Genetic Code – DNA, RNA and the Genetic Code

by Kevin Ahern, PhD

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    00:01 Now the genetic code is the instructions, by which the information in the sequence of a messenger RNA, is converted into a sequence of amino acids in making protein. You can see on the screen the genetic code. The genetic code is written in three base sequences. You can see for example, the first base being U, the second base being U and the third-base being U, corresponds to a UUU and as you see there that would tell the ribosome, this is the place to put a phenylalanine in. So when the ribosome translates the messenger RNA sequence, it will move down the messenger RNA, three bases at a time, looking and reading that sequence and then putting the appropriate amino acid in during that synthetic process.

    00:52 The genetic code is very interesting. When we think about the genetic code, we think about some secret and of course it's not a secret, we've known the genetic code since the early 1960s, but the information in the genetic code tells how the proteins are made.

    01:08 As I noted, the code is for the codons that are found in the messenger RNA, three base sequences. Because there are three bases and each base has four possibilities, the possible number of codons then, is four to the third and which corresponds to 64 codons. Well we don't have 64 amino acids, we have 20 amino acids, which means, as you saw in the last slide, that some amino acids are specified by more than one codon, this is called redundancy.

    01:43 Redundancy may be important because that means that any given amino acid may have more than one codon. If we look for example, at leucine, leucine can be coded by CUU, CUA, CUG or CUC. You'll notice that in each case it starts with a CU and the third base can be quite variable. In this case it could be any of the four nucleotides, now that's not true for all of the amino acids, but we do know that in looking at the genetic code that the third base frequently can vary without having any implication for which amino acids is being coded. We refer to that tendency as having the third base as what's called the wobble base, the wobble isn't as important as the first two is in specifying that.

    02:31 That means for example that if a mutation happens in the third base of a codon, it's less likely to have impact on the amino acid that's actually specified.

    02:42 Now the genetic code also what we call punctuation. Punctuation means that it has places that tell the ribosome, 'Here is the place to start making the protein and here's the place to stop making the protein', that punctuation is important. The start codon is known as AUG, that codes for the amino acid methionine and methionine is almost always used as the start codon, the place to start the synthesis of a protein. Stop codons are pretty important, in fact the genetic code has three different stop codons, UAA, UAG and UGA. Those stop codons, when the ribosome encounters them, tells the ribosome, 'Quit making protein, release what you've made, and take yourself apart,' and that's exactly what happens.

    03:36 The genetic code is also interesting in that it's universal, meaning that the bacteria in my gut, use exactly the same code as the cells in my skin. Universal means that essentially, all the way across all the biological kingdoms, the same code is used. There are minor variations from place to place, but essentially the same code is used everywhere.

    04:04 That turns out to be really useful, because if I want to synthesize a human protein in a bacterial cell, I simply need to take the human coding sequence for that, put it into a bacterial cell, set things up properly and the bacterium can synthesize that protein.

    04:20 That's at the root of process that's used in biotechnology. The last thing about the genetic code is that it's actually 'read' and the reading of the genetic code occurs by the base pairing between the anti-codon of the transfer RNA with the codon in the ribosome. If those match properly, the ribosome knows this is the right amino acid and it puts it into the protein. If they don't match properly, the transfer RNA is kicked out, until the right one comes in.

    About the Lecture

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

    Included Quiz Questions

    1. It contains 20 codons, one for each amino acid.
    2. It includes one start codon and three stop codons
    3. It is redundant
    4. It is almost exactly the same in every cell
    5. It is read in groups of three nucleotides at a time.
    1. Two nucleotides in RNA molecules that do not follow Watson-Crick base pairing rules
    2. A and T nucleotides in RNA molecules
    3. A and U nucleotides in RNA molecules
    4. Two nucleotides in RNA molecules that strictly follow Watson-Crick base pairing rules
    5. C and G nucleotides in RNA molecules

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

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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