Genetic code

In the central dogma of molecular biology DNA contains the information for the primary sequences of proteins.

This information is encoded using the four nitrogenous bases of DNA A,T,G and C.

The information in the DNA is transcribed into information in the mRNA.

The information in the mRNA is translated into the primary sequence of a protein.

aacrfyinorderforthistooccurthestartoftranslationmustbefound

Just as it is difficult of understand the above sentance without the correct punctuation, a messenger RNA must have the punctuation for the "start of translation" in order to be translated correctly.

The code word for the start of translation is the first base triplet from the 5' end of the message that has the sequence AUG. Thus this sequence AUG is known as the "start translation" codon. A codon being a series of three bases that specifies an amino acid or a translational punctuation mark. The start word (AUG) also sets what is referred to as the "translational reading frame" of the message because the bases after the start translation code word are read thee at a time. Each three bases is a code word and specifies either an amino acid or a translational punctuation mark.

The relationship between codons and amino acids specified by each codon is shown in the table below.

You will notice that:

 

 first base

 second base

 third base

 

 U

C

A

G

 

 U

 phen UUU  ser tyr cys UGU

 U

C

A

G

 U

 phen UUC ser tyr cys UGC

 U

leu UUA ser STOP STOP

 U

leu UUG ser STOP trp

 C

leu CUU pro his arg

 U

C

A

G

 C

leu CUC pro his arg

 C

leu CUA pro gln arg

 C

leu CUG pro gln arg

 A

 ile thr asn ser

U

C

A

G

 A

 ile thr asn ser

 A

 ile thr lys arg

 A

 meth(start) thr lys arg

 G

 val ala asp gly

U

C

A

G

 G

 val ala asp gly

 G

 val ala glu gly

 G

 val ala glu gly