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Lecture 6

Cell Biology - Lecture 6 -Video 2.2 - Notes

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Department
Biology
Course
CAS BI 203
Professor
Martin Steffen
Semester
Spring

Description
Lesson 6 – Video 2b [00:00:00.00] 69 [00:00:01.03] SPEAKER: Hi. We continue our 2497 discussion of protein translation.And now we 2498 will look at the ribosome in detail. 2499 [00:00:11.25] So the ribosome, the complete one here, is a very large protein complex, 4.2 2500 megadaltons in size. It has two major subunits-- the large subunit, and the small subunits. These 2501 sizes are characterized by their sedimentation rate.And as you can see, that's not a linear 2502 property. But S stands for svedbergs. 2503 [00:00:41.28] The large subunit is comprised of approximately 49 proteins and 3 ribosomal RNA 2504 molecules. The small subunit is made up of 33 proteins and a single ribosomal RNA molecule. 2505 And the mass contribution to the final weight is approximately equal between protein and 2506 ribosomal RNA. 2507 [00:01:16.52] Ribosomes are one of the most abundant proteins. In a cell in bacteria, it can be 2508 2% to 3% of total protein mass. So the cell is always geared up for making a lot of them. 2509 [00:01:28.96] In eukaryotes, they're made in the special part of the nucleus called the nucleolus. 2510 The nucleolus is in blue, the nucleus is in yellow.And its genes for the proteins and the RNA 2511 exist in nearly every chromosome.And parts of every chromosome loop into this particular 2512 region, where there is a lot of synthesis of RNAand of protein. 2513 [00:01:58.57] Now, the RNAfor the proteins will get transported out into the cytosol. They will 2514 be turned, the RNA, will be turned into proteins by ribosomes. Many of the ribosomal proteins 2515 will go back, be transported back into the nucleolus.And subunits will be assembled inside the 2516 nucleus and then exported in order to form more ribosomes. 2517 [00:02:27.23] The only reason telomerase is included here is because it's also a complex of 2518 protein and RNA.And it is also assembled in the nucleus-- not pertinent to the translation story, 2519 however. 2520 [00:02:40.61] On this slide, we see two of the three RNAmolecules all wound up from the large 2521 sub unit.And we also see, just for reference and size, one of the 49 proteins. So if all the proteins 2522 were present and we drew everything to scale, the final size of the ribosome complex would not 2523 be that significantly different. 2524 [00:03:06.48] Now, we think mostly of proteins doing the biochemistry. But the RNA is very 2525 much a part of the machinery that's actually doing chemistry. It's not just structural here. And so 2526 that's why it can be referred to is as a ribozyme. 2527 [00:03:26.00] So here, we see a picture of large subunits, small subunit, and three transfer RNA 2528 molecules.Again, no proteins are present here.And as the diagram in the middle shows that the 2529 messenger RNAthat is being decoded is running through the ribosome approximately there. 2530 [00:03:49.34] Now, there are three sites. The three tRNAs are sitting in three sites. The exit site, 2531 the peptidyl site-- so this is where the tRNA sits that has the peptide attached to it-- and the 70 aminoacyl site. This is the tRNAthat's bringing the 2532 new amino acid to be connected. You will 2533 need to memorize the names of these sites. 2534 [00:04:14.75] Now, here we see the four steps of one cycle of peptide elongation. In the peptidyl 2535 site, the p site, you have an existing polypeptide chain attached to a single tRNA molecule. The 2536 correctly base paired tRNA, the one with the proper anticodon for the existing codo
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