Transcription and Translation, excerpt 1 | MIT 7.01SC Fundamentals of Biology


PROFESSOR: Now, onward. Transcription. So, we’ve got DNA, we’ll do two,
three, probably three by now, transcription. So we have DNA goes to DNA. DNA makes RNA, RNA
makes protein. This, by the way, gets the
name the central dogma of molecular biology. Due to Francis Crick, and as
an aside, Francis actually never said DNA goes to
RNA goes to protein. What he said was nucleic
acids go to protein. The information flows from
nucleic acids to proteins. He never actually said DNA goes
to RNA goes to protein, and that’s an important point. And we’ll come to it at some
point, probably next time. So, transcription. Here’s my genome. Here’s my double helix. I’m going to stop wrapping
around itself, just because it’s tedious. And here’s a chunk of DNA
that encodes a gene. Maybe it’s a gene that makes
our enzyme for arginine biosynthesis. Remember, we had our arginine
genes and all that. But what happens is it has a
starting point, it has a stopping point. Five prime to three prime. What happens is there is a
signal in the DNA that the cell knows how to read
called a promoter. And under certain circumstances,
this promoter invites an enzyme to sit down,
and the enzyme starts copying. Which direction does
this enzyme go? Five prime to three prime. They all go five prime
to three prime. But it makes RNA. Okay? And then it gets to certain
point, and it stops copying. This process of copying is
called transcription, because it’s just a direct
transcribing. So what’s the difference
between DNA and RNA? Two differences. One, this is two prime
deoxyribose. This is ribose. It’s not two prime deoxy. It’s truly ribose. The other difference, where DNA
has T, RNA, has U, uracil. The difference, what’s the
difference between T and U? The difference is
a methyl group. It’s a methyl group in an
unimportant position for the base paring. It doesn’t matter. It has an extra methyl group. You can look it up
in your book. So for all practical purposes,
you, in thinking about polymerization and five prime
to three prime, and everything, could imagine the
DNA and RNA are the same basic structure, because it’s got one
little methyl group that distinguishes T and U. And it’s
got a hydroxyl on the two prime position in the carbon,
in the sugar, which actually isn’t anything we use. We never use the two prime. So none of what I’ve told you is
affected by t versus u, or deoxyribose versus ribose. Now, it turns out it does
make a difference in the overall structure. It’s harder to base pair with
the ribose there as opposed to the deoxyribose because they
stack differently, et cetera. It makes a difference
to the cell. RNA is less stable, all
sorts of things. But for your practical purposes,
apart from having to know that it’s T versus U, and
deoxyribose versus ribose, you won’t see in this course
actually see any real strong reasons why it matters, but it
does matter to the cell. All right. So this enzyme comes along, and
it copies a segment of the DNA, starting at a promoter. It knows which strand it’s on. Remember, this is stranded. It’s not like it’s going
back this way. It has a directionality to it. And in reaches what’s called a
transcriptional stop signal. Transcriptional stop, which is a
certain sequence in the DNA, and it comes to an end. And it makes an RNA transcript, which then floats away. Which we’ll talk
next time, gets translated into a protein. And how do you think
this works? It takes nucleotides, RNA
nucleotides here, with their triphosphates, and sticks them
on, just like we saw with DNA. And it makes a polymer of RNA. And the enzyme is called
RNA polymerase, right? This is all pretty
logical stuff. RNA polymerase comes along
and does that. So we get RNA polymerase. Now, when I am a cell, and this
is my genome, I have a gene that goes this way. Here’s its promoter, here’s
its transcriptional stop. I could also have a gene
that goes this way. Here’s its promoter, here’s
its transcriptional stop. Directionality could go
in either direction. RNA polymerase comes along, and
with the help of friends, knows where to start. Those friends could be other
proteins that are sitting down there that RNA polymerase
likes to associate with. And which strand is
being transcribed? The bottom strand, or
the top strand? Matters. You get a different single
stranded RNA. So RNA, when it floats off,
is single stranded. This is going to make a single
stranded RNA that, five prime to three prime RNA that is
complimentary to, matching to, the bottom. This guy, however, will make an
RNA that is complimentary to the top. Much of the business of running
your cell is figuring out which genes you should
be transcribing into RNA. It turns out your liver is
making different transcripts. It’s transcribing different
segments of your genome than say, a muscle cell. Than say, a brain cell. All of that machinery of
figuring out how one genome gets read out in different ways
by RNA polymerase is the problem of gene regulation. And we will talk about that
in a little while. What we’re going to talk about
next time is how that RNA transcript gets translated
into a protein. And you guys probably,
again, all know this. But nonetheless, we’ll talk
a little bit about it. And then we’ll talk about some
variations on that theme. Until next time.