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Genetic Code

First
Base

Second Base

 Third
Base

 U

 C

 A

 G

 U

 phe

 ser

 tyr

 cys

 U

 U

 phe

 ser

 tyr

 cys

 C

 U

 leu

 ser

 stop

 stop

 A

 U

 leu

 ser

 stop

 trp

 G

 C

  leu

 pro

 his

 arg

 U

 C

  leu

 pro

 his

 arg

 C

 C

  leu

 pro

 glu

 arg

 A

 C

  leu

 pro

 glu

 arg

 G

 A

 ile

 thr

 asn

 ser

 U

 A

 ile

 thr

 asn

 ser

 C

 A

 ile

 thr

 lys

 arg

 A

 A

 met-start

 thr

 lys

 arg

 G

 G

 val

 ala

 asp

 gly

 U

 G

 val

 ala

 asp

 gly

 C

 G

 val

 ala

 glu

 gly

 A

 G

 val

 ala

 glu

 gly

 G

Example: Read and translate the codons on m-RNA into the appropriate amino acids.
mRNA: U U A U C G A C G
Solution: Divide the mRNA bases into the triplet codons and consult the genetic code table for the translation. The first base (U) is located on the extreme left of the table, while the second base is located in the horizontal grouping on the top of the table. This should give the upper left box. Finally, read the third base from those in that box only. This analysis gives UUA as leu. Similarly UCG is ser (2nd box on top) and ACG is thr (3rd box down in 2nd column.)

 U U A

 U C G

 A C G

 leu

 ser

 thr

Types of RNA

Messenger RNA:

Messenger RNA (mRNA) is synthesized from a gene segment of DNA which ultimately contains the information on the primary sequence of amino acids in a protein to be synthesized. The genetic code as translated is for m-RNA not DNA. The messenger RNA carries the code into the cytoplasm where protein synthesis occurs.

Genetic Code:

Each gene (or distinct segment) on DNA contains instructions for making one specific protein with order of amino acids coded by the precise sequence of heterocyclic amines on the nucleotides. Since proteins have a variety of functions including those of enzymes mistakes in the primary sequence of amino acids in proteins may have lethal effects.

How can a polymeric nucleotide with only four different heterocyclic amines specify the sequence of 20 or more different amino acids? If each nucleotide coded for a single amino acid, then obviously only 4 of the 20 amino acids could be accommodated. If the nucleotides were used in groups of two, there are 16 different combinations possible which is still inadequate.

It has been determined that the genetic code is actually based upon triplets of nucleotides which provide 64 different codes using the 4 nucleotides. During the 1960's, a tremendous effort was devoted to proving that the code was read as triplets, and also to solving the genetic code. The genetic code was originally translated for the bacteria E. Coli, but its universality has since been established. The genetic code is "read" from a type of RNA called messenger RNA (mRNA). Each nucleotide triplet, called a codon, can be "read" and translated into an amino acid to be incorporated into a protein being synthesized. The genetic code is shown in Figure 7.

Several distinctive features of the genetic code are clearly evident. First, all of the 64 codons or triplets have a known function, with 61 coding for amino acids and the other 3 serving as a stop or termination signal for protein synthesis. Secondly, the code is degenerate, meaning that there are usually several codons for each amino acid. Only methionine and tryptophan have a single codon. More specifics on the importance of the degeneracy of the genetic code will be discussed in a later section.

Quiz: Read and translate the codons on mRNA into the appropriate amino acids.
G U A C G A A A A


Click for larger image 

Ribosomal RNA:

In the cytoplasm, ribsomal RNA (rRNA) and protein combine to form a nucleoprotein called a ribosome. The ribosome serves as the site and carries the enzymes necessary for protein synthesis. In the graphic on the left, the ribosome is shown as made from two sub units, 50S and 30 S. There are about equal parts rRNA and protein. The far left graphic shows the complete ribosome with three tRNA attached.

The ribosome attaches itself to m-RNA and provides the stabilizing structure to hold all substances in position as the protein is synthesized. Several ribosomes may be attached to a single RNA at any time. In upper right corner is the 30S sub unit with mRNA and tRNA attached.

30S Subunit Ribosome - Chime in new window

Note: The coordinates used in this display have only the alpha carbons of the proteins (CA) and the DNA backbone atoms.

Link with 70S Ribosome
Link with 50S Ribosome
Link with 30S Ribosome - Dr. William McClure, Carnegie Mellon University

Ribosome - Molecule of Month, PDB - several pages, David S. Goodsell

Credit: 30S Ribosome: Yusupov, M. M., Yusupova, G. Z., Baucom, A., Lieberman, K., Earnest, T. N., Cate, J. H. D., Noller, H. F. (2001) "Crystal structure of the ribosome at 5.5 Å resolution" Science 292:883.



Click for larger image

Transfer RNA:

Transfer RNA (tRNA) contains about 75 nucleotides, three of which are called anticodons, and one amino acid. The tRNA reads the code and carries the amino acid to be incorporated into the developing protein.

There are at least 20 different tRNA's - one for each amino acid. The basic structure of a tRNA is shown in the left graphic. Part of the tRNA doubles back upon itself to form several double helical sections. On one end, the amino acid, phenylalanine, is attached. On the opposite end, a specific base triplet, called the anticodon, is used to actually "read" the codons on the mRNA.

The tRNA "reads" the mRNA codon by using its own anticodon. The actual "reading" is done by matching the base pairs through hydrogen bonding following the base pairing principle. Each codon is "read" by various tRNA's until the appropriate match of the anticodon with the codon occurs.

In this example, the tRNA anticodon (AAG) reads the codon (UUC) on the mRNA. The UUC codon codes for phenylalanine which is attached to the tRNA. Remember that the codons read from the mRNA make up the genetic code as read by humans.

tRNA-phe - Chime in new window

More Detail - Dr. William McClure, Carnegie Mellon University

Quiz: Read and translate the codon on mRNA into the appropriate amino acid.
AGA. What is the anticodon on the appropriate tRNA?


Credit: Sussman, J.L. et al. (1978) "Crystal Structure of Yeast Phenylalanine Transfer RNA I. Crystallographic Refinement" J. Mol. Biol. 123 607.