HAZEL SIVE: The
last thing I want
to tell you about in this
class is probably the coolest.
And that is that macromolecules
carry information.
And they carry
information because they
have got different
ends and because they
have got direction.
And if you think about
it, all languages
have got some kind of start
and stop signals to them.
And there is some direction
associated with them.
And the language of life
is exactly the same.
So let us write down
that macromolecules
may have order and polarity.
The polarity would give
some kind of direction.
And these two
attributes together
comprise information that
can be used by the cell.
The two classes
of macromolecules
that do this par excellence
are the nucleic acids
and the proteins.
And so we're going to
talk about both of those--
nucleic acids and proteins.
The nucleic acids as I
said carry the information
of hereditary.
The proteins carry
the information
of just about everything else.
So these two classes
are what we'll discuss
over the next few minutes.
In order to do that, we need
to come back and consider
the nucleic acid polymer.
The nucleic acid polymer
has two different ends.
It has got a so-called 5 prime
and a so-called 3 prime end.
And you can see
that if I draw you
a very rough schematic
of how a nucleic acid
polymer is put together.
I'm going to write 5 prime.
And next to it, I'm going
to put P for phosphate.
The phosphate is covalently
bonded to a sugar abbreviated
S which is covalently
bonded to another phosphate,
another sugar,
another phosphate,
another sugar in my
schematic, and then I'm
going to write 3 prime.
And sticking off
from the sugar, I'm
going to write the bases
base 1, base 2, and base 3.
So let's look at
what I've drawn here.
PS refers to a covalently-joined
sugar phosphate backbone.
It's the thing that holds the
nucleic acid polymer together.
The bonds that join the
sugar and the phosphate
have a special name.
They're called
phosphodiester bonds.
And they join the phosphate
to one of the hydroxyl groups
on the sugar.
Let's think now going
back to that polymer.
You can see I've written
5 prime and 3 prime.
At one end of the polymer--
this has to do with polarity.
At one end of the polymer,
there's a 5 prime end.
And the other end,
there's a 3 prime end.
Those are two different ends.
They're chemically different.
And the cell can distinguish
them from one another.
And that gives you
part of the information
coding in nucleic acids.
So the 5 prime end
is the phosphate
that's joined to the 5
prime carbon on the sugar.
And the 3 prime end is
the 3 prime hydroxyl group
that, again, is on the sugar.
The second thing
that you need to know
is that this polarity
is superimposed
with the bases and the sequence
of the bases along the polymer.
So base 1 is near
to this 5 prime end,
then base 2 and base 3.
So there is base order
along the polymer.
And then the last thing
that you should know
is that a new base will
add onto this 3 prime end.
So this base 3 was the last one
that was added to the polymer.
And when a new one comes
along to extend the polymer,
it will add to that 3 prime end.
So the 3 prime is
the last base added.
And the next base adds to
the 3 prime hydroxyl group
on the sugar.
Let's take a look at a
slide to help us there.
This is a fragment
of a DNA polymer.
And there is the 5 prime
phosphate and the 3
prime hydroxyl circled in blue
on either ends of the polymer.
And I've also circled this
complex phosphodiester
bond that holds the sugar
and the phosphate together.
And the bases are
kind of separate.
They're hanging off the
sugar phosphate backbone.
If we look at a
schematic of this,
I've drawn you the sugar
phosphate backbone, the base
order, the first nucleotide,
the last nucleotide,
the free phosphate group
at the 5 prime end,
and the free hydroxyl group
on the last nucleotide.
The direction of polymerization
is from 5 prime to 3 prime
because you always add
onto the 3 prime end.
But when we write the sequence
of the bases on nucleic acid,
we don't write the sugar
phosphate backbone.
We just take that as a given.
What's really important are
the base order, B1 to B6
in my diagram--
I've put a real example there--
and the 5 prime and
the 3 prime ends.
And you always, always,
always write those 5 prime
and those 3 prime ends.
Doesn't matter if you've been
working with nucleic acids
for 30 years.
You always have to write 5
prime 3 prime and the base order
between.
There is no exception
ever, ever, ever.
It's one of the most important
things I can tell you.
The order of the bases relative
to this 5 prime and 3 prime
is the information
in nucleic acid.
It's what the genes are.
It makes you who you
are because of these two
attributes of nucleic acid.
The polarity, the 5 prime
and the 3 prime end,
shows the first to
last nucleotide added
and the direction in which
to read the information.
We will have more
to say about this.
But what I want you to do
now is to go to this exercise
and practice your
nucleic acid polarity
skills that use these
pointers and these rules
that I've just given you.
