Messenger RNAs – DNA, RNA and the Genetic Code

by Kevin Ahern, PhD

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    00:01 off of the DNA there as well, terminating transcription.

    00:02 Now transcription process is very important and we most commonly think of it associated with the synthesis of messenger RNAs. Messenger RNAs are the molecules that are used by the ribosome to translate into protein. It turns out that the synthesis and the form of the messenger RNAs between prokaryotic cells and eukaryotic cells is quite different. For example, on the top, I list the characteristics of the eukaryotic messenger RNAs and in the bottom I list the characteristics of the prokaryotic. You can see starting at the prokaryotic at the bottom, that the five prime end of a messenger RNA, they have a triphosphate. At the eukaryotic messenger RNA by contrast, they have an unusual structure called a Cap, and that Cap in the eukaryotic messenger RNA plays a couple of functions, prokaryotes don't have that.

    00:54 The functions it plays in a eukaryotic cell is protection of the messenger RNA from degradation, and it turns out that eukaryotic messenger RNAs are much stabler then prokaryotic messenger RNAs for that reason and second, it helps the process of translation to occur more efficiently.

    01:13 Moving further into the messenger RNA molecule, we come to a region called the untranslated region, and this untranslated region is different between prokaryotes and eukaryotes as well.

    01:24 In prokaryotes, there is a special sequence that's there that helps the ribosome know where to start translating the protein. That sequence is called a Shine-Dalgarno sequence and is found in virtually every prokaryotic messenger RNA. Eukaryotes use a slightly different process and don't have a Shine-Dalgarno sequence. The green region of the messenger RNA here, the CDS, is known as the coding DNA sequence, and the coding DNA sequences is where the gene that will be translated into protein is actually contained. In eukaryotic cells, this gene sequence may arise from the splicing together of several portions of a bigger messenger RNA.

    02:07 Prokaryotic cells don't have this phenomenon occurring. In eukaryotic cells, this happens through a process called splicing, prokaryotic cells do not have splicing.

    02:19 Moving further down the messenger RNA, we can see the three prime untranslated region, which is like the untranslated region of the five prime end, the difference being that the sizes of these vary quite a bit between prokaryotes and eukaryotes. In eukaryotes they can be quite large and they may have important functions in helping to stabilize the structure of the messenger RNA, as well as to help the translation process to occur. Prokaryotic cells are much more efficient in their use of nucleic acid and they usually have a fairly short three prime untranslated region. The far three prime end of the messenger RNA has important functions and also is quite different between prokaryotes and eukaryotes. In eukaryotes, they have sequence called the poly-A tail. This refers to the fact that at the three prime end of eukaryotic messenger RNAs, there's a long chain of adenosine residues that are linked to the end of the messenger RNA. These appear to play roles in the stability of the messenger RNA and also in translational efficiency, helping the ribosome to translate more efficiently.

    03:20 The poly-A tail may also play roles in the exit of the messenger RNA from the nucleus.

    03:25 Remember that eukaryotes have a nucleus and prokaryotes don't. Prokaryotes at the three prime end of their messenger RNA typically don't have any additional sequences like a poly-A for example, but they may have additional genes that are located down a stream from there. When they have such an orientation, the prokaryotic messenger RNA is called an operon. The operon has multiple coding sequences within there.

    03:50 With this I've now concluded the discussion about the structure of nucleic acids, a little bit about the DNA replication, and transcription of a process occurring in the eukaryotes in prokaryotic cells. Translation will follow in an additional lecture.

    About the Lecture

    The lecture Messenger RNAs – DNA, RNA and the Genetic Code by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. It begins near a promoter
    2. It involves synthesis of protein from RNA
    3. It proceeds in the 3’ to 5’ direction
    4. It produces a duplex RNA
    1. It operates in a transcription bubble
    2. It binds to RNA to make DNA
    3. It uses a rho protein in bacteria to identify the promoter
    4. It copies the coding strand to make mRNA

    Author of lecture Messenger RNAs – DNA, RNA and the Genetic Code

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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