22
FRIEDHELM HILDEBRANDT
1st position
2nd position
Jrd posi tion
15' end)
u c A G
13' end)
!
-!,
u
Phe
Ser
Tyr
Cys
u
Phe
Ser
Tyr
Cys
c
Leu
Ser
STOP
STOP
A
Leu
Ser
STOP
Trp
G
c
Leu
Pro
His
Arg
u
Leu
Pro
His
Arg
c
Leu
Pro
Gin
Arg
A
Leu
Pro
Gin
Arg
G
A
lle
Thr
Asn
Ser
u
lle
Thr
Asn
Ser
c
lle
Thr
Lys
Arg
A
Met
Thr
Lys
Arg
G
G
Val
Ala
Asp
Gly
u
Val
Ala
Asp
Gly
c
Val
Ala
Glu
Gly
A
Val
Ala
Glu
Gly
G
Fig. 13. The genetic code. (Modified from [1].)
Differential transcripts from the same gene
An economic means for the diversification of gene products has been introduced by nature through the use of alternative promoters (P1 and P2)
and alternative splicing (splicing alternatives 1 and 2, see Figure llB).
Translation and the genetic code
Translation (protein biosynthesis) (Figure 12) occurs at ribosomes (Figure
12A). During translation, the A-site of the ribosome is occupied by a tRNA
that contains an anticodon, which is complementary to the respective codon on mRNA. A peptide bond is formed between the amino acid carried by
the tRNA and the nascent polypeptide chain, which occupies the P-site of
the ribosome (Figure 12B). Translation always starts at the start codon AUG
(methionine) and ends at one of the 3 stop codons UAA, UAG, UGA.
The genetic code (Figure 13) is called degenerate, since more than one
triplet may encode the same amino acid. There is a correlation between the