mutations: G to A, G to C, G to T, A to G, A to C, A to T, etc... The 12 mutations are not observed in equal numbers. It's like a 12-sided dice with some sides bigger than others. Purine to purine and pyrimidine to pyrimidine changes are more likely than purine to pyrimidine or pyrimidine to purine, for just one example. Then, in addition to single point base changes, we find insertions and deletions of one or more bases, and larger gene segment rearrangements. Then there are mobile DNA elements like transposons that really do a lot of evolutionary work.

Much of what is called "junk" DNA in eukaryotic genomes is actually the stuff that allows or causes eukaryotes to be able to evolve more quickly. Moving blocks of genes around (for example to coordinate their regulation) usually requires homologous recombination. There needs to be a chunk of DNA that is identical in the old site and the new site. So a genome with 20,000 copies of the same element, like an ALU repeat is very flexible for this type of recombination.

Also, the more DNA we have, the more targets we have for mutation. The "extra DNA" is not like a sponge that can soak up the mutations. If anything, it makes mutations more likely, because more than 99% of all damaged bases get repaired by DNA damage and repair enzymes. So the more DNA we have, the more damage there is, and the more repair needs to take place. If we added new copies of repair genes to our genome faster than we added "junk DNA" we could slow down evolution. But adding "junk DNA" speeds up evolution.


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