9.05: Genetic Code

Can You Code?

If someone asks you whether you can code, you probably assume they are referring to computer code. The image in Figure \(\PageIndex{1}\) represents an important code that you use all the time but not with a computer. It's the genetic code, and it is used by your cells to store information and make proteins.

Figure \(\PageIndex{1}\): FACIL genetic code logo

What Is the Genetic Code?

The genetic code consists of the sequence of nitrogen bases in a polynucleotide chain of DNA or RNA. The bases are adenine (A), cytosine (C), guanine (G), and thymine (T) (or uracil, U, in RNA). 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. With four bases forming three-base codons, there are 64 possible codons. 61 codons are more than enough to code for the 20 amino acids, thus more than one codon codes for a single amino acid. Please find genetic codes in Table \(\PageIndex{1}\) or in appendix 1.

Table \(\PageIndex{1}\): Codon Chart. To find the amino acid for a particular codon, find the cell in the table for the first, second, and third bases of the codon. Once you have found the codon, you can find the corresponding amino acid in the adjacent cell on the right side of the codon cell. For example CUG codes for leucine (Leu), AAG codes for lysine (Lys), and GGG codes for glycine (Gly). Text only version of the codon chart.

	Second base U	Amino acid	Second base C	Amino acid	Second base A	Amino acid	Second base G	Amino acid
First base U	UUU	Phe	UCU	Ser	UAU	Tyr	UGU	Cys	Third base U
First base U	UUC	Phe	UCC	Ser	UAC	Tyr	UGC	Cys	Third base C
First base U	UUA	Leu	UCA	Ser	UAA	(stop) no amino acid	UGA	(stop) no amino acid	Third base A
First base U	UUA	Leu	UCG	Ser	UAG	(stop) no amino acid	UGG	Trp	Third base G
First base C	CUU	Leu	CCU	Pro	CAU	His	CGU	Arg	Third base U
First base C	CUC	Leu	CCC	Pro	CAC	His	CGC	Arg	Third base C
First base C	CUA	Leu	CCA	Pro	CAA	Gln	CGA	Arg	Third base A
First base C	CUG	Leu	CCG	Pro	CAG	Gln	CGG	Arg	Third base G
First base A	AUU	Ile	ACU	Thr	AAU	Asn	AGU	Ser	Third base U
First base A	AUC	Ile	ACC	Thr	AAC	Asn	AGC	Ser	Third base C
First base A	AUA	Ile	ACA	Thr	AAA	Lys	AGA	Arg	Third base A
First base A	AUG	Met (start)	ACG	Thr	AAG	Lys	AGG	Arg	Third base G
First base G	GUU	Val	GCU	Ala	GAU	Asp	GGU	Gly	Third base U
First base G	GUC	Val	GCC	Ala	GAC	Asp	GGC	Gly	Third base C
First base G	GUA	Val	GCA	Ala	GAA	Glu	GGA	Gly	Third base A
First base G	GUG	Val	GCG	Ala	GAG	Glu	GGG	Gly	Third base G

Reading the Genetic Code

If you find the codon AUG in Table \(\PageIndex{1}\), you will see that it codes for the amino acid methionine. This codon is also the start codon that establishes the reading frame of the code. The reading frame is the way the bases are divided into codons. It is illustrated in Figure \(\PageIndex{2}\). After the AUG start codon (not shown in the image), the next three bases are read as the second codon. The next three bases after that are read as the third codon, and so on. The sequence of bases is read, codon by codon, until a stop codon is reached. UAG, UGA, and UAA are all the stop codons. They do not code for any amino acids.

Figure \(\PageIndex{2}\): Reading the Genetic Code. The genetic code is read three bases at a time. Codons are the code words of the genetic code.

Characteristics of the Genetic Code

The genetic code has a number of important characteristics:

• The genetic code is universal. All known living things have the same genetic code. This shows that all organisms share a common evolutionary history.
• The genetic code is unambiguous. This means that each codon codes for just one amino acid (or start or stop). This is necessary so there is no question about which amino acid is the correct one.
• The genetic code is redundant. This means that each amino acid is encoded by more than one codon. For example, in the table above, four codons code for the amino acid threonine. Redundancy in the code helps prevent errors in protein synthesis. If a base in codon changes by accident, there is a good chance that it will still code for the same amino acid.

Review

1. Describe the genetic code.
2. Explain how the genetic code is read.
3. Identify three important characteristics of the genetic code.
4. Summarize how the genetic code was deciphered.
5. Use the table entitled The Genetic Code, shown above, to answer the following questions.
   1. Is the code depicted in the table from DNA or RNA? Explain your reasoning.
   2. Which amino acid does the codon CAA code for?
   3. Does UGA code for an amino acid? Why or why not? If so, which one?
   4. Look at the codons that code for the amino acid glycine. How many of them are there? What are their similarities and differences from each other?
   5. Imagine that you are doing an experiment similar to the one performed by Nirenberg and Matthaei with 20 test tubes, each containing bacterial cell contents and all 20 amino acids, with one type of amino acid labeled in each tube. If you added synthetic RNA containing only the base cytosine, a polypeptide chain consisting of which amino acid would be produced? Explain your answer.
6. True or False. One codon can encode for more than one amino acid.
7. True or False. The codons for tyrosine in plants are the same as ones that encode for tyrosine humans.
8. True or False. The start codon encodes for an amino acid, in addition to its function establishing where the reading frame starts.
9. How many possible codons are there?
   1. 64
   2. 20
   3. 3
   4. It depends on the species
10. How many common amino acids are there in proteins?
    1. 64
    2. 20
    3. 3
    4. 4

Explore More

https://bio.libretexts.org/link?16750#Explore_More

Comparing DNA sequences is vital to understanding evolutionary relationships between organisms. Check out more here:

Attributions:

1. Genetic Code logo by Bas E. Dutilh, et al, licensed CC BY 2.5 via Wikimedia Commons
2. Genetic code by Madprime, public domain via Wikimedia Commons
3. Text adapted from Human Biology by CK-12 licensed CC BY-NC 3.0