Introduction to nucleic acids and nucleotides | High school biology | Khan Academy

We are now going to talk about what is perhaps the most important macromolecule in life, and that is known as nucleic acid. Now, first of all, where does that name come from? Well, scientists first observed this in the nucleus of cells, and so that's where you get the "nucleic" part, and it has some acidic properties, and so that's where you get the "acid" part.

Perhaps the most famous of the nucleic acids is deoxyribonucleic acid, or DNA for short, and we'll go into some depth in this as we go through our journey in biology. But you might already know that this is the molecule that stores our hereditary information. This DNA, to a large degree, makes you you, and it's known as a macromolecule. We've talked about macromolecules in other videos; we've talked about carbohydrates, and we have talked about proteins.

DNA is a macromolecule because it can be made of many millions of atoms. Just to get a sense of it, you can see right over here the double helix of DNA, where you have one side of your helix right over there, and then another one right over here. Then, you kind of have these rungs of this twisted ladder. A DNA molecule, let's say in the human genome, a chromosome for example, is primarily a really long DNA molecule, and it can have on the order of a hundred million rungs to this ladder.

Now, another thing to appreciate, like many other macromolecules, DNA or nucleic acids in general, they are polymers in that they are made up of building block molecules. Those building blocks for nucleic acids—and DNA is the most famous nucleic acid and RNA (ribonucleic acid) would be a close second—but the building blocks of them are known as nucleotides.

Nucleotides, and we see some examples of nucleotides right over here. This is d-oxy adenosine monophosphate, which would be a nucleotide found in DNA. You can see the various parts of it; you have a phosphate group right over here, you have a five-carbon sugar, which in this case is ribose, and then you have what is known as a nitrogenous base.

And why is it called nitrogenous? Well, those blue circles represent nitrogen, and we've seen this before. The grays are carbons, and the reds are oxygens, and the whites are hydrogens. This part of the molecule has some basic characteristics, while this phosphate group at the end has some acidic characteristics. What happens is they get stacked onto each other, where the ribose phosphates alternate to form the backbone of this DNA molecule.

You can see it right over here, where you have a phosphate and a ribose, and a phosphate and a ribose, and then you have the nitrogenous base forming part of the rung of the ladder. The way that DNA stores information is that every one of these nitrogenous bases right over here, this is adenine; it has a complementary nitrogenous base on the other to complete that rung of the ladder. So, adenine matches with thymine in DNA, and we'll see in future videos in RNA it's a nitrogenous base known as uracil, and guanine matches with cytosine.

Don't worry too much about this now; we'll go into some depth in this in future videos when we talk about DNA and how information is stored in it. But for the sake of this video, just appreciate that the monomer for a nucleic acid like DNA is a nucleotide. So, monomer.

To be very clear, this would not be the only monomer; the analogous nucleotide in RNA, which stands for ribonucleic acid, would be adenosine monophosphate. Right over here, you can see the difference between the two: that we have an oxygen right over here and we don't have an oxygen right over here. That's why this is called deoxy, and that's why it's deoxyribonucleic acid—you're missing one of those oxygens on your five-carbon sugar.

But adenine, as I mentioned, is not the only nitrogenous base. You could have a nucleotide where the nitrogenous base is thymine. Once again, this looks very similar, but notice what is going on over here. You could have a nucleotide that looks like this. Once again, you have your five-carbon sugar here, you have your phosphate group, but the nitrogenous base here keeps on changing, and it's the order of these different nucleotides that actually encodes the information in DNA.

Now, one question you might say is, “Well, look, if I have this part of the molecule that has basic characteristics, why is it considered an acid?” Well, look at how this molecule is structured. The basic parts form the rungs of this ladder, so they're not going to be as reactive because they're really tight; they're closer to the inside of the molecule, while the acidic parts, the phosphate groups, are on the outside, so they're going to be more reactive.

The molecule as a whole is going to have an acidic characteristic. I'm going to leave you there. In future videos, we're going to go into a lot more depth in the importance of nucleic acids, especially DNA and RNA, and to just appreciate how amazing these molecules are. Some folks theorize that the first signs of life were nothing more than self-replicating RNA molecules that eventually got enclosed in a cell in some type of membrane structures, which eventually started forming the genetic machinery of a cell, etc., etc., etc.

So this is why these nucleic acids are sometimes considered the most fundamental macromolecule to life.