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4.6: Genetic Code

Difficulty Level: At Grade Created by: CK-12
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How do you go from four letters to 20 amino acids?

You need a code. And the code that changes the information embedded in DNA and RNA into ordered amino acids and proteins is the genetic code. And every living organism uses the same genetic code.

The Genetic Code

How is the information in a gene encoded? The answer is the genetic code. The genetic code consists of the sequence of nitrogen bases—A, C, G, U—in an mRNA chain. The four bases make up the “letters” of the genetic code. The letters are combined in groups of three to form code “words,” called codons. Each codon stands for (encodes) one amino acid, unless it codes for a start or stop signal. There are 20 common amino acids in proteins. There are 64 possible codons, more than enough to code for the 20 amino acids. The genetic code is shown in Figure below. To see how scientists cracked the genetic code, go to this link: http://www.dnalc.org/view/16494-Animation-22-DNA-words-are-three-letters-long-.html.

The Genetic Code. To find the amino acid for a particular codon, find the cell in the table for the first and second bases of the codon. Then, within that cell, find the codon with the correct third base. For example CUG codes for leucine, AAG codes for lysine, and GGG codes for glycine.

Reading the Genetic Code

As shown in Figure above, the codon AUG codes for the amino acid methionine. This codon is also the start codon that begins translation. The start codon establishes the reading frame of mRNA. The reading frame is the way the letters are divided into codons. After the AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. This is illustrated in Figure below. The mRNA molecule is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all stop codons. They do not code for any amino acids. Stop codons are also known as termination codons.

Reading the Genetic Code. The genetic code is read three bases at a time. Codons are the code words of the genetic code. Which amino acid does codon 2 in the drawing stand for?

Characteristics of the Genetic Code

The genetic code has a number of important characteristics.

  • The genetic code is universal. All known living organisms use the same genetic code. This shows that all organisms share a common evolutionary history.
  • The genetic code is unambiguous. Each codon codes for just one amino acid (or start or stop). What might happen if codons encoded more than one amino acid?
  • The genetic code is redundant. Most amino acids are encoded by more than one codon. In Figure above, how many codons code for the amino acid threonine? What might be an advantage of having more than one codon for the same amino acid?


  • The genetic code consists of the sequence of bases in DNA or RNA.
  • Groups of three bases form codons, and each codon stands for one amino acid (or start or stop).
  • The codons are read in sequence following the start codon until a stop codon is reached.
  • The genetic code is universal, unambiguous, and redundant.

Making Connections

Practice I

Use this resource to answer the questions that follow.

  • http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: Decoding RNA
  1. What is a polypeptide?
  2. How do the 4 nucleotides code for 20 amino acids?
  3. What is the meaning of a "triplet code"?
  4. Describe the 4 properties of codons.
  5. Describe how the genetic code was "cracked."
  6. What is the start codon?
  7. How many stop codons are there? What do these codons signal?
  8. What is a reading frame? Why are reading frames important?
  9. What amino acid do the following codons encode: UUC, UCU, CUU?

Practice II


1. What is the genetic code? What are codons?

2. Use the genetic code to translate the following segment of RNA into a sequence of five amino acids: GUC-GCG-CAU-AGC-AAG

3. The genetic code is universal, unambiguous, and redundant. Explain what this means and why it is important.

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codon A group of three nitrogen bases in an mRNA molecule; makes up a code word of the genetic code; codes for an amino acid, start, or stop.
genetic code The universal code of three-base codons; encodes the genetic instructions for the amino acid sequence of proteins.
reading frame The three base frame that divides the bases into codons.
start codon Codon that begins translation; AUG.
stop codon Codon that does not code for an amino acid; UAG, UGA, and UAA; also known as a termination codon.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Sep 04, 2016
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