RNA Processing

by Kevin Ahern, PhD

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    00:01 The forms and functions of RNA are incredibly diverse, as we shall see in this presentation.

    00:07 In the first part I will talk about the processing of RNA and how RNA can actually help process other RNAs.

    00:14 Then I will talk about RNA interference, an important process that exists in eukaryotic cells whereby RNAs again help to control the synthesis of other RNAs.

    00:23 And finally I will talk about genetic code considerations which will include how transfer RNAs are best prepared for the process of translation.

    00:33 Most RNAs that exists in cells end up in a form different from what they started and this process is called RNA processing.

    00:41 The processing of RNAs can involve a couple of things. It can involve for example chemical alteration of the bases that happens in some RNAs like transfer RNA.

    00:50 It can also involve removal, deletion, and alteration of the bases within an RNA sequence as we shall see. Now enzymes called RNAses participate in the processing of ribosomal and transfer RNAs in most cells.

    01:06 The ribosomal RNAs are usually made as larger molecules called pre-ribosomal RNAs and these are processed in the nucleolus of eukaryotes.

    01:15 There are small RNAs called snoRNAs that help in this processing, as I will describe.

    01:22 Now transfer RNAs are also processed but interestingly transfer RNAs are made on the pre-ribosomal RNA, meaning that in some cases they are made with the ribosomal RNA.

    01:34 And the individual transfer RNAs are chopped out and then modified as appropriate for their function.

    01:41 The processing of transfer RNAs involves quite a few enzymes.

    01:45 But one of the enzymes is actually not an enzyme meaning that one of the things that catalyzes a reaction is actually another RNA called a ribozyme.

    01:55 This ribozyme is known as RNAase P and, as we will see, it helps to form the proper 5 prime end of a transfer RNA.

    02:04 Now transfer RNAs, of course, are the RNAs molecules that carry the amino acid to the ribosome for translation.

    02:11 They are single strand of RNA that you can see on the figure in the upper right, having a 5 prime end and a 3 prime end.

    02:17 The 3 prime end is that part that sticks up a little bit further on right on that image.

    02:22 That 3 prime end with the sticking up part is actually a specific sequence that ends in CCA, and that CCA is added by a polymerase after the RNA has been processed.

    02:34 Now chemical modifications of ribosomal RNAs and transfer RNAs occur in prokaryotes and also in eukaryotes and these chemical modifications are quite common. They will actually change what the structure of an individual nucleotide base is within these individual molecules.

    02:51 Now you can see for example in this figure shown on the right, the conversion of uridine which of course is a U within a transfer RNA to a pseudouridine, as shown on the right.

    03:02 And this transformation has added another nitrogen to the ring, as you can see.

    03:09 Now ribosomal RNA modifications in addition to some of the general ones involving clipping out of sequences are also help to long by some small nuclear RNAs that are given the name snoRNAs.

    03:20 So these small RNAs actually help to process ribosomal RNAs to their final form.

    03:26 Base alterations meaning the actually editing and changing of base sequence within an RNA can also occur and this happens in some eukaryotic messenger RNAs particularly among the lower eukaryotes on the evolutionary ladder. But it also occurs in the human as well.

    03:45 Now messenger RNAs in eukaryotes are very different than messenger RNAs in prokaryotes in comparing the ways in which they get processed.

    03:54 Messenger RNAs in prokaryotes get very little processing and the reason that this happens is because, translation in prokaryotes is occurring almost at the same time as the RNA is being made.

    04:06 And this is possible in prokaryotes; because, prokaryotes don't have a nucleus and transcription and translation are occurring in the same place.

    04:14 In eukaryotes, transcription is occurring in the nucleus and translation is occurring in the cytoplasm.

    04:21 So between the nucleus and the cytoplasm , there is a significant amount of alteration that can and does happen.

    04:28 Now I have drawn on the screen here a representation of a eukaryotic mRNA and it includes some of those modifications that I talked about.

    04:36 First of all at the 5 prime end we can see that the eukaryotic mRNAs have a structure that we call a cap.

    04:43 We don't see that structure of a cap in a prokaryotic messenger RNA.

    04:48 At the 3 prime end, or the other end of the messenger RNA, we see what's called a poly A tail.

    04:54 The poly A tail is the addition of a large number of adenine residues or adenosine residues to the 3 prime end of the eukaryotic messenger RNA.

    05:04 This polyadenylation appears to play a very important role in the stability of the messenger RNA and also improving the efficiency of translation of the messenger RNA as it interacts with the ribosome.

    05:18 The chemical modification at the 5 prime end to put the cap on is an interesting one.

    05:22 It involves the addition of a 7-methyl guanosine to the 5 prime end of the messenger RNA.

    05:28 But that 7-methyl guanosine is added in an unusual way.

    05:32 It's joined in a 5 prime to 5 prime linkage, as you can see in the image there.

    05:37 That 5 prime to 5 prime linkage is very unusual in biochemistry and this is the only place it occurs.

    05:43 And that cap also appears to play two functions.

    05:47 First, it protects the mRNA against degradation by nucleases.

    05:53 Second, the cap plays a very important role in helping to start the process of translation in the ribosome.

    06:00 Now the long string of the As at the end is as I said are probably there to help stabilize the messenger RNA and function in transport.

    06:10 There is a sequence within the messenger RNA as it's being made that goes AAUAAA.

    06:17 And when that sequence is encountered it is a signal to the cell to chop the RNA and begin adding the poly-A tail to the end of that.

    06:28 Now in the middle of the messenger RNA, we are seeing the final messenger RNA right here.

    06:33 But the pre-messenger RNA that existed may well have a had other sequences that are not present in this final version.

    06:43 The removal of those other sequences from the internal parts of an RNA is called splicing and splicing produces a product that has those internal parts removed and the final product shown as you see on the screen.

    About the Lecture

    The lecture RNA Processing by Kevin Ahern, PhD is from the course RNA and the Genetic Code.

    Included Quiz Questions

    1. They frequently arise from splicing
    2. They contain a poly-A tail at the 5’ end
    3. They use a different genetic code
    4. They lack a cap
    1. In eukaryotic cells, the mRNA does not require any chemical modifications, whereas in prokaryotic cells the polycistronic mRNA needs splicing before translation
    2. Poly A tail in the mRNA at the 3’ end helps in transportation from the nucleus to the cytoplasm and provides stability
    3. The AAUAA sequence in the 3’ end of the immature mRNA molecule acts as a signal for chopping off and the addition of poly A-tail to form a mature mRNA
    4. The capping process involves the addition of 7-methylguanosine to 5’ end of the mRNA
    5. The addition of a cap at 5’ end of mRNA helps in the starting the translation process and protects mRNA from nuclease attack
    1. 5’ to 5’ linkage
    2. 3’ to 5’ linkage
    3. 5’ to 3’ linkage
    4. 5’ to 1’ linkage
    5. 3’ to 1’ linkage

    Author of lecture RNA Processing

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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