DNA & RNA Codons

  1. Strands and Directions of Synthesis
  2. Explanation of the Codons Animation
  3. 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 residues are added) (red).

DNA Coding Strand
(Codons)
5' > > > - - - - - - T T C - - - - - - > > > 3'
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

3. The Genetic Code

The Genetic Code (mRNA)
1st position
(5' end)
2nd position (middle) 3rd position
(3' end)
U C A G
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
  1. One codon: Met, Trp.
  2. Two codons: Asn, Asp, Cys, Gln, Glu, His, Lys, Phe, Tyr.
  3. Three codons: Ile, stop ("nonsense").
  4. Four codons: Ala, Gly, Pro, Thr, Val.
  5. Five codons: none.
  6. 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.