Codon: Definition & Sequences

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  • 0:04 What Are Codons?
  • 1:06 Sequence Reading Frame
  • 2:27 Start & Stop Codons
  • 3:31 Redundantly Unambiguous
  • 4:05 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

DNA is written in a language that uses 3-letter words. Each 3-letter word is called a codon. A full sentence in this DNA language is a gene. This lesson defines codons and discusses the different sequences that give rise to amino acids.

What Are Codons?

The genetic code is contained within genetic material, otherwise known as DNA or mRNA sequences. This code is translated into proteins by living cells. Ribosomes decode mRNA sequences to construct proteins. Ribosomes can read the coding, which is written using 3-letter words called codons. The genetic code is highly similar among all organisms and can be expressed in a diagram.

Codons and their corresponding amino acids
Image of the codons diagram

Transfer RNA, or tRNA, molecules carry amino acids to the ribosomes and read the mRNA three nucleotides at a time, or one codon at a time. tRNAs can read codons because they have an anti-codon as part of their sequence. Just like in mRNA, the anti-codon of tRNA codes for a specific amino acid, just in the reverse order of the mRNA. This diagram shows the relationship between mRNA and codons.

mRNA and corresponding codons
Image of mRNA and codons

Sequence Reading Frame

Because codons have three letters, the genetic code can be interpreted three ways. These three different ways of interpretation are called reading frames. As an example, the gene CGAGCCTCC, if read from the first position, or the first frame, contains the codons CGA, GCC, TCC. If read from the second position, or the second frame, it contains the codons GAG and CCT. If read from the third position, or the third frame, it contains the codons AGC and CTC. Notice that because the code is read in codons of three letters each the second and third reading frames only contain two complete codons.

As a result of the different reading frames, every DNA sequence, or gene, can be read three different ways. Each different frame will produce a different amino acid sequence when translated. Only one frame is actually the correct frame and will produce a viable protein. The other two frames will not. Luckily, in our cells the actual frame in which a protein sequence is translated is defined by a start codon. The start codon is usually the first AUG codon in the mRNA sequence.

Start and Stop Codons

Translation starts with, you guessed it, a start codon. However, this is not the only sequence that is needed to start translation. In addition, there are initiation factors that are needed to start translation. AUG is the most common start codon, but there are others. AUG always codes for methionine. GUG or UUG, which usually code for valine and leucine, respectively, can also be start codons. When they are, they code for methionine or formylmethionine.

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