Is DNA the genetic material?

DNA, DNA, DNA......... It's all about the DNA!

Remember the time when I showed you how the famous DNA duo Crick and Watson made the first DNA structure? Sure you do!

I have something to ask, is DNA the genetic material?

All of you who learned biology would say "Of course, who in the world would not know that?" That's right, it's now too obvious that DNA is the genetic material.

However, in 1869 after DNA was first showed to biologists, some of them still thought that the proteins was the guy who was carrying all the information because DNA is a lot more simpler(only has four base pairs and some phosphates) then proteins do.

Luckily, some great scientists showed evidence and experiments that DNA was our little genetic material guy - And I'll tell you this: Crick and Watson did not solve this problem.

Wanna know who did this and what experiments it was? Let's start!

Now, somebody. A guy named Griffith had an experiment with mice and pneumonia bacterias and did it quite well.

First, look here: https://en.wikipedia.org/wiki/Bacterial_capsule

This will show you some structures of bacteria cells. 

The pneumonia cells also have characters in their DNA, and the one we are going to use is something called the rough and smooth cells.

The bacterias has a polysaccharide coat which acts like a protection gear protecting the bacteria from enemies. For example, a mammal immune system.

Bacterias with the smooth side progeny(or gene) has polysaccharide coats while the rough side progeny(gene) doesn't have them.

What Griffith did in his experiment is to have four examples of pneumonia with different gene types or progenies in each one. Then he injected those to a mouse.

The first one was the smooth type pneumonia which was highly virulent because they have almost no problem entering the body with the help of their polysaccharide coat. As a result, the mouse died.

The second one is a rough type pneumonia which was not that virulent because unlike the smooth pneumonia, the rough one didn't have the polysaccharide coat with them. The mouse lived in result.

The third one was the smooth type, only the difference now is that Griffith had heat - killed this bacteria. The mouse has lived again, and it's nonvirulent.

The last one is a mixture of a rough type pneumonia with a heat - killed smooth pneumonia, a mixture of two nonvirulent bacterias. But guess what? The mouse has died from this mixture!

Weird, we would expect the mouse to live in this one because both of the mixture is nonvirulent.... So Griffith examined the blood from the dead mice from the fourth experiment, And a shocking news came to him.

There was suddenly a live smooth pneumonia bacteria living!

But what we had injected to the mouse was the rough bacteria with the heat killed smooth bacteria, so if we say mutations are not possible in this experiment, this means that there was a chance that the mixture could have changed to become into a smooth bacteria. 

The heat killed smooth guy has already lost it's polysaccharide coat, and it's dead. So there's no way it would have rebirthed and lived.

But the rough type bacteria could have picked up something from the heat - killed bacteria and turned into a smooth bacteria! That could work!

Griffith has discovered that the rough pneumonia has been transformed into a smooth bacteria by a "transformation principle". These days, we can now know that the transformation principle was actually a happening by the rough type pneumonia who has taken the DNA of the heat killed pneumonia when it lived from the heat, making it from a rough type into a smooth type.

And then, for further more of this, Avery, McLeod and McCarty has experimented with the heat killed pneumonia to be with some special enzymes and mixed it with the rough type pneumonia to inject it to a mouse. 

In this one, we are going to have two enzymes. A enzyme that eats proteins and an enzyme that eats DNA. 

The experiment has told us this: When the heat killed bacteria was with the protein eating enzyme and did the steps above, the mouse died. So the transformation still occurred.  

However, when it was with the DNA eating enzyme and did the same steps, the mouse had lived! The transformation has not occurred!

These data has proved well enough for the "DNA is the genetic material" argues, but there is just one more experiment I want to tell you. 

Something called a Hershey - Chase experiment has filled up some few gaps of the Avery - McLeod - McCarty experiment, making it a stronger proof that DNA is the genetic material.

First things first, do you know about phages?

Phages, short for "Bacteriophage" are tiny viruses that hunts for bacterias. Once they found their target, phages enters their DNA inside the bacteria, and the DNA will form into new phages, which will go outside from the bacteria, making the bacteria unable to do it's original job.

Phages has a protein body(or phage coat) on the outside, but in the head part, they have their DNA inside the protein phage coats. That's a basic intuition of phages.

The second thing to know is that proteins has sulphur, but not phosphorus inside them while DNA has phosphorus, but not sulphur inside them.

Hershey and Chase wanted to label the phages whether to know if the genetic material is the protein or the DNA, so they kept two 'sides' of phages.

One side was to keep and grow phages in radioactive 35 - sulphur and the other side was to do the same thing in radioactive 32 - phosphorus.

Within this, Hershey and Chase could label the protein side has the sulphur, and the DNA side as the phosphorus.

Hershey and Chase kept and grew the phages from the S side and the P side, then they let the phages attack a bacteria for just enough time for the phage DNA to enter the bacteria. Then they blended it to separate the phage coat from the bacteria and centrifuged it.

The results had a supernatent and a precipitate, the supernatent was the phage coat and the precipitate was the bacteria pellet. Look here for more:

https://en.wikipedia.org/wiki/Precipitation_(chemistry)

I'll give you some interesting news: When the bacteria and the phage was blended, the bacteria had to give out it's DNA. So the bacteria pellet must have either of the 'labels' which is either sulfur or phosphorus. 

So that means depending on what came out of the bacteria pellet will determine the result of the experiment, and will also proof to the protein side or the DNA side for the question "What is the genetic material?".

Guess what? The radioactive 35 - Sulphur came out of the supernatent while the radioactive 32 - Phosphorus came out of the bacteria pellet!

It means that DNA is indeed, the genetic material! 

The experiment of Griffith, the Avery - McLeod - McCarty experiment, and the Hershey - Chase experiment all tells us that DNA is the genetic material that we need to have in order to live. And that's how we now "obviously" know that DNA is our most important guy in our cell, the genetic material.

It's pretty fascinating, the structure of four nitrogeneous pairs and phosphorus has some sort of mechanism to code the start of life... The why of this still hasn't been solved yet, but i'm sure somebody(who can be me or you) could figure it out for a another dimension!

That's all for now in this article, and happy biologying!

                               By Jane Kim,   <<해인이가>>