Lesson 6 – Video 3
2580 [00:00:01.40] SPEAKER 1: To conclude our discussion of gene expression and
2581 translation, we have a few topics that will sort of round out our perspective on the entire
2582 That includes talking about the way that introns are spliced out of the initially
2583 eucaryotic mRNAs, the transport of mRNA from the nucleus to the cytoplasm. We'll
2584 how protein folding can be assisted by chaperones.
2585 [00:00:32.09] We'll talk about how proteins are degraded. We'll talk a little bit about the
2586 proteasome which is the protein complex that degrades the cellular protein sort of the
partner the ribosome. And then we'll just talk a little bit about how antibacterial drugs target
2588 ribosome as a way of trying to cause bacterial cell death.
2589 [00:01:02.03] Here I'd like to just briefly show some of the data that was used for the
2590 of introns.As you can see in this case where you're going to hybridize some mRNAto
2591 find out where the gene is, if there are no introns, the mRNA sequence will be an exact
2592 the genomic DNA.
2593 [00:01:25.78] However, if the mRNA have introns spliced out, part of the mRNAwill
2594 to part of the genomic DNAfor exon one. Genomic DNAcorresponding to the intron
2595 hybridize to anything in the mature mRNA, but then this other part of the mRNA will
2596 to part of the genomic DNAfor exon two.And these kinds of structures are observed
2597 instance, here.And that was how the existence of introns was deduced.
2598 [00:02:06.53] Just like the sequences that help proteins recognize where transcription
2599 start and stop, there are sequences that help other proteins and RNAmolecules
2600 these are exon/intron boundaries and that there should be a splicing event here. Now not
2601 the exons are used and not all splices are simple splices with the contiguous regions
2602 spliced together. Sometimes you might skip exon two and go to exon three. The way
that that is
2603 determined by the cell that is not entirely clear as far as I'm aware.
[00:02:52.33] And there is an eight base pair sequence at the 2604 5' end and 12 base pair
2605 the 3' end that indicates where the splicing should occur.And here is where the excised
intron 2606 would have been.
2607 [00:03:15.67] Now in the sequences here, in addition toACGU, there's some other
2608 here, other letters. That's Y is for pyrimidine. R is for purine, meaning it could be either
2609 Pyrimidine means it could be either U or C.
2610 [00:03:37.17] In the center here, you see some of the sequences that are involved, in
2611 theAwhich does some of the chemistry of the splicing reaction which is also done by
2613 [00:03:49.64] Splicing is performed by a protein and RNAcomplex called the
spliceosome. It's a
2614 very large, complex machine.And an example of the part of the RNAdoing some of the
2615 chemistry involved with the splicing of exons is that base pairAthat was in red in the
2616 slide attacking the upstream exon intron boundary forming this laureate structure which
2617 cleaved our. The two exon ends are joined.And now you have a spliced boundary
region of your
2619 [00:04:39.43] Now this is one of the first examples of RNAhaving enzymatic activity.
2620 fact that RNAis a molecule that can both code for information and perform lots of
2621 has led some people to speculate on the possibility of RNAbeing one of the very
2622 molecules present at the origin of life.And that's as good a hypothesis as any right now.
2623 [00:05:11.73] One of the differences between eucaryotic and procaryotic RNAthat
2624 pointed out was a poly(A) tail at the three prime end of eucaryotic mRNAs. Once the
2625 sequence for RNApolymerase has been passed, the RNApolymerase will dissociate
2626 DNA, dissociate the RNAfrom the complex, and an enzyme called poly(A)
2627 polymerase which does not need a template, will just start addingA,A,A,A,Ato the
end of a
2628 synthesized mRNA.
2629 [00:06:07.08] This poly(A) run will be covered with poly(A) binding protein, and it will
2630 involved in the transport of mRNA out of the nucleus. The fact that all eucaryotic genes,
2631 end in poly(A)-- those that are protein coding-- has been tremendously useful because it
2632 scientists to take a poly(T) primer which will bind to all the RNAs and synthesize them
2633 making molecules called CDNA.And we'll see that in a future lecture.
2634 [00:06:47.89] Once synthesized and processed in the nucleus, mRNAs need to be
transported out 2635 into the cytoplasm. In the nucleus, they're bound by the cap-binding protein, the
2636 proteins, and a complex called the exon junction complex. Movement inside the nucleus
2637 partially aided by diffusion once it goes through the nuclear pore large openings in the
2638 membrane that separates the nucleus from the cytosol. Proteins will bind to the cap
2639 the mRNAs and help transport them to ribosomes at the sites where they are
synthesized into or
2640 read for protein synthesis.
[00:07:42.09] We saw earlier in the course that proteins could be 2641 unfolded with urea full
2642 protein sequences, could be unfolded with urea, and then refold on their own.As they're
2643 synthesized, the amino end of a protein is exposed prior to completion of the entire
2644 chain. So proteins can start to fold in domains before the entire sequence is even
2645 [00:08:13.63] This shows you that kinetic factors potentially can play an important role
2646 final folded structure. If this portion of the protein starts to lock into a particular
2647 the once full length protein is present, it may not be able to unfold what's already been
2648 performed. And this is one of the reasons why we're not certain that most proteins fold
2649 global thermodynamic minimum because these proteins can get kinetically trapped into
2650 minimum by the process where part of the protein starts to fold before the rest is
2651 [00:09:02.66] Sometimes proteins don't get folded correctly due to ra