How to Translate the Genetic Code

by Georgina Cornwall, PhD

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    00:01 RNAs involved in the process of transcription and translation or gene expression.

    00:01 How do we go from the nucleotide sequence to an amino acid sequence or what is the decoding for this translation? That is the genetic code. The genetic code is we know on DNA, but in the beginning, there was some question because we have this issue of having twenty amino acids that we need to code for and we have four bases to explain the language and initially they knew that there were proteins and nucleic acids involved in DNA structure.

    00:41 You know there is histone cores and such associated with chromosomes and we knew that both proteins and nucleic acids were involved and there were a number of different experiments that were done to clear the fact that it was indeed a code made from nucleic acids.

    01:02 The problem was though how do we get a four base code of DNA of As, Cs, Gs and Us into coding for the proteins, which are twenty different amino acids. Present little bit of a problem, a one letter code certainly does not do it and so they propose that perhaps it was a two letter code. If we raise the four bases to the second power, we could have 16 different codons or 16 different codes for the 20 different amino acids and that doesn't work out. Then we have to look at a three letter code. If it is a three letter code, we have 64 codons or 64 codes for amino acids, but that is way too many. But it is okay, though. It is the minimum number of letters in the code that would be required to code for all of the amino acids that we have. That is how we came up with the genetic code. Sixty of these codons specify amino acids and one of them is going to specify a start codon and three of them will specify to stop. These signal where the ribosome should get on to the messenger RNA and begin and then specify where it should stop and jump off. We will look at the details of that later. But how do we read this code? We are going to read it in three letters. In this figure, it is in a circular format often we have it in a grid format, but either way, we can look at the first letter say we pick G and then it is split into four other letters for the second letter and each one of those is split into four further letters to acquire the third letter in that code for the specific amino acid or the start or stop codon. This is the genetic code decoded. What other three letter sequences that specify each amino acid? Now you can see that there are perhaps a few too many of those codons and so that brings us to the idea that the genetic code is very specific; however, it is degenerate. By specific, we mean that each codon codes for a specific amino acid and no other amino acid, but by degenerate we mean that it has many ways to say the same amino acids. In some amino acids, you will recognise are coded form by more than one codon. Although it is specific, we see that there is some degeneration in that. There is multiple ways to say the same thing. Now we know what the genetic code is. We have to ask the question is it really universal? It turns out that the genetic code is universal except for one exception. We do know that mitochondria also have their own genetic code.

    04:11 It is mostly similar to our genetic code; however, they have made some modifications to it such that different codons code for slightly different amino acids. But other than that all organisms are coding for the same codons and the same amino acids. This is a huge link that all of life is somehow connected by this genetic code. It is one language, one translation and we can take DNA code from messenger RNA and make it into polypeptides which later fold into proteins and there you have it. We are going to explore in future lectures the specifics of transcription and specifics of translation as well as how gene expression is regulated. Now at this point, you should be able to distinguish between transcription and translation as well as discuss the one gene one polypeptide hypothesis and list the different roles of RNA. We will look at them in much more detail and then describe what the genetic code is. At this point, we are good to move on. I will look forward to seeing you in the next lecture.

    About the Lecture

    The lecture How to Translate the Genetic Code by Georgina Cornwall, PhD is from the course Gene Expression.

    Included Quiz Questions

    1. 64
    2. 4
    3. 12
    4. 61
    5. 20
    1. AUG codon leads to the termination of the translation process and hence is referred as a non-sense codon.
    2. The genetic codon is unambiguous and hence a specific codon codes for one amino acid only.
    3. A genetic codon is non-punctuated as the message is read in the continuous sequence of nucleotide triplet.
    4. UAA, UAG, and UGA code for no amino acids and serve as stop codons during translation.
    5. A genetic codon is redundant as many codons can code for the same amino acid during translation.

    Author of lecture How to Translate the Genetic Code

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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