Squid sperm to the rescue

By Madeline McCurry-Schmidt

340px-Dried_squid_3

A dried squid. Magnificent.

I’m in research mode today. I was reading The Disappearing Spoon last week (it’s a fun book, I recommend it), and the author mentioned that Rosalind Franklin used squid sperm in her DNA studies. The author didn’t dwell on the reason for squid sperm. In fact, most texts skip over the detail.

But squid sperm ain’t a footnote! As a person obsessed with squid, I refuse to stand for that. We owe our knowledge of DNA to squid (and scientists, I suppose). So my question today is: Of all creatures, why did Franklin pick squid?

According to a 1951 paper by A.E. Mirsky and Hans Ris in the Journal of General Physiology, squid sperm is special because of its ratio of DNA to sperm. Mirsky and Ris discovered this by washing sperm cells with citric acid to remove everything but the sperm heads. After counting the sperm cells, they put they cells in a centrifuge and let the DNA spin free.

Mirsky and Ris weren’t doing this research for Franklin’s sake. At the time, scientists wondered if more “sophisticated” organisms had more DNA. Would an intelligent, many-armed squid have more DNA than a bottom-dwelling sea sponge?

In fact, squids did have more DNA.

“The squid is a far more highly developed animal than are the limpet, snail and chiton; and the squid has far more DNA per sperm than is found in the lower molluscs,” wrote Mirsky and Ris.

Their squid sperm had about three times more DNA per nucleus than sperm from their runner up, cliff crabs. Sea sponges had barely a dash of DNA.

Like good scientists, Mirsky and Ris decided they need more data before making a rule that more complexity equals more DNA. They were right to be cautious. The complexity trend is not true in all organisms. Plus, amount of DNA is not the same as the number of genes. You’d figure that human beings must be more complicated, genetically, than fruit flies. But scientists have found that humans and fruit flies have about the same number of genes.

In 1951, complexity was still a puzzle, but Mirsky and Ris squid showed that sperm was at least a good source of DNA. Scientist began wondering if they could use it to solve the other mystery of their day: the shape of DNA.

Scientists used squid sperm DNA in more and more experiments. In 1953, English researchers Maurice Wilkins and John Randall reported that squid sperm DNA was packaged nicely for use in X-ray diffraction.

watson-crick1

As seen in every biology textbook ever

So squid DNA was in the spotlight when Franklin did her work at King’s College London in the 1950s. It was squid sperm DNA on those famous X-ray diffraction pictures that Franklin shared with Francis Watson and James Crick. And when Watson and Crick posed next to their model of DNA, there was a tiny piece of squid sperm posing for the world.

Want to learn more about DNA? Read about the weird genetics of hybrid animals

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About Madeline McCurry-Schmidt

I'm a science writer specializing in biological sciences and animal behavior.
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