Though what happens in cells may appear to
be chaotic, in fact the overall process is
highly, tightly regulated. We know the central
dogma specifies that DNA, makes RNA, makes
protein. In this module I will talk about
some considerations of that to help us better
understand the process. First I'll talk about
nucleic acid structure because understanding
structure is important to understanding function.
Next I will talk about how the DNA is replicated
in very general terms. And then last I'll
talk about the synthesis of RNA from DNA,
the process of transcription.
Now when we consider the nucleic acids, DNA
and RNA, we have to first of all understand
some things about their structure. The building
blocks of DNA and RNA are nucleotides and
these nucleotides can be broken into two different
groups, deoxyribonucleotides and ribonucleotides,
you can see them on the screen here. Now these
nucleotides have some common features associated
with them. First each has a high energy triphosphates,
the three phosphates linked together hold
a lot of energy. They each have a sugar, in
the case of the deoxyribonucleotides, that
sugar is called deoxyribose, in the case of
the ribonucleotides, that sugar is known as
ribose. Last, each of these have a base and
the base can be one of four different bases,
in the case of the deoxyribonucleotides, the
bases are adenine, guanine, cytosine or thymine.
Ribonucleotides have three of the same bases,
adenine, guanine and cytosine, but instead
of thymidine, they have uracil.
This figure shows the structure of the four
deoxyribonucleotides, deoxyadenosine triphosphate
known as the dATP, deoxyguanosine triphosphate
known as the dGTP, the deoxycytidine triphosphate
known as the dCTP and deoxythymidine triphosphate
known as dTTP. The deoxy in front of the thymidine
is sometimes omitted. Now, the deoxyribonucleotides
are broken into two groups, the purines contain
either adenosine or guanosine as you can see
on the very top and these groups have bases
that are very large. The pyrimidines by contrast
have either cytidine or thymidine containing
a basis and these are very small. When base
pairing occurs, C pairs with G, which means
a pyrimidine pairs with a purine or A pairs
with T, which means a pyrimidine pairs with
a purine again. Ribonucleotides are very similar,
the difference being that we have uridine
triphosphate, instead of deoxythymidine triphosphate.
In each case you'll notice the deoxy
is missing, because in this case we have ribose
in place of deoxyribose, in making ribonucleotides.
Ribonucleotides of course are used to make
RNA, deoxyribonucleotides are used to make
DNA. Again, we have the purines because we
have adenosine and guanosine on top and we
have the pyrimidines because we have cytidine
and uridine on the bottom.
Now, the structure of nucleic acids is depicted
schematically on the screen, we see for example
the individual nucleotides but now they have been
joined by phosphodiester bonds. Phosphodiester
bonds are the joining forces that hold together
the bases on one strand of a DNA molecule.
You can see that the deoxyribose sugars have
a numbering scheme associated with them, such
as I talked about in the carbohydrate lecture.
Now the numbering scheme of the deoxyribose
or the ribose, if we're talking about RNA,
allows us to define polarity within a nucleic
acid. So for example we see at one end that
we have a five prime, the five relating to
the five at the end of the deoxyribose sugar.
At the other end of the molecule, we see a
free three prime, we have a hydroxyl down
there. So this defines two ends of a nucleic
acid molecule. You will frequently hear people
referring to five prime and three prime in
this way, and five prime and three prime turn
out to be very important because the five
prime to three prime direction is the way
the DNA is made, it's the way that RNA is
made, and it's the way that messenger RNA
is translated into protein.