March 6 – Stranding Around

Today’s Factismal: There are 4,401,091 different ways to combine the DNA in human chromosome 1, which contains 4,316 different genes.

When talking about DNA it is easy to get lost in the numbers. The four “letters” of DNA are written into the twenty-three “books” of paired of human chromosomes, separated into more than 20,000 “chapters” of genes. The largest human chromosome has 249,250,621 of those letters that can be written 4,401,091 different ways, whereas the smallest human chromosome (the “Y” or male chromosome) has a paltry 59,373,566 letters that can be written just 286,812 ways. (The “X” or female chromosome has 155,270,560 letters that can be written 2,174,952 different ways. So it is true – women are far more complicated than men.)

The twisted ladder of DNA (Image courtesy National Institute of Health)

The twisted ladder of DNA (Image courtesy National Institute of Health)

But back in 1952, very little was known about DNA. At the time, it wasn’t even considered a serious contender for the genetic codebook; most biologists thought that the information was carried by the proteins and that DNA had, at best, a support role. A large part of that was because they didn’t understand how DNA was organized. Was it a messy blob or was there a structure to it? If DNA were a blob of organic goo then it would probably be useless for carrying genetic information. But a regular structure would imply that it had a more central role.

And it was that structure that Francis Crick and James Watson discovered. Based on the work of Linus Pauling, who has discovered that amino acids had a helical structure, and more controversially on the work of Rosalind Franklin, whose unpublished discoveries they used without her knowledge or permission, Crick and Watson were able to deduce that DNA has a structure like a twisted ladder with straight rungs, the famous “double helix”. This structure allows DNA to “unzip” as it is used to create templates for proteins and then to zip back into place without losing information or being scrambled. It also allows for DNA to replicate itself simply by completely unzipping and allowing the complementary parts of the “ladder” to fall into place.

On March 6, 1953, Watson and Crick sent their paper describing the structure of DNA into the magazine Nature; it would be published less than a month later and would completely rewrite biology as we knew it. They would go on to win the 1962 Nobel Prize in Medicine for the discovery (Franklin had died in 1958 and so was not eligible), and biologists would go on to sequence the entire human genome in 2003 (which is where those numbers in the first paragraph came from). Thanks to their discovery and the hard work of thousands of biologists, new medicines and new treatments for old diseases are now possible.

If you’d like to help out with making some of those discoveries, then why not join the Personal Genome Project:

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