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:

several amino acids are specified by more than one code word
several code words do not specifify any amino acids. These are the stop translation code words
the codon AUG specifies the amino acid methionine and the start of translation. Does this mean that every protein is synthesized with a methionine at the N terminal end? yes. Does this mean every protein has a methionine at the N terminal end? no. Many proteins have their N terminal ends cleaved off.

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

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