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  • PROFESSOR: Hi.

  • 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.

PROFESSOR: Hi.

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B2 中高級

轉錄與翻譯|麻省理工學院 7.01SC 生物學基本原理 (Transcription & Translation | MIT 7.01SC Fundamentals of Biology)

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    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
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