In this video, we're going to talk about the difference between a nucleotide and a nucleoside. And then we're going to talk about the difference between purines and pyrimidines. So what is the difference between these two?
What is the difference between a nucleoside and a nucleotide? What would you say? A nucleoside has two components to it.
It contains a 5-carbon ribose sugar and it contains a nitrogenous base. So that is a nucleoside. A nucleotide has three units. It has the 5-carbon ribose sugar and it has the nitrogenous base. In addition to that, it contains a phosphate group.
So that is a nucleotide and this is a nucleoside. Now there are five nitrogenous bases that you need to be familiar with. A, G, T, C, and U.
A stands for adenine, G is guanine, T is thymine, C is cytosine. and U is uracil. The first two, adenine and guanine, are known as purines. Purines are nitrogenous bases that contain two rings. The other three, thymine, cytosine, and uracil, these are known as pyrimidines.
And the reason for that is that they contain only one ring. So pyrimidines are nitrogenous bases with one ring. Purines are nitrogenous bases with two rings.
Now, it's important to know that DNA, deoxyribonucleic acid, contains the nitrogenous bases A, G, T, C. RNA, ribonucleic acid, contains the bases A, G. Now instead of thymine, RNA has uracil. So make sure you understand that. DNA contains thymine, but RNA contains uracil.
So here we have two purines. On the left we have adenine and on the right this is guanine. One key difference between the two is that guanine has a carbonyl group, whereas adenine does not. Adenine does have this aromatic ring. It looks like a benzene ring, but it's a heterocyclic aromatic ring due to the nitrogen atoms.
Whenever you see a six-membered ring with alternating double bonds, such a ring is very stable due to aromaticity. So here are the structures of the three nitrogenous bases that are pyrimidines. They contain one ring. On the left, we have cytosine.
In the middle, thymine. And on the right, uracil. Notice that thymine and uracil look very similar.
The only difference is that thymine has a methyl group, whereas uracil does not. Now, both thymine and uracil contain two carbonyl groups. whereas cytosine only contains one.
So that's how you can tell them apart. Cytosine has one carbonyl group, thymine has the methyl group, and uracil just have two carbonyl groups and two nitrogen atoms. Now the primitive molecules are quite stable because if we draw the resonance structure you could see that they could form an aromatic ring.
Let's take a lone pair from nitrogen And let's use it to form a pi bond. And let's break the pi bond of the carbonyl group. Now let's do the same thing with the lone pair on the other nitrogen atom.
This oxygen also has two lone pairs. But once we break the pi bond, it's going to have three lone pairs. And so we're going to get a resonance structure that looks like this. So now each oxygen atom will carry a negative charge, and now we have two additional double bonds, and there's going to be a positive formal charge on the nitrogen atom.
Anytime nitrogen has four bonds, it carries a positive formal charge. Each oxygen will now have two lone pairs. So in this form, notice that the ring looks like a benzene ring, with the exception of the two nitrogen atoms in the ring.
So what we have here is a heterocyclic aromatic ring. Aromatic rings are highly stable due to the alternating single and double bonds, and the fact that the pi electrons can move freely around that ring. So this resonance form is highly stable, except the charge separation part.
But that's the resonance structure of your cell, at least one of the resonance structures that you can draw. But that's it for this video. So thanks for watching, and now you know the difference between nucleosides and nucleotides, and purines and pyrimidines.