Codons

Hey guys! Welcome to this Mometrix video over codons. In our last video we looked at genes and alleles, and before that we looked at DNA and RNA. Well, having that previous information will help to make this new information, over codons, come together.

Building Blocks

So, remember DNA provides the instructions on what to make, and RNA (more specifically messenger RNA) attaches to the DNA and translates (or reads the genetic instructions and puts it into action in our bodies).

When we talked about genes, we compared them to a set of written instructions. The whole of our DNA, or genetic code, is like the entire written instruction manual, and a gene is like a sentence or paragraph instructing for a specific part.

Codon Structure

Well, a codon can be thought of as a word within that paragraph. The information contained in DNA and RNA is broken up into a series of three nucleotides and these three nucleotides are called codons. A codon codes for an amino acid, and this sequence of codons codes for a protein. So, three of these DNA or RNA base pairs make up an amino acid, and a sequence of codons make up a protein.

Let’s take a closer look at what this looks like.

Remember, DNA and RNA have 4 base pairs that connect to the sugar phosphate backbone. Adenine, Guanine, Thymine (or Uracil for RNA), and Cytosine.

A codon is a portion of DNA or RNA which has the information necessary to identify a single unique amino acid in the sequence of a protein. As you should know the “genetic code” is embodied by the sequence in DNA molecules of four bases, Adenine, Guanine, Thymine, and Cytosine, usually listed only by their first letters: A, T, G, or C. The “code” consists of strings of codons in a continuous line making up a single gene, so for example AAT GAC TTC AAA GTG etc (this is not a real sequence, I just made something up for example). Each codon represents an amino acid and the sequence of codons dictates the order of the amino acids in the linear sequence of the protein.

The role of transfer RNA, or tRNA, molecules is to bring the amino acids to the ribosomes, and interpret the mRNA per codon, or three nucleotides at a time. The reason that tRNAs are able to interpret (or read) codons is, because they contain an anti-codon within their sequence. Which means this: We talked about how messenger RNA or mRNA codes for a certain amino acid, well tRNA has an anti-codon that, likewise, codes for an amino acid, but in the reverse order. That is where it gets its name as the anti-codon in the tRNA sequence.

We compared DNA to a whole instruction manual, a gene to a paragraph or sentence within that instruction manual, and then we compared a codon to a word within that sentence. Well, the whole DNA sequence is made up of three-letter words (which are our codons). And, since codons have three letters (or nucleotides), there are three different ways that the genetic code can be read. This is what we call reading frames: the three different ways the code can be read.

Let’s take a look at an example. Let’s say that you have the gene GCAACGTGC. If you began to read this from the first frame, then your codons would be GCA, ACG, and TGC. However, if your were to start to read the sequence from the second frame then the codons would be CAA, and CGT. And if you started from the third frame, then the codons are AAC, and GTC. It’s important to notice that, since the codons are made up of three nucleotides each there are three complete codons when you start reading in the first frame, but there are only two complete codons when you start in the second, and third reading frames.

Since there are three different reading frames, this means that a DNA sequence could be interpreted three different ways; and each frame would then code for a different amino acid. But of the three ways that the sequence could be read, only one way actually codes for a usable protein. The other ways will not code for a usable protein.

But there is good news. When our DNA is being translated, there is actually a built-in order to how this should be done. The beginning frame which the sequence should be translated is signaled by a start codon, which is typically the very first AUG codon within the mRNA sequence.

A start codon always signifies when translation should begin, along with other initiating factors. Just like the process of translation needs to know where to start, it also needs to know when to stop, and this signifier is called a stop codon.

I hope that this video over codons was helpful for you. Be sure to check out the next video in our series over Mendelian genetics.

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