Famous Females - Barbara McClintock
REARRANGING GENES!
VARIATION IN OUR DNA
You probably know that half of your genes come from mom, and half of your genes come from dad. But this isn't the only way we get our differences.
But why be different? Well, living things benefit from being different because it helps their species survive over time.
Maybe a bird species needs to have a differently shaped beak to get at certain seeds, or maybe a scorpion species needs to be able to withstand higher temperatures to live in a desert.
Imagine you and your friends are making toy block towers on a table, then someone comes and shakes the table you're working on, like an earthquake!
Some of the towers would fall, but the more stable ones would stay standing.
Had you made them all the same, the quake could have wiped out all the towers. Like these toys, species need to be different so that some will survive changes to their environment.
Today's famous female is geneticist Barbara McClintock. She helped us to understand just how genes make us so different before we even knew the structure of DNA.
BARBARA McCLINTOCK (1902 - 1992)
Barbara McClintock was born at the turn of the 20th century in Hartford, Connecticut. She was originally named 'Eleanor', but her no-nonsense, independent nature convinced her parents that it was too feminine a name, and renamed her Barbara.
Her father was a doctor and her mother raised the kids and enjoyed art and poetry. Barbara and her mother had some uneasy relations however, which would show itself as Barbara grew.
In high school, her teachers praised her intelligence and saw her becoming a college professor - an idea that horrified her mother who felt that professors were an odd bunch that would make her daughter unattractive to potential suitors.
Her mom forbid Barbara from going to college, but at the last minute her dad came to the rescue. Barbara enrolled and graduated from Cornell University in New York.
She took the only course in genetics available to undergrads, and afterwards got a call from the professor inviting her to his graduate level class. She reflected, "Obviously, this telephone call cast the die for my future. I remained with genetics thereafter."
A BETTER WAY TO 'SEE' GENES
Now, genes are very, very small - you have to have a microscope to see them. And even then, you have to dye them with a stain so that you can see their structures.
Well, McClintock developed a staining technique that was better than any before.
With it, she proved that genes are physically located on little threads called chromosomes.
She also proved a previously suggested idea that sex cells (like sperm and eggs) actually swap genetic material when making new cells. It's called chromosome crossover.
This is when the chromosomes actually swap bits of themselves with each other, so that there's a whole new combination of genes on each one! Sort of like if teams swapped a few players before a new game - the game would have the same people overall, but each team would have a slightly different makeup.
COLOR IN CORN
Now, the kind of corn you're used to eating in the summer has some pretty uniform colors. Usually yellow or white. In Indian corn, kernels are different colors!
McClintock was studying one species whose kernels were sometimes white, sometimes purple, or sometimes mottled with purple streaks or spots. She wondered how this happened.
Now, corn is a great organism to use in breeding experiments because each kernel is an individual embryo produced from an individual fertilization.
That's great for scientists because they get a lot of genetic results from just one ear of corn. Instead of getting just one set of genes from one ear, you actually get about 800!
JUMPING GENES
Now, keep in mind that in the late 1940s, when McClintock was studying purple corn kernels at the Carnegie Institution of Washington, it was thought that genes didn't change much. They just stayed on their chromosomes and didn't move, right?
McClintock though, using her staining techniques, was able to see that the chromosomes of a purple kernel were similar to the spotted kernel, with just a little difference. It looked as though someone had taken a bit of one chromosome and stuck it somewhere else.
Turns out, that's exactly what is happening. McClintock discovered that some genes are actually moving around, as if scissors had come to a strand of DNA, cut some out, and then glued it back in the middle of a different set of DNA.
When this happens in corn, this extra bit can land in the middle of the genes for color, which ends up affecting what color the kernel will be.
We call these moving elements transposons, or transposing elements, or 'jumping genes'.
Turns out this is in more than just corn. It happens in animals too, which of course, includes us. This was revolutionary! McClintock said,
"You can see why I have not dared publish an account of this story. There is so much that is completely new and the implications are so suggestive of an altered concept of gene mutation that I have not wanted to make any statements until the evidence was conclusive."
OTHER SCIENTISTS DON'T UNDERSTAND
McClintock knew this was a big discovery - her most important yet. However, when she first presented it to other scientists in 1951, they didn't understand, and some even got a little hostile.
Disappointed, after a few years she stopped publishing papers on the subject altogether. She kept working though, she loved her job. She said, "I never thought of stopping."
It would take 20 more years before transposition was seen by some biologists in bacteria and viruses, and McClintock's work would be back in the limelight. Today we know that transposable elements make up over half of the human genome, and about 90% of the maize (corn) genome!
Acknowledged at last, Barbara McClintock won a Nobel Prize in 1983 for Physiology or Medicine. It just goes to show, if you know something and you have the evidence to back you up, don't give up! Everyone might just be too mind blown to see your genius and hard work just yet!
Chelsea Schuyler
Author