The first Tuesday of the month brings Nottingham Skeptics in the Pub to The Canalhouse. April’s speaker is Kat Arney here to talk about “Herding Hemmingway’s Cats – Understanding How Our Genes Work”
“Hemmingway’s cats” have six toes. Writer Ernest was very fond of them and there are a number of them still living on his old Florida estate. The sixth toe is caused by a faulty DNA “switch”. There is no such thing as a toe gene (nor is there a gene for autism, schizophrenia etc)
The double helix is the blueprint of life, the DNA within it is information, a series of recipes. The rungs of the double helix ladder are made of four different bases (A, C, G and T) Genes makes things inside cells – using our recipes they create proteins. This is the very stuff that builds us. Hence we are all unique and how we are built reflects our chances of getting cancer or having high intelligence. There is no one gene for these things. For example there are hundreds of genes that affect the connections in our brain cells.
So, DNA is the cell’s instruction manual. All of our cells are different eg skin cells, brain cells, heart cells, etc. Your genome (the sum total of all of you DNA) is kept in the nucleus of the cells. Protein is manufactured in the cytoplasm of the cells. Molecular cookery changes DNA into RNA and then into protein. But how does the DNA get out of the nucleus into the cytoplasm? This is where the RNA comes in – it’s a “photocopy” of one half of the helix from a gene. This is then able to leave the nucleus and get into the cytoplasm to create the protein.
There is 2.2 metres of DNA in each cell in your body, meaning that if you stretched out all of the DNA in your body it would reach to the moon and back 1500 times. DNA is split into 23 chromosomes and there are 3 billion pairs of bases in your body (if you printed these out it would be the equivalent to 200 thousand page phone books) and it takes 20,000 genes to build a human. All of your cells contain the same DNA but each cell will make different proteins. So, how does a skin cell know that it’s a skin cell?
We make 100,000s of different proteins in our cells but the recipes used for this aren’t as precise as Delia’s. In fact less than 2% of the genome is actually made up of genes. The rest of it is non-coding DNA. For a long time this was thought of as “junk” DNA but it now turns out that 80% of it looks like it might actually be functional as it unilaterally interacts. Despite this is does seem like most of it is dull and repetitive and made up of harmless DNA that does nothing. In many ways it’s like all that stuff that you have in your house that you can’t bring yourself to get rid of and some of the non-coding DNA is actually junk. However, 7-8% of the non-coding DNA is control switches that turn the DNA on and off.
The control switches are what turns on the genes in the right place at the right time and this happens from the very beginning of life. Which brings us back to Hemmingway’s cats. The author was fond of three things – fighting, fishing and erm, female company. At the intersection of that particular Venn diagram, you’ll find cats. According to legend, Hemmingway was given a six toed cat by a sea captain. They were popular on ships as their extra fingers meant that they were better rodent catchers, had increased prehensility so that could hold on better in stormy seas and because sailors are just really superstitious.
The reasons that these cats have an extra toe is all down to the limb bud control switch. This controls a gene called the Sonic Hedgehog gene which enables cells to decide whether they are going to be “toes” or “not toes”, If the switches don’t turn on or off at the right time then you can end up with too many toes just like Hemmingway’s cats.
These control switches are evolution’s playground. For example the difference between salt water and freshwater sticklebacks. During the last ice age many sticklebacks found themselves stuck in freshwater, unable to get back to the sea. 10,000 years later (which isn’t very long in evolutionary terms) the freshwater sticklebacks had lost their “death spikes” due to the lack of predators. In fact, due to the PITX-1 gene and how the control switch turns it on, they had lost their entire pelvis. Meanwhile male chimps have a penis that is covered in spikey nodules that help them to “hang on” during intercourse. A number of other mammals also have these but humans have evolved to a stage where we don’t require them. However, it is possible to turn on the control switch in humans, although I’m not quite sure why we’d want to.
To drive home just how little we really know about genes, Kat tells us about an experiment on nematode worms where they made a change to one of the worm’s genes. This mutation should have killed all of the worms. However, despite the worms all being genetically identical, half of them survived. After changing a second gene, which again on its own should have killed the worms, 10% of them were still alive. They had to mutate a third gene to finally kill all of them. In fact even in humans we are walking around with 30-40 mutations that should kill us but don’t.
Finally a look at how one small change can make such a huge difference. In humans we have a gene called KITLG which is responsible for making blood cells and semen amongst other things. In hair, if you replace one of your A bases with a G base in this gene, it’s the difference between being a blond and a brunette. Similarly, one letter different (in those 3 billion) makes a difference to your skin colour which should surely bury racism on scientific grounds even if we can’t bury it on moral ones. It does just got to show, that from a genetic point of view, we’re all mutants
Next up at Nottingham Skeptics in the Pub is Alistair Coleman who is coming to talk about “North Korea – How Much Do We Really Know?” on Tuesday May 10th at 7:30pm in The Canalhouse
By Gav Squires