HAZEL SIVE: At this
point, you should
have had practice with these
aspects of DNA replication.
You should understand
which the template strands
are, both of them.
You should be able to,
again, use complementarity,
or base pairing, to fill
in the opposite strand.
You should have some
sense of what a primer is.
Really, what you
need to know is it's
the 3-prime end you add onto.
And you should have a sense of
the direction of DNA synthesis,
the 3-prime and addition rule.
So bearing those rules in mind,
and DNA replication in mind,
let us go on to our next topic,
which is that of transcription.
If you like, transcription
is the first step
in the information transfer.
Transcription produces RNA that
is copied from a DNA template.
The difference between
transcription and DNA
replication are really twofold.
Firstly, the RNA uses uracil,
where DNA uses thymine.
And secondly, unlike the
case for DNA replication,
only one of the strands
is used as a template.
So there is only
one template strand.
Let's see how it works.
Let's start off again with our
double-stranded DNA, our gene,
going in antiparallel
arrangement--
5-prime to 3-prime,
3-prime to 5-prime.
So this is our gene.
It's DNA.
As before, for DNA
replication, the strands
are going to separate.
Here they are--
5-prime to 3-prime and
3-prime to 5-prime.
The top strand is not the
template strand in our example,
so we're going to call
this non-template.
The bottom strand is
the template strand.
I just made that up.
I could have done it
the other way around.
It wouldn't have mattered, OK?
But you'll see in this example
why I've done it this way.
That template
strand is now going
to be copied into RNA in
much the same way that DNA
replication takes place--
a different set of components,
but the complementary idea
is the same.
So this template strand
is now copied into RNA,
and the RNA will go
5-prime to 3-prime.
The RNA will then
leave the template.
The RNA leaves the template.
And the two strands,
the template
and the non-template strand,
will come together again,
and they will make their
complementary base-paired
arrangement.
That is transcription.
Let's look at a
couple of slides.
Let's firstly talk
about the difference
between the nucleotides
in RNA and DNA.
I've told you, one, that there's
uracil instead of thymine.
The other one that
I'll point out to you
is that ribonucleotides,
the ones that make RNA,
have got an extra
hydroxyl group on them.
They've got that 2-prime
hydroxyl on the sugar.
That is not there in
the deoxyribonucleotides
that make DNA.
The "deoxy" in
deoxyribonucleic acid
refers to that
2-prime hydrogen that
is a hydroxyl in the ribose.
OK?
So there is a difference
of hydroxyl group
as well as this uracil versus
thymine in the nucleotides.
The reason that you care at all
about this extra hydroxyl group
is that it makes the
ribonucleotides more reactive.
It makes RNA less
stable than DNA.
That hydroxyl group on the
2-prime is a reactive group,
and RNA is much less
stable than DNA.
So DNA is a better storage
material for the genes,
because it doesn't break
down as much as RNA,
because it doesn't have this
reactive 2-prime hydroxyl.
Here's a schematic
of transcription.
I haven't put the exact
nucleotides in here.
But you can see template
and non-template strand.
You can see that
the RNA is copied
from just one of the
strands, and after it's
copied it leaves the template,
and the template strands
go back together again.
You should note on
the slide that I've
indicated a start site and a
stop site for RNA synthesis.
There is a choice, and there
are particular DNA sequences
that regulate where
RNA is made from,
where transcription begins,
and where transcription ends.
And one thing I want to
really emphasize here--
and you'll have practice
in a moment on this--
is that the complementary
DNA strands are transcribed
into different RNAs.
You just have to do the
exercise to know this.
Here's a nucleic acid
sequence, double-stranded DNA.
If you use the top
strand as the template,
you get a piece of RNA made.
If you use the bottom strand,
you get another piece of RNA
made.
If you compare the RNAs that
are made by flipping them around
so that the 5-prime end is
on the same side for each
of the RNAs--
then you can compare them
with the correct polarity--
you'll see, obviously,
that the two
strands made from the
two different templates
are different.
So it really matters
in transcription
which strand you're copying from
as to what the outcome will be.
I want you now to go
to this class exercise
and practice some
rules of transcription.
They'll be your nucleic
acid rules applied now
to RNA synthesis.
