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Codon Recognition: How tRNA and Anticodons Interpret the Genetic Code

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  • 0:06 Codon Recognition Review
  • 1:06 Transfer RNA (tRNA)
  • 3:09 The Anticodon
  • 5:23 Codon Recognition by tRNA
  • 8:19 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

How does codon recognition work at the molecular level? Can you use tRNA and anticodons to decipher the genetic code? Learn the mechanics of codon recognition and build a polypeptide from a sample genetic code.

Review of Codon Recognition

Translation is the process of converting the genetic information in the mRNA strand to the form of a protein. The basic unit of this genetic information is a codon. Earlier, we described a codon as a sequence of three nucleotide bases that specifies a particular amino acid. Codon recognition describes the process of matching codons to the correct amino acids. Codons are read down the length of the mRNA strand and translated into an appropriate polypeptide chain.

Codon recognition describes the process of matching codons to the correct amino acids
Codon Recognition Review

But when we talk about 'reading' the codons, what does that actually mean? Who is reading the sequence of mRNA codons? How are the codons being interpreted as instructions for amino acids? And who is responsible for bringing those amino acids together in the correct order specified by the genetic code? In this lesson, we're going to talk about how codons are recognized with the help of a new type of RNA. We'll study the molecular mechanics involved, and we'll practice using these molecules to make our own polypeptides.

Transfer RNA (tRNA)

Let's begin by reminding ourselves what the word translation describes. Translation of mRNA to protein is similar to translation of one language to another. We're converting the language of mRNA, a sequence of nitrogenous bases, to the language of a protein, a sequence of amino acids. Translation between languages always requires the work of an interpreter, an agent responsible for recognizing both languages and drawing the connections between the two.

For example, let's say that I took a summer vacation in France and that I didn't know how to speak any French. But if I brought along my cousin, who knows both French and English, then I would have an interpreter to help me communicate. If I wanted to buy some cheese from a grocer, I could just tell my cousin, 'I want to buy some cheese.' My cousin would use her translational powers to convert my English sentence into a French one and say, 'Je veux acheter du fromage.'

In the case of genetic translation, we have a molecule that acts as an interpreter between codons and amino acids. It's a special type of RNA called transfer RNA, or tRNA for short. Transfer RNA is the type of RNA that interprets the mRNA code during translation.

Transfer RNA acts as an interpreter between codons and amino acids
Transfer RNA Interpreter

When a cell is ready to have its genetic code translated into polypeptides, it first has to be prepared with all the essential amino acids. Remember, there are 20 different amino acids involved in making our proteins. Proteins can only be made correctly when the amino acids are assembled in the right order. The job of tRNA is to match up the amino acids with the correct codons in the mRNA strand.

We can look at our codon chart to get an idea of what tRNA's job is like. Let's see here. If we were tRNA and we read the codon UUU, then we would know to grab a phenylalanine. If we read the codon AGC, then we would know to grab a serine. Transfer RNA reads a gene's codons from start to finish and matches the amino acids in the correct order.

The Anticodon

That sounds simple enough, but how does tRNA go about matching the codons to the correct amino acids? How does it know when it's found a codon and an amino acid that are supposed to go together? Like all RNAs, tRNA is a single strand of nucleotide bases. But a tRNA is short - only about 80 nucleotides long - and it folds up on itself so that some parts are actually double-stranded. It twists into a funny shape that sort of looks like a four-leaf clover, or a T-shape. That's convenient, because it helps to remember that tRNA is shaped like a T.

At one end of the T is the attachment site for a certain amino acid. At the other end is a set of three nucleotides that match the codon that specifies the same amino acid. Note that these nucleotides on the tRNA do not make up a codon. They are, in fact, the exact opposite - an anticodon. The anticodon is a perfect complementary match to a codon. So, if the codon for serine is AGC, then the anticodon for serine is UCG. The tRNA that contains the anticodon UCG will also be the tRNA that attaches to a serine amino acid.

Okay, maybe I should back up for just a second. How did I get the anticodon for the codon AGC? Well, it's really simple; it's just like how we matched the complementary bases when we made copies of DNA in DNA replication. We also matched complementary bases when we transcribed DNA to mRNA. In translation, we're using two different kinds of RNA: mRNA and tRNA. So, we're only using the letters A, G, C, and U. G and C always pair together, and A and U always pair together. Just remember that G and C are both consonants, and A and U are both vowels.

To get an anticodon, switch the consonants G and C or the vowels A and U
Anticodon

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