Lesson 6 – Video 2a
[00:00:00.95] OK, this is the first of two videos on translation, a 2354 s we continue our
2355 gene expression-- which is turning specific genes on-- and translation, specifically, the
2356 making protein out of RNA. We'll learn that when we read the mRNA, the reading
2357 crucial. That codons of three nucleotides specify amino acids.
2358 [00:00:24.62] We'll be introduced to tRNAmolecules, which are the adapter molecule
2359 the nucleic acid code and the protein code. We'll emphasize that proteins are
synthesized in the
2360 amino to carboxy direction, also written as N to C.
2361 [00:00:42.83] In the second video we'll be introduced to the fact that the ribosome is the
2362 molecular machine of many proteins and RNAmolecules, that it's responsible for
2363 proteins, and it's quite a remarkable machine.
2364 [00:00:57.16] And we will learn the four step process of making proteins from amino
2365 [00:01:07.40] In this slide we want to emphasize a major difference between eukaryotes
2366 prokaryotes. For making proteins in eukaryotes here in the nucleus, in the white circle,
2367 transcribed into RNA. The RNA is spliced and capped. It'll be exported out of the
2368 the cytosol. And here is where the protein is made from the mRNA, the separation of
2369 RNAis made, and where the protein is made.
2370 [00:01:36.59] Prokaryotes don't have a nucleus, so DNAis made to RNA, and protein's
2371 from RNAin the same compartment, and it can happen even at the same time. While
the is RNA
2372 polymerase is making part of the mRNA molecule, the ribosomes may have already
2373 the other end and started to make protein.
2374 [00:02:00.54] Now on this slide we see the beginning of mRNA being converted into
2375 And the first thing you'll notice is that there are three nucleotides, C-U-C in this case,
2376 a single amino acid, leucine. Now you might ask, why should that be? So I want you to
2377 pause the video and think about it for a few seconds. Why you need three nucleotides to
2378 one amino acid?
2379 [00:02:27.94] OK, I hope you gave that some thought. The answer is, that's the smallest
number 2380 of nucleotides which can be used. If you only used one nucleotide-- well there's four
2381 nucleotides that could be there-- you could only code for four different amino acids.
2382 [00:02:43.18] So if you used two nucleotides, you have four that could go here, four
2383 go here, that's 16 different combinations. You can only code for 16 amino acids then.
It's only by
2384 using the third, in which case you have four by four times four-- or 64 different
2385 that you have enough coding potential to code uniquely for 20 amino acids.
2386 [00:03:10.64] The second important thing to notice is, the way you group the three
2387 crucial. You can see that different groupings of the same nucleotide sequence-- I'll say
2388 five bases-- C-U-C-A-G, C-U-C-A-G, C-U-C-A-G-- so this is the same sequence, just
2389 differently. In this case we're grouping the first three, in this case we grouping two three
four, in this case we're grouping nucleotides three, four, and five. The 2390 y code for
2391 acids, leucine, serine, glutamine.
2392 [00:03:46.53] Normally the cell, and you, when presented with a sequence would have
2393 out what the reading frame is. We'll see later in the course some of the tricks for doing
2394 computational approaches for doing that. For this class, for now, you can always
2395 when you're given a mRNA sequence.And you have to convert to protein, you can
2396 assume the first three is a codon, and then four, five, and six is a codon, and seven,
2397 [00:04:23.27] Now here we see the 64 different codons, the 64 ways of taking three
2398 at a time. They're organized by the 20 amino acids, and there are three special codons
2399 for stop codons. This is the signal to the ribosome to stop making the protein. I'll just
2400 that these three codons have special names, sometimes you'll see in the literature amber,
2401 and opal. I don't know which is which, you don't need to know that either.
2402 [00:05:01.85] We can also see here that this is the three letter code for the various
2403 Underneath we have the one letter code for amino acids. If you plan on going on in
2404 biomedical research, you might as well learn the single letter codes, but you're not
required to for
2405 this class. You do need to know the three letter codes.
2406 [00:05:30.63] And as we mentioned earlier, what class of amino acid they are.Alanine
is a 2407 hydrophobic amino acid.Arginine is a positively charged amino acid.Aspartic acid is a
2408 negatively charged amino acid, and so on.
2409 [00:05:47.63] Now let's focus in on alanine. There are four different ways to code for
C-A, G-C-C, and G-C-U. You'll notice that in all cases the first two codons are G-C. This is
2411 generally typical of all the variation in the coding system. It's in the third position where
2412 tolerant of any base. For this reason, the third position is called the wobble position, and
2413 generally less important for specifying an amino acid.
2414 [00:06:28.64] The first two being the same doesn't always hold. For instance, there's six
2415 ways of doing serine. The first two can either beA-G or U-C.
2416 [00:06:41.14] Let's talk about the six different ways of coding for leucine. In principle,
all six of
2417 these are interchangeable. The cell can use any of these at any time, and they will
2418 leucine. Now in practice, in actuality, the cell uses different codons at different times.
2419 [00:07:02.73] Sort of the most prominent example is, some of these codons are used in
2420 abundant proteins. They have highly abundant tRNAs, they are incorporated quickly