Genome complexity: DNA is all sizes (Introduction)

by David Turell @, Friday, November 10, 2017, 23:03 (129 days ago) @ David Turell

Looking at all organisms, DNA comes in all sizes totally unexplained:

'Take an onion. Slice it very thin. Thinner than paper thin: single-cell thin. Then dip a slice in a succession of chemical baths cooked up to stain DNA. The dyed strands should appear in radiant magenta — ­the fingerprints of life’s instructions as vivid as rose petals on a marital bed. Now you can count how much DNA there is in each cell. It’s simply a matter of volume and density. A computer can flash the answer in seconds: 17 picograms. That’s about 16 billion base pairs — the molecular links of a DNA chain.

'Maybe that number doesn’t mean much to you. Or maybe you’re scratching your head, recalling that your own hereditary blueprint weighs in at only 3 billion base pairs. “Huh?” joked Ilia Leitch, an evolutionary biologist at the Royal Botanic Gardens, Kew, in England. Her reaction mimicked the befuddlement of countless anthropocentric minds who have puzzled over this discrepancy since scientists began comparing species’ genomes more than 70 years ago. “Why would an onion have five times more DNA than we have? Are they five times more clever?”

'Of course, it wasn’t just the onion that upended assumptions about a link between an organism’s complexity and the heft of its genetic code. In the first broad survey of animal genome sizes, published in 1951, Arthur Mirsky and Hans Ris —pioneers in molecular biology and electron microscopy, respectively — reported with disbelief that the snakelike salamander Amphiuma contains 70 times as much DNA as a chicken, “a far more highly developed animal.” The decades that followed brought more surprises: flying birds with smaller genomes than grasshoppers; primitive lungfish with bigger genomes than mammals; flowering plants with 50 times less DNA than humans, and flowering plants with 50 times more; single-celled protozoans with some of the largest known genomes of all.


'Mammals are not especially diverse when it comes to genome size. In many animal groups, such as insects and amphibians, genomes vary more than a hundredfold. By contrast, the largest genome in mammals (in the red viscacha rat) is only five times as big as the smallest (in the bent-wing bat). Many researchers took this to mean that mammalian genomes just don’t have much going on. As Susumu Ohno, the noted geneticist and expert in molecular evolution, put it in 1969: “In this respect, evolution of mammals is not very interesting.”


'Or perhaps genomes are unpredictable in the same way life is unpredictable — with exceptions to every rule. “Biological systems are like Rube Goldberg machines,” said Jeff Bennetzen, a plant geneticist at the University of Georgia. “If something works, it will be done, but it can be done in the most absurd, complicated, multistep way. This creates novelty. It also creates the potential for that novelty to change in a million different ways.'”

Comment: I've skipped all the theorizing about why sizes are what they are. There is no coherent theory. Transposons play a key role:

"By then, scientists had learned that B chromosomes are only a tiny fraction of the molecular parasites making genomes fat. The most prolific freeloaders are mobile strings of DNA called transposons, identified in 1944 by Barbara McClintock, the groundbreaking cytogeneticist who was honored with a Nobel Prize for that discovery. Transposons are popularly known as “jumping genes,” although they are rarely in fact true genes. They can get passed down from one generation to the next or transmitted between species, like viruses, and they come in several flavors. Some encode enzymes that snip a transposon out of its place in a genome and paste it elsewhere. Others copy themselves by manufacturing RNA templates or stealing enzymes from other transposons." (plucked out of the article)

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