Structure of TRNA and RRNA – DNA, RNA and the Genetic Code

by Kevin Ahern, PhD

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    00:01 In the central dogma, the process of translation is the most important part, because that's where the protein is synthesized using the information encoded in the DNA and brought in by the messenger RNA. In this module I'll talk about that process, starting first with the structure of the three different RNAs that play important roles and then moving to the actual translation process itself.

    00:23 Now, in the process of a transcription and translation, particularly translation, there are three major RNAs that are necessary. One of those RNAs is transfer RNA as can be seen in this slide. Transfer RNAs have an interesting and distinctive structure. First of all, they're not very long, you can see the entire sequence of a transfer RNA on the screen here. In addition you'll notice that the transfer RNA existing as one strand is actually forming base pairs with itself and the base pairs it forms with itself gives the transfer RNA a distinctive structure. Now the transfer RNA has different regions that have different names. So we can see in this transfer RNA for example, that it has a five prime and on the left at the top and a three prime end on the right at the top. The three prime end terminates in the sequence CCA and that's true for virtually every transfer RNA. Attached to that A at the three prime end is the place where an amino acid is attached. Now that's critical because the transfer RNA of course carries amino acids to the ribosome for the purpose of translation. The stem by which it is located is known as the acceptor stem, the loop to the right is called the pseudouridine loop because it's named for one of the unusual bases that's found in transfer RNAs. Transfer RNAs are commonly, chemically modified in order to function, it's not completely understood why that's the case. Next we have the variable loop, which is a very small loop that does vary in size from one transfer RNA to another.

    02:02 The anti-codon loop is shown next and the anti-codon loop is very important. The anti-codon is a three base sequence that you can see at the bottom in dark blue. That region of the transfer RNA will pair with the messenger RNA codon during the process of translation.

    02:21 The specificity of that pairing is what determines the proper placement of the proper amino acid into a protein. Last we have on the left what's called the D-Arm of the transfer RNA.

    02:34 Now, ribosome RNAs are the third RNA that we find in the translation process. These are components of ribosomes and at first it was believed that the components were simply there as sort of a scaffolding to hold the proteins that are there. We now know there's more functions associated with those ribosome RNAs. The size of the ribosomal RNAs is measured in what's called Svedberg units or as people more commonly call them, S units and we will see that in just a moment. The extensive secondary structures that we saw in the transfer RNAs are even more extensive in the longer ribosomal RNAs. And the largest ribosomal RNA is actually catalytic. It's what we call a ribozyme, now it's not a ribosome, but a ribozyme. A ribozyme is an RNA the catalyze a reaction and it turns out that the ribozyme, that the ribosomal RNA is, makes peptide bonds for proteins.

    03:35 So the ribosomal RNAs, there are groups of them. In bacteria there are three of them specifically. One called the 5S which is located in the large ribosomal subunit, it provides structural support, that is, a service scaffolding for the ribosome to exist. The next largest one is called the 16S ribosomal RNA and it's found in the small ribosomal subunit. We will see that ribosomes have a big subunit and a small subunit, so the 16S ribosomal RNA is found in the small ribosomal subunit and its purpose is to align the messenger RNA in the right place, so that translation can begin. The largest of the ribosomal RNAs, as I mentioned in the last slide, has the responsibility for catalyzing the formation of peptide bonds. Now eukaryotes are very similar, but they actually have four ribosomal RNAs, not three, and they perform many of the same functions that are found in the prokaryotic cells, and I wont go through those here.

    04:37 Now to give you an idea about some of the structures that we see associated with the ribosomal RNAs, I'm showing on the screen here, two ribosomal RNAs from eukaryotic cells. And you can see that just like we saw in the transfer RNAs, the ribosomal RNAs have extensive secondary structure. When I use that term associated with a nucleic acid, I'm talking about pairing with itself. So we see regions that are pairing with themselves and we see some pretty exotic looking structures within these. These small ribosomal RNAs probably help the ribosome to bind to the proteins properly. The next largest ribosomal RNA and bacterial cells is the 16S ribosomal RNA and the slide is a zoomed way out, so you're not going to see individual bases in there, but from the distance that we're showing in this slide, you can see the extensive base pairing that's occurring within this ribosomal RNA. This ribosomal RNA, as I noted, helps to align the messenger RNA during the process of translation and it probably also helps the ribosome to bind proteins in the right orientation, so that they can function in the process of translation.

    About the Lecture

    The lecture Structure of TRNA and RRNA – DNA, RNA and the Genetic Code by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. They contain many chemically modified bases.
    2. They have a codon loop opposite of the part of the molecule where the amino acid is carried.
    3. They have an amino acid attached to the 5’ end.
    4. They carry ribosomes to the protein being made.
    5. Amino acids attach to thymine on the tRNA.
    1. They have extensive secondary structures.
    2. They catalyze the formation of themselves.
    3. They code for protein using the genetic code.
    4. They are smaller than tRNAs
    5. Only the large subunit of the ribosome consists of rRNA.
    1. The anticodon present at the 5’ end of the acceptor end helps the tRNA to recognize and bind to the correct amino acid floating in the cytoplasm of the cell
    2. The tRNA is an adaptor molecule, which serves as a physical link between mRNA and the amino acid sequence of the protein.
    3. The large subunit rRNA present in the ribosomes is an example of a ribozyme that helps in the formation of peptide bonds during protein synthesis.
    4. Eukaryotes have four rRNA types.
    5. The amino acid gets attached to the base A of CCA tail present at the 3’ end of the tRNA at the acceptor stem.

    Author of lecture Structure of TRNA and RRNA – DNA, RNA and the Genetic Code

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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