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Lecture 24, part 2 of 2
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II. WHY HAVE SO MUCH REPETITIVE DNA?

Charlesworth, B., Sniegowski, P. and Stephan, W. (1994) The evolutionary dynamics of repetitive DNA in eukaryotes. Nature 371:215-220.
Genetic diversity can be generated by unequal crossing over. If there are two copies of a gene, some crossover events can lead to duplication of the gene on one chromosome, and deletion on the other. Successive duplication or deletion of copies can lead to increases or decreases in the size of multigene families.
 
 
 
 
 
 
 
 
 
 
 


If a gene duplication does occur, one copy of the gene is free to mutate, while the other can retain its original function. This allows evolution to experiment with variants of a protein until it stumbles upon a beneficial variant.

 
 
 
 
 
 

2. Perhaps most important reason: Decreases the potential risks of recombination.


Unequal crossing over could have deleterious effects if it occurs within a coding region. If all recombination occurred in coding regions, then you would have a high frequency of inactivation of genes.
This may be tolerated in unicellular organisms (bacteria and fungi) that have very little repetitive DNA. In this case their high reproductive rates my be such that, a) even if a fraction of individuals gets deleterious mutations due to recombination, the rapid growth rate is sufficient to prevent a population bottleneck: b) Furthermore, there is probably a selective advantage for having a small genome if you have a rapid growth rate.

 
 

3. Mid rep. DNA could play a role in chromatin packaging.

By definition, things like matrix attachment sites, and perhaps sites (if they exist) that govern higher levels of chromatin packageing, must be present in hundreds or thousands of copies. Therefore, such sites fall into the middle repetitive fraction of the genome.

4. Mid rep. could simply be selfish DNA.

Some sequences may just be more efficiently duplicated by the cellular machinery that duplicates sequences. Those sequences that lend themselves to being duplicated by the DNA replication machinery will tend to propogate throughout the genome. Darwinian selection operates even at the molecular level.Transposons are a good example of selfish DNA.

5. Mid. rep DNA might play no significant role whatsoever

Not everything has to have a selective advantage to get fixed in the population.
Point about equilibrium: Even if a particular trait or structure in the genome is selectively disadvantageous, it may take time to lose it after speciation occurs (speciation usually implies a population bottleneck).
Caveat: Most of the domesticated species of plants and animals are not at equilibrium. In many cases, their genetic diversity is greatly limited by artificial selection. We have to be careful about inferring much about evolution from domestic species.




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previous  page PLNT3140 Introductory Cytogenetics
Lecture 24, part 2 of 2
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