DNA & RNA Codons

DNA & RNA Codons

Strands and Directions of Synthesis
Explanation of the Codons Animation
The Genetic Code

1. Strands and Directions of Synthesis

All strands are synthesized from the 5' ends > > > to the 3' ends for both DNA and RNA.
Protein chains are synthesized from the amino ends > > > to the carboxy ends.
Color mnemonic: the old end is the cold end (blue); the new end is the hot end (where new residues are added) (red).

DNA	Coding Strand (Codons)	5' > > > - - - - - - T T C - - - - - - > > > 3'
DNA	Template Strand (Anti-codons)	3' < < < - - - - - - A A G - - - - - - < < < 5'
mRNA	Message (Codons)	5' > > > - - - - - - U U C - - - - - - > > > 3'
tRNA	Transfer (Anti-codons)	3' < < < A A G < < < 5'
Protein	Amino Acid	Amino > > > Phenylalanine > > > Carboxy

As shown schematically above, messenger RNA is synthesized complementary and antiparallel to the template strand (anticodons) of DNA, so the resulting mRNA consists of codons corresponding to those in the coding strand of DNA. The anticodons of tRNA adapt each three-base mRNA codon to the corresponding amino acid, following the genetic code:

2. Explanation of the Codons Animation

Most images show 17 base pairs. For the Codons animation, the left-most two base pairs are hidden, leaving exactly five 3-base codons (15 base pairs).
The coding strand turns gray and then disappears, leaving the template strand (see strands above).
Anti-codons in the template strand are identified as groups of three bases, moving from the right end (5' end) to the left end (3' end); that is, moving in the direction of synthesis. (For more on how to identify 5' and 3' ends, press the Back button of your browser, and view the graphics page on Ends and Antiparallelism.)
The mRNA codons are now shown as white text only, complementing the anti-codons of the DNA template strand. These are displayed from left to right, namely, in the direction in which the mRNA would be synthesized (5' to 3' for the mRNA) antiparallel to the DNA coding strand.
Finally, after each mRNA codon is added "=" and the amino acid coded for (see code below).

3. The Genetic Code

The Genetic Code (mRNA)

1st position (5' end)	2nd position (middle)				3rd position (3' end)
1st position (5' end)	U	C	A	G	3rd position (3' end)
U	Phe F Phe F Leu L Leu L	Ser S Ser S Ser S Ser S	Tyr Y Tyr Y STOP STOP	Cys C Cys C STOP Trp W	U C A G
C	Leu L Leu L Leu L Leu L	Pro P Pro P Pro P Pro P	His H His H Gln Q Gln Q	Arg R Arg R Arg R Arg R	U C A G
A	Ile I Ile I Ile I Met M	Thr T Thr T Thr T Thr T	Asn N Asn N Lys K Lys K	Ser S Ser S Arg R Arg R	U C A G
G	Val V Val V Val V Val V	Ala A Ala A Ala A Ala A	Asp D Asp D Glu E Glu E	Gly G Gly G Gly G Gly G	U C A G

One codon: Met, Trp.
Two codons: Asn, Asp, Cys, Gln, Glu, His, Lys, Phe, Tyr,
Three codons: Ile, STOP ("nonsense").
Four codons: Ala, Gly, Pro, Thr, Val.
Five codons: none.
Six codons: Arg, Leu, Ser.

I am grateful for help provided by Steve Sandler and Klaus Nüsslein in the Microbiology Department in Life Sciences at the University of Massachusetts, Amherst.