In this clip, we're going to go through problem two of the

transcription and translation unit.

We're also going to review all of the information you would

need to fill out problem one, the table.

If you haven't tried these problems yet on your own,

please pause the video now and try them, and then come back

and watch this explanation.

OK.

So here we have, as in problem two, the double stranded DNA.

Now, this is a piece of the eukaryotic genome, and so this

DNA is in the nucleus of the cell.

Now what we're going to do is we're going to go through

transcription and translation with this gene.

So the first thing we need to do is we need to figure out

where we want to start transcription and in what

direction we want to go and what strand we want to use as

our template.

So in the problem, we're given our promoter region and we're

given that our transcription starts right here at this

arrow and moves in this direction.

So we know where we're starting and what direction

we're moving in.

All transcription will proceed reading a template in the

three prime to five prime direction and will polymerize

the RNA made in the five prime to three prime direction.

So what we need to do now is figure out what strand is

going in the three prime to five prime direction and the

direction that we're going to be transcribed.

So that means that this strand, which is going from

three prime over here down to five prime at the end, this is

going to be our template strand.

So our template strand is going to be at the bottom.

OK.

So when our RNA polymerase starts, it's going to be

reading this template strand and adding nucleic acid,

adding nucleotide triphosphates, or NTPs, one by

one together to make the RNA.

Now it's going to add these together by adding a

phosphodiester bond in between all of the monomers.

So we're going to start our transcription.

And basically, what I need to do is just use the

complimentary base pair to my template strand.

So I'm going to write it out here.

So I've done a lot of transcription here.

Now where am I going to stop my transcription?

I'm not going to stop just randomly in

the middle of nowhere.

What my RNA polymerase is going to do is it's going to

stop when it gets to the end of this terminator sequence.

So I'm going to keep transcribing until I get to

the end of my terminator here.

OK.

So this is my RNA strand.

And as I said before, we're going to polymerize this mRNA

in the five prime to three prime direction.

OK, we've got our mRNA.

Since this is a eukaryotic cell and the mRNA right now is

in the nucleus, what we need to do is transport it out of

the nucleus and into the cytoplasm where it will be

translated by a ribosome.

Now what the ribosome is going to do is it's going to take

this mRNA, which is a message that encodes the sequence of

amino acids that we need to put together to make the

protein, and we're going to take this message and we're

going to find a place to start translation.

So what the ribosome is going to do is it's going to start

at the five prime end of the template and read towards the

three prime end of the template.

It's going to read until it gets to a start codon.

The start codon is AUG.

So this is where we're going to start, and AUG encodes

methionine.

We're then going to read in three base pair codons down

from the five prime end of our template to the three prime

end of our template.

And so we're going to keep going in this translation

until we reach what's called a stop codon.

Now there are three stop codons.

Stop codons are UAA, UGA, and UAG.

So if when my ribosome finds any of these three codons in

frame, it's going to stop.

So I get all the way down here before I hit a stop codon on

this transcript.

So what my ribosome is going to be doing as it's reading

these codons is it's going to insert amino acid by amino

acid, pairing them together with a peptide bond.

So we've got methionine bonded to histidine, tyrosine,

leucine, and so on and so forth.

OK, so this is the protein chain that's encoded by this

gene in the DNA.

And in proteins, we always synthesize from the N terminus

to the C terminus.

OK, so we've gone through both transcription and translation.

Now what we want to do is we want to take into account what

would happen if we had a mutation in our sequence.

So in this case, in this problem, we're going to insert

a base pair right here.

So we're going to insert an extra T here

and an extra A here.

Now, if this is happening in the DNA, when our DNA is

transcribed to make our RNA, what's going to

happen to the RNA?

Well, we're going to also add the corresponding base into

our RNA, so we're going to have an extra U here.

So in addition to only adding one nucleotide in our RNA,

this is also going to throw off our frame that we're

reading when our ribosome is reading our RNA.

So instead of the frame that we had previously, we're now

going to have a new frame.

So it's going to start at the same start codon and then read

this codon just as normal, but then this U will make UUA and

then will read in codons of three from there.

So again, I need to translate until I hit a stop codon.

And so the stop codon is going to be different in this case,

because this stop codon is no longer in frame.

I need to find an in frame stop codon.

And the first stop codon that I hit is going to be this you

UAG right here.

And so our new protein sequence is now going to be N

terminus, and then the beginning is going to be very

similar, and then our amino acids are going to be

completely different because we're reading this in a

different frame.

OK, so we can see how this one mutation, addition of one base

pair into our DNA, has caused us to create a completely

different peptide.

It's probably not going to perform the same function.

OK.

So we've gone over transcription and translation.

You should be able at this point to answer all of

question two and you should also be able to fill in

all of table one.

Thank you for watching.