Elongation of mRNA

00:01 And now, it’s time to elongate the growing strand.

00:04 We'll go back to the simple representation of polymerase and that RNA polymerase has a transcription bubble.

00:16 And in the transcription bubble, we have the opened DNA strands and the messenger RNA being produced.

00:23 Now, the RNA polymerase, just like DNA polymerase, is going to read the template strand from 3’ to 5’ and make a messenger RNA transcript that is 5’ to 3’.

00:38 So the beginning end of the messenger RNA is the 5’ end.

00:43 Now, it also is going to add nucleotides to the tail using U instead of T.

00:51 However, these are just like they are with DNA replication, nucleotide triphosphates.

00:59 So all of the nucleotides that are being polymerized actually come with their own energy.

01:04 So the nucleotide triphosphates will come in as single nucleotides.

01:08 The two phosphates fall off, providing some energy to catalyze the whole process, just as it does in replication.

01:16 So I don’t know that I mentioned the triphosphate part of nucleotides there, so keep note of the fact that it’s similar for both So once the elongation cycles have finished, we’ve added more and more and more nucleotides to the sequence and made our messenger RNA, we need to release the messenger RNA from the DNA and that happens when we reach a terminator sequence.

01:43 These terminator sequences are kind of interesting.

01:47 It is a whole sequence of G and C base pairs and those G and C base pairs have a pretty strong association for each other and so that results in forming a hairpin loop in the messenger RNA transcript.

02:01 And after those G and C pairings will see that there are some A and T pairings, and then we'll see that there’s a string of uracils.

02:15 So the string of uracils actually ends up being quite weak.

02:19 U-A bonding is quite weak and so when those pairs pair, the hydrogen bond breaks between them pretty easily and the messenger RNA falls out of the holoenzyme.

02:35 And the whole thing is released and now we have a transcript of DNA in the form of messenger RNA.