Mendelson and Stahl Experiment

Labelled for reuse https://pixabay.com/static/uploads/photo/2013/07/18/10/55/microbiology-163470_960_720.jpg 

Credit to my biology teacher for teaching me basically all the biology I know, and thus this as well.

I'm back! At least for now. Apologies in advance, the next posts for a while will be primarily scientific concepts...

So let's start off with a bang, shall we? Deoxyribonucleic acid, or rather DNA. Our DNA is essentially in charge of controlling everything a cell does and what it does or does not produce. Every single one of your billions of cells (well nearly every one, red blood cells don't have DNA because they need the space to carry oxygen) has a nucleus. And within that quaint nucleus you house your DNA.  All of it that is. The only real difference between DNA in your different cells is the parts which are activated the the parts which are not.  Anyway, when you started off, and I mean single cell start off, you had one cell with one copy of your DNA. Now, how are you going to get a copy of this DNA into your other cells. Well, that my friends, is the process of DNA replication (I will get into that in a later post). What I want to get into today is how it was proven that this process is semi-conservative. 

Essentially it is semi conservative because in each new cell one old parent strand of DNA is located. Recalling that DNA is a double helix made up of two strands. Because of this process there is always half of a preexisting DNA molecule in each new cell. So how did Methelson and Stahl (I can't recall these distinguished gentlemens' first names) prove that this was true? Let me tell you.

Number one) they took a common bacteria, E. Coli and placed it in nitrogen 15 isotope (which is a heavy isotope- the atomic mass of nitrogen is 14.01). They then let the E. Coli grow in this heavy isotope of nitrogen for four generations.

Numero dos) These cells were taken, and transferred to a new "growing area" (not really sure what you would want to call it...), this time with a lighter isotope of nitrogen, namely nitrogen 14. Now samples were taken from this population periodically. 

Numero tres) Once DNA was extracted from the samples they were dissolved int a solution of cesium chloride.

Numero quatro) Solution was centrifuged (the spinny thing), allowing for a concentration gradient to be built up. 

Numero cinco) Last one. I promise. The DNA molecules would then move around in the created gradient and would "stop" at the point where the density equaled that of the cesium. 

The first sample taken at step one, all DNA was heavy, as they were grown in nitrogen 15, and thus sat at the bottom of the test tube. 

Once the solution was transferred into the nitrogen 14 medium and a sample was taken of the first generation, the bacteria had DNA, all of which were located at a density between nitrogen 14 and 15, indicating that each DNA had one strand of the "old" nitrogen 15, and a new strand made when in nitrogen 14. 

When a second sample of DNA was taken (from the second generation) from the E. Coli in the nitrogen 14 mixture, there were two areas where "bands" of DNA could be found. One was at higher up in the test tube indicating that both strands of DNA had the lighter nitrogen 14. The second band was once again between nitrogen 15-14, indicating that one strand had nitrogen 14 and the other stand nitrogen 15. 

Labelled for reuse https://upload.wikimedia.org/wikipedia/commons/2/2a

Do excuse the Chinese, I believe... But this image is essentially showing what I just wrote. The first generation grown in the heavy nitrogen isotope has all DNA in one band near the bottom. Once the bacteria was then place into the lighter nitrogen isotope solution all of the first generation DNA was located in one band, which was between the band of nitrogen 15 and nitrogen 14. This indicates that one strand of each was nitrogen 14 and the other nitrogen 15. This is due to semi conservative DNA replication! Anyway, hope it makes a little more sense now.