The Genetic Code
The primary purpose of DNA is information storage. This purpose is expressed in what is called "the Central Dogma" (Crick's term) of molecular biology (that is, biological chemistry).
- DNA is the repository of all of the information necessary for the construction of an organism; that is, DNA is the genetic material
- A gene is a segment of DNA that codes for a specific protein or functional RNA
- DNA can replicate itself by complementary base pairing of deoxyribonucleotides to each strand of the double helix
- Complementary base pairing of ribonucleotides to segments of DNA produces RNA; the process is called transcription
- Messenger RNA (mRNA) carries the code for the protein to be synthesized
- Transfer RNA (tRNA) collects the individual amino acids for incorporation into protein
- Complementary base pairing of tRNAs to mRNA produces protein; the process is called translation
Like most dogmas, this one is only correct in a very broad sense.
The central dogma required some revision in 1970 with the discovery of reverse transcriptase enzymes in RNA viruses (retroviruses), independently by Howard Temin and David Baltimore who shared a Nobel Prize for the discoveries..
Furthermore, much DNA does not code for protein at all.
Nonetheless the basic information flow is as shown above and in this Table:
| DNA |
mRNA |
Polypeptide |
5'----3'
C----G
G----C
T----A
G----C
G----C
A----T
T----A
A----T
C----G
A----T
C----G
T----A
T----A
T----A
T----A
G----C
C----G
C----G
G----C
T----A
T----A
T----A
C----G
T----A
3'----5' |
5'
C
G
U
G
G
A
U
A
C
A
C
U
U
U
U
G
C
C
G
U
U
U
C
U
3'
|
Arg
Gly
Tyr
Thr
Phe
Ala
Val
Ser
|
Here is the entire code:
- The code is non-overlapping; no "letter" is part of two "words" in the same "message"
- Nonetheless, each letter represents a potential starting point for translation of a different polypeptide, creating a series of "reading frames"
- The code is unambiguous: in a given organism, each three-letter code (codon) corresponds to only a single amino acid
- The code is degenerate: some amino acids are coded by more than one triplet
- The format of the Table makes it easy to see that NNU and NNC always code for the same amino acid
- This is an example of "wobble" - the lesser importance of the third base in a codon
- Three codons signal "stop"; they are termination codons
- "Release factors" recognize stop codons and catalyze the disassembly of the translation machinery
- However, under some circumstances (not yet understood), the UGA stop codon codes for a new amino acid, selenocysteine
- Furthermore, almost all organisms have tiny quantities of tRNAs that bind to stop codons in competition with release factors and deliver amino acids
- These are sometimes called "suppressors" since they suppress termination
- The methionine codon, AUG, is the universal start code
This page last modified 12:35 PM on Wednesday January 14th, 2009.
Webmaster, Department of Chemistry, University of Maine, Orono, ME 04469