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PROFESSOR: Now, onward.

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Transcription.

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So, we've got DNA, we'll do two,
three, probably three by

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now, transcription.

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So we have DNA goes to DNA.

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DNA makes RNA, RNA
makes protein.

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This, by the way, gets the
name the central dogma of

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molecular biology.

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Due to Francis Crick, and as
an aside, Francis actually

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never said DNA goes to
RNA goes to protein.

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What he said was nucleic
acids go to protein.

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The information flows from
nucleic acids to proteins.

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He never actually said DNA goes
to RNA goes to protein,

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and that's an important point.

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And we'll come to it at some
point, probably next time.

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So, transcription.

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Here's my genome.

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Here's my double helix.

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I'm going to stop wrapping
around itself, just because

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it's tedious.

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And here's a chunk of DNA
that encodes a gene.

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Maybe it's a gene that makes
our enzyme for arginine

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biosynthesis.

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Remember, we had our arginine
genes and all that.

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But what happens is it has a
starting point, it has a

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stopping point.

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Five prime to three prime.

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What happens is there is a
signal in the DNA that the

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cell knows how to read
called a promoter.

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And under certain circumstances,
this promoter

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invites an enzyme to sit down,
and the enzyme starts copying.

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Which direction does
this enzyme go?

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Five prime to three prime.

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They all go five prime
to three prime.

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But it makes RNA.

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Okay?

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And then it gets to certain
point, and it stops copying.

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This process of copying is
called transcription, because

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it's just a direct
transcribing.

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So what's the difference
between DNA and RNA?

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Two differences.

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One, this is two prime
deoxyribose.

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This is ribose.

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It's not two prime deoxy.

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It's truly ribose.

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The other difference, where DNA
has T, RNA, has U, uracil.

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The difference, what's the
difference between T and U?

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The difference is
a methyl group.

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It's a methyl group in an
unimportant position for the

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base paring.

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It doesn't matter.

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It has an extra methyl group.

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You can look it up
in your book.

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So for all practical purposes,
you, in thinking about

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polymerization and five prime
to three prime, and

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everything, could imagine the
DNA and RNA are the same basic

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structure, because it's got one
little methyl group that

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distinguishes T and U. And it's
got a hydroxyl on the two

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prime position in the carbon,
in the sugar, which actually

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isn't anything we use.

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We never use the two prime.

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So none of what I've told you is
affected by t versus u, or

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deoxyribose versus ribose.

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Now, it turns out it does
make a difference

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in the overall structure.

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It's harder to base pair with
the ribose there as opposed to

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the deoxyribose because they
stack differently, et cetera.

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It makes a difference
to the cell.

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RNA is less stable, all
sorts of things.

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But for your practical purposes,
apart from having to

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know that it's T versus U, and
deoxyribose versus ribose, you

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won't see in this course
actually see any real strong

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reasons why it matters, but it
does matter to the cell.

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All right.

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So this enzyme comes along, and
it copies a segment of the

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DNA, starting at a promoter.

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It knows which strand it's on.

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Remember, this is stranded.

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It's not like it's going
back this way.

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It has a directionality to it.

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And in reaches what's called a
transcriptional stop signal.

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Transcriptional stop, which is a
certain sequence in the DNA,

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and it comes to an end.

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And it makes an RNA transcript,

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which then floats away.

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Which we'll talk
next time, gets

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translated into a protein.

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And how do you think
this works?

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It takes nucleotides, RNA
nucleotides here, with their

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triphosphates, and sticks them
on, just like we saw with DNA.

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And it makes a polymer of RNA.

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And the enzyme is called
RNA polymerase, right?

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This is all pretty
logical stuff.

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RNA polymerase comes along
and does that.

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So we get RNA polymerase.

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Now, when I am a cell, and this
is my genome, I have a

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gene that goes this way.

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Here's its promoter, here's
its transcriptional stop.

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I could also have a gene
that goes this way.

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Here's its promoter, here's
its transcriptional stop.

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Directionality could go
in either direction.

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RNA polymerase comes along, and
with the help of friends,

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knows where to start.

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Those friends could be other
proteins that are sitting down

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there that RNA polymerase
likes to associate with.

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And which strand is
being transcribed?

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The bottom strand, or
the top strand?

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Matters.

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You get a different single
stranded RNA.

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So RNA, when it floats off,
is single stranded.

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This is going to make a single
stranded RNA that, five prime

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to three prime RNA that is
complimentary to, matching to,

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the bottom.

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This guy, however, will make an
RNA that is complimentary

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to the top.

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Much of the business of running
your cell is figuring

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out which genes you should
be transcribing into RNA.

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It turns out your liver is
making different transcripts.

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It's transcribing different
segments of your genome than

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say, a muscle cell.

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Than say, a brain cell.

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All of that machinery of
figuring out how one genome

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gets read out in different ways
by RNA polymerase is the

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problem of gene regulation.

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And we will talk about that
in a little while.

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What we're going to talk about
next time is how that RNA

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transcript gets translated
into a protein.

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And you guys probably,
again, all know this.

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But nonetheless, we'll talk
a little bit about it.

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And then we'll talk about some
variations on that theme.

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Until next time.