Hey it's professor Dave, let's discuss nucleic acids.
We learned about proteins and carbohydrates as two of the main polymers in the body,
but there is one more type we need to learn about, and that's nucleic acids.
We've probably heard of these since they include DNA and RNA, and we probably know
a little bit about DNA, but it's time to know a lot more, because this is the
molecule that gives an organism its identity, and we want to comprehend all
of the intricate processes associated with DNA, as this will allow us to combat
cancer and other diseases. Just like the other polymers we learned about, first we
have to understand the monomers they are comprised of, so let's take a look at
nucleotides. A nucleotide is a molecule that has three sections.
First there is a monosaccharide, which is always either D-ribose or 2-deoxy-D-ribose
and we can number the carbons 1' through 5'. The difference
between these has to do with the presence or absence of a hydroxyl on
carbon 2, and these sugars will specifically occur in RNA, or ribonucleic
acid, and DNA, or deoxyribonucleic acid respectively. Next, extending from the
anomeric carbon is a heterocyclic base. It's a heterocycle because there is an
element other than carbon inside the rings, in this case nitrogen, and it's a
base because of the lone pairs on the nitrogen atoms. These bases can be either
purines or pyrimidines, depending on whether there is one ring or two, and in DNA the
bases are adenine, guanine, cytosine, and thymine which are abbreviated as
A, G, C, and T. In RNA instead of thymine there will be uracil, or U, which is almost the
same as T, it's just missing this methyl group. These bases always have beta
glycosidic linkage to the anomeric carbon. The sugar and the base combined
are called a nucleoside, and these have names related to the bases.
If the base is adenine we get either adenosine or 2-deoxyadenosine
and so forth.
Now if we add the third section we mentioned earlier, which is a phosphate
group on carbon 5 of the sugar, we will get a nucleotide. Nucleotides include ATP
which is the currency of cellular energy that we will talk more about later, as
well as cyclic AMP, a second messenger molecule released in signal transduction.
But most importantly nucleotides are the monomers that make up nucleic acids.
In nucleic acids the monomers are connected by phosphate esters that link the 3'
prime hydroxyl of one nucleotide to the 5' hydroxyl of another. As these
come together we can see that there is a backbone comprised of identical sugars
and phosphate groups with the varying bases protruding from the chain. In this
way we can list the sequence of bases, like GCAT, just the way we list the
primary structure of a protein when we list the sequence of amino acids.
Now comes the interesting part. Before we fully understood the structure of DNA we
noticed that pairs of bases were always present in roughly equal amounts, those
pairs being A and T as well as C and G. After much research we finally
understood that this is because DNA exists as two strands that pair up in
base-specific manner, meaning that the two strands are complementary. Everywhere
that there is an a C on one strand, there is a G right across on the other strand, and
everywhere there is an A, there is a T on the other side. This specific pairing
happens for two reasons: first, the geometry of DNA is such that one purine and
one pyrimidine will fit nicely between the strands, whereas two purines would be too
wide and two pyrimidines would be too narrow. But the specific pairing out of
the two spatially viable possibilities is because of hydrogen bonding that
happens between complementary bases. We can see here that an AT pair has this
carbonyl interacting with this proton and this other proton
interacting with the lone pair on this nitrogen atom. A CG pair makes three such
interactions. If we tried to bind A with C and G with T these interactions would
be impossible, so base pairing is highly specific, and a DNA molecule is millions
of these base pairs extending in a double helix that looks like this, with one
strand being a perfect complement to the other, allowing for an unimaginable
number of possible sequences. These two strands are antiparallel, meaning they
run in opposite directions, one going from what we call the 5' end to
the 3' end, whereas the complementary strand will go 3' to 5'.
By contrast, RNA, which as we said is comprised of ribose sugars rather
than 2-deoxyribose, will tend to be single-stranded, and we will learn about
certain kinds of RNA later. Later we will also learn about how DNA is the genetic
code that makes you what you are, and this code has to be inside every single
cell in your body. But because there is so much information to code, the DNA
molecule is really, really, long. If you took the DNA from just one of your cells
and stretched it out, it would be over a meter long, and if you took all of the
DNA molecules in all of the trillions of cells in your body, unwound them, and lined
them up end to end, it would stretch to the end of the solar system and back.
How is there room for all of this stuff in your body?
Well DNA is stored by coiling around proteins called histones. Then these
coils undergo supercoiling to save even more space. One long supercoiled DNA
molecule with all of the histones is called a chromosome, and all of the
genetic material in the nucleus of a cell is collectively referred to as
chromatin. And while this kind of image may have been familiar before, now we
know how to zoom in and look at the individual monomers of this huge
molecule, which will allow us to understand all the things that DNA can do.
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