Elongation of Translation

00:00 So now, let’s take a look at how the ribosome actually goes about the elongation cycle.

00:10 This happens in a fairly cyclic way.

00:11 But before we get into how the cycle works, let’s look at some specific details of how things move through these sites.

00:20 So we have the A, P, and E sites.

00:24 Here, we are looking at the A-site and the P-site.

00:28 We have a tRNA holding onto an amino acid in the P-site, which I call the polypeptide site, right? And then we have a new tRNA arriving in the arrival site, the A-site.

00:45 That tRNA in the A-site is going to get the polypeptide chain from the tRNA in the P-site.

00:54 So the tRNA in the P-site will pass its polypeptide chain onto the top of the amino acid on the tRNA in the arrival site.

01:07 So, little bit confusing, but you’ll be with me here shortly.

01:13 This tRNA that’s in the P-site is going to pass the polypeptide chain, the one on the A-site, but not only that, it’s very specific about the way that it binds it on there.

01:27 We’re going to take the carboxyl end of the first amino acid, which came from P-site, and stick it to the amino end of the newly arriving amino acid such that chain grows in a direction where the front of the chain is the amino end and the end of the chain we’re adding on to essentially the carboxyl terminus through a little bit of a roundabout way of passing the chain over to the A-site.

01:58 When the ribosome translocates as we’ll see in the elongation cycle, then we will have the polypeptide chain right back into the polypeptide or P-site.

02:08 So let’s look at that part of the story.

02:11 We have this elongation cycle.

02:13 Naturally, we are going to have some other factors involved.

02:17 We’ll call them elongation factors.

02:19 Specifically in this case, it’s elongation factor Tu, T-U.

02:23 And we have to use some energy, in this case, in the form of 2 GTPs.

02:28 So we begin the cycle.

02:31 We have the formylmethionine in the P-site.

02:36 We have a newly arrived tRNA with the next amino acid in the A-site.

02:44 What happens now is we will invest a little bit of energy using an elongation factor.

02:51 The elongation factor helps cleave the GTP and release some energy in order to help the polypeptide chain move from the tRNA in the P-site over to the tRNA in the A-site.

03:07 So it simply passes its chain to the guy sitting in the A-site.

03:11 Then, some magic happens.

03:14 The chain is on the tRNA in the A-site and we have a little bit more energy because something has to happen.

03:23 Elongation factor comes in again.

03:24 It helps us cleave some energy from the GTP molecule and that energy is used to help the ribosome move along, translocate along the messenger RNA strand.

03:38 So once that ribosome translocates, we will end up with an empty A-site because the tRNA from the A-site has now been moved into the P-site.

03:48 So those guys stay put on the messenger RNA.

03:52 After all, they are hydrogen-bonded to it, with the codon and anticodon pairing.

03:57 So the polypeptide chain is just back in the P-site and the guy who was in the E-site is going to get kicked out.

04:06 So the guy in the E-site gets kicked out as the polypeptide chain moves back to the P-site.

04:14 A-site’s empty.

04:14 The whole cycle can repeat.

04:15 We’re going to have someone arrive in the A-site, the polypeptide chain gets passed to the A-site and the ribosome translocates and so on and so forth.

04:27 So it’s a cyclic fashion of arrival, drop off an amino acid, transfer it to A, kick the guy out of the exit site, so on and so forth, adding amino acids to the chain, and growing it towards the carboxyl terminus.

04:45 So spend some time thinking about that and perhaps diagramming what happens – pen on paper again – what happens as the ribosome translocates which way is the messenger RNA moving or the ribosome moving in order to accomplish the goals of growing this chain.

05:04 Again, peptidyl transferase is the enzyme that’s involved in helping this transfer of the polypeptide chain from P to A, ribosome translocates, and we’re back in the P or polypeptide site.

05:19 Pretty exciting cycle, right? So one little thing, we were alluding to earlier the idea of wobble pairing.

05:29 So we have tRNAs that are associated with specific amino acids.

05:36 But you’ll recall from our lesson where we learned about the genetic code that some amino acids can be coded for by multiple codes.

05:45 You’ll notice in this figure, the messenger RNA on the normal pairing matches the third position nucleotide on the anticodon of the tRNA.

05:56 However, in wobble pairing, we have kind of an odd pairing going on in there.

06:00 G is not supposed to pair with U.

06:03 But in this case, it can because the rules about pairing of codons and anticodons in the third position seem to be a little bit less stringent, which allows for identical tRNAs carrying their amino acid to bind to less identical triplet sequences on the messenger RNA.

06:28 So that is the nature of wobble pairing and it can be the basis when it’s not supposed to happen of some mutations in the function of proteins, but for this case, let’s just call it normal.