Re: Length of DNA

Jan Allbright wrote:



Bob ... Could you expand a bit on "unusual recombination events"? And how would you differentiate between "total amount of DNA" vs "length of DNA"?


unusual recombination events can include translocations, inversions, translocated inversions...

Scott ... How would horizontal transfer function in higher forms?

Horizontal gene transfer is not as significant in higher forms. You would be talking about adding a minimal amount of DNA this way ie. thousands of bp to billions = not very much.

Some more background. I thought I was on the right trail in looking at the history of wheat by which 7-Chrom wild grasses outcrossed first to a 14-Chrom wheat and then to 21-Chrom wheat. But this appears to be a case of polyploidy as opposed to a 1N increase.


But genome duplication doesn't mean that they remain polyploid. Over time since multiple copies of genes exist, many genes can mutate because the selective pressure to maintain a functional gene is spread over all the copies resulting in mutation. Over time, the 'polyploid' state becomes 1N.

This happens in plants commonly. You can even force plants to double their genetic complement using colchicine to disrupt the segregation of chromosomes, then take the cells with double the genetic complement and grow them.

There is a guy that does this regularly with pitcher plants, and the results are sometimes interesting.


The example of multiple copies of genes resulting from erroneous recombination events has more than likely led to the evolution of our diverse immune systems. Natural selection is the most important factor in genome expansion.

Lets say hypothetically you have an essential protein that bursts a single type of bacteria, and your population lives in an area where they are impossible to avoid. There would be strong selective pressure to maintain this protein as it is.

Then, since humans are diploid, lets say a recombination event results which causes two copies of the gene to be contributed to the offspring by one parent ( for example: by translocation of a chromosome fragment caused by damage to DNA), and one copy by the other parent through normal gametogenesis. Now the offspring has three copies. This initial mutant will probably have no significant advantage over her peers with the extra copy, and as her genetic contribution is passed over and over and over, the chance of mutation in one of her copies gets larger.
What are the results? perhaps a single nucleotide change introduces a stop codon in one copy, no problem, there are two more copies. But lets say a single nucleotide mutation in one copy instead of resulting in a dud (the most likely circumstance), results in a single amino acid change in the protein. Now this sort of mutation changes the biochemical properties of the protein. Perhaps this new single amino acid variant of the original protein results in the ability to combat more than one species of bacteria, all of the sudden this 'mutant' has an advantage over her peers.

Natural selection will therefore favour this 'mutant' over her peers who lack this new gene, and she will reproduce like mad while her peers will have more trouble because they suffer from bacterial infections which their original protein does not combat.

Duplication events are favourable because they allow for greater diversity amongst related genes.

ie. the immunoglobulin genes which have enough diversity to allow humans to combat all sorts of nasties (complexity is in greater than 10^8 depending on what you talk about) These inevitably formed from duplication events.



To give you a better example of how totally messed up recombination can happen, consider that of the human genome we have lots of junk DNA which used to be functional transposons. We have many copies of these things all over our genome in every chromosome.

These archaic transposons all have the same DNA sequence( for simplicities sake).

Now this allows unrelated sections of different chromosomes to pair up transiently and undergo homologous recombination events due to the archaic transposons homology between chromosomes. Now you can get a translocation between these chromosomes, this event could happen amongst different areas of the same chromosome resulting in duplication of DNA in the same chromosome rather than a translocation between different ones.

This sort of thing is not overly mysterious. natural selection punishes errors in DNA replication and segregation more than it favours them though. If you have an unusual number of chromosomes, or they have no partner, you will most likely die.

There are estimates on spontaneous abortion in humans, you should look them up, a lot of these abortees are genetic freaks whom nature selects against before they are even born let alone allowed to pass on their genes.

The cases of successful addition of genetic information can only happen when people can successfully breed.

Anyways it's lunch time for me.. Chew on that





Thanks folks .. the light is getting brighter! 
.

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