Most of the time, biologist in the lab aren’t working with things that they can see. You need to look at cell under a microscope, proteins using X-Ray crystallography (for example), and DNA and RNA by various methods like gel electrophoresis or cell staining (see more about visualizing DNA here). Many experiments require scientists to manipulate DNA, RNA or proteins in small liquid volumes to do experiments to understand the sequence of these molecules, to discover what other molecules they bind to, or to change them to figure out how they work in cells. To manipulate these small volumes, we use a tool called a pipette (shown at left) – also called a micropipette to be even more precise. It’s similar in concept to the baster you have in your kitchen that sucks up liquid when basting your turkey, except it’s for much smaller volumes and is exquisitely precise.
Pipettes come in different sizes, and the size of the pipette determines the volume that they can dispense (see picture at right). They dispense microliter (abbreviated as “ul”) volumes of liquid. To give you an idea of scale, 1 drop of water is about 50ul. One milliliter (ml) is1000ul, which is the largest micropipette volume you can dispense, and there are about 44ml in a shot of vodka. These are small volumes we are talking about!!
The sizes shown to the right dispense up to 2ul (called a “p2”), 200ul (“p200”), or 1000ul (“p1000”). By spinning the plunger on the pipette, you can change the volume that is dispensed within the range that the pipette allows. You then put disposable tip to the end of the pipette and suck up that volume of liquid. As an example, check out the photo below of the p1000 dispensing 600ul of water and the p200 dispensing 20ul of water. The image to the left is the pipette tip filled with that volume and to the right is the water in a tube. This type of tube is used every day in the lab and is called an eppendorf tube. IT can hold 1.5ml (or 1500ul) of water. These small plastic tubes have attached caps that can snap the tube closed to allow you to mix the contents or do whatever is needed for the experiment (like heat it up, cool it down, spin the contents, etc). And if you’re wondering why they are called “eppendorf” tubes, it’s after the name of a German company called Eppendorf that is one manufacturer of this kind of tubes (similar to how the brand Kleenex is now synonymous with tissues)
Maybe you’re wondering why scientists have to use such small volumes? One reason is because there isn’t a large amount of DNA or RNA or proteins in cells, so when it’s isolated, it doesn’t take up a large volume. As an example, if you isolate the genomic DNA from 5 million cells, you would isolate 10-30ug of DNA. That’s MICRO grams of DNA – or 0.00003 grams. As comparison, a grain of salt weighs more than 1200x more than the amount of DNA that you would isolate from 5 million cells! And this is all in just 50ul of liquid. Very small amounts! The good news is that working in large volumes isn’t really needed because most experiments don’t need a large quantity of materials to get the answer. Not to mention that working with larger volumes for experiments would be more expensive. If you are doing an enzymatic reaction, a larger volume reaction would require more enzyme. To get an idea of what this would cost, a very common reaction cuts DNA with restriction enzymes, which are essentially super-specific DNA scissors. These restriction enzymes cost several hundred dollars per tube and you use 0.5-1ul for every 50ul reaction. If you did a reaction in a large test tube of 10ml, you’d need 200 times more enzyme and would go through hundreds or thousands of dollars if enzyme for each experiment. Labs are not that rich!
Fortunately scientists have developed techniques and equipment, like the micropipette, that can manipulate, detect and analyze incredibly small amount of DNA or RNA or proteins. For most bench scientists, learning how to use a micropipette is done on day one (I learned how to use one in high school at City Lab) and pretty soon becomes second nature. In grad school, I pipetted so much that there were days I’d go home with a sore thumb or pipetting calluses. However all this practice did pay off. I picked up a pipette for the first time in 8 years a few weeks ago to start doing experiments in my new lab. After all these years, it was just like riding a bike – or in this case just like “using a pipette”.