Lesson 6 – Video 2b
[00:00:01.03] SPEAKER: Hi. We continue our 2497 discussion of protein translation.And
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,
2500 megadaltons in size. It has two major subunits-- the large subunit, and the small
2501 sizes are characterized by their sedimentation rate.And as you can see, that's not a
2502 property. But S stands for svedbergs.
2503 [00:00:41.28] The large subunit is comprised of approximately 49 proteins and 3
2504 molecules. The small subunit is made up of 33 proteins and a single ribosomal RNA
2505 And the mass contribution to the final weight is approximately equal between protein
2506 ribosomal RNA.
2507 [00:01:16.52] Ribosomes are one of the most abundant proteins. In a cell in bacteria, it
2508 2% to 3% of total protein mass. So the cell is always geared up for making a lot of
2509 [00:01:28.96] In eukaryotes, they're made in the special part of the nucleus called the
2510 The nucleolus is in blue, the nucleus is in yellow.And its genes for the proteins and the
2511 exist in nearly every chromosome.And parts of every chromosome loop into this
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.
2514 be turned, the RNA, will be turned into proteins by ribosomes. Many of the ribosomal
2515 will go back, be transported back into the nucleolus.And subunits will be assembled
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
2518 protein and RNA.And it is also assembled in the nucleus-- not pertinent to the
2520 [00:02:40.61] On this slide, we see two of the three RNAmolecules all wound up from
2521 sub unit.And we also see, just for reference and size, one of the 49 proteins. So if all the
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
2525 much a part of the machinery that's actually doing chemistry. It's not just structural here.
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
2528 molecules.Again, no proteins are present here.And as the diagram in the middle shows
2529 messenger RNAthat is being decoded is running through the ribosome approximately
2530 [00:03:49.34] Now, there are three sites. The three tRNAs are sitting in three sites. The
2531 the peptidyl site-- so this is where the tRNA sits that has the peptide attached to it-- and
aminoacyl site. This is the tRNAthat's bringing the 2532 new amino acid to be connected.
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
2535 site, the p site, you have an existing polypeptide chain attached to a single tRNA
2536 correctly base paired tRNA, the one with the proper anticodon for the existing codo