38 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
hundred amino acids in length, and since any 
one ot 20 amino acids can fill each slot, the num- 
her of possible proteins is enormous. Neverthe- 
less, each protein requires the strictest ordering 
of amino acids in its structure. Changing a 
single amino acid in the entire sequence can 
drastically change the protein's character. 
It was now possible for scientists to move 
nearer to an appreciation of how genes func- 
tioned. First had come the recognition that DNA 
determined protein; now it was evident that the 
sec|uence of nucleotides in DNA determined a 
linear sequence of amino acids in proteins. 
By the early 1980’s, the way proteins were 
manufactured, how their synthesis was regu- 
lated, and the role of DNA in both processes 
were understood in considerable detail. The 
process of transcribing DNA’s message— carry- 
ing the message to the cell’s miniature pi'otein 
factories and building proteins— took place 
through a complex set of reactions. Kach amino 
acid in the protein chain was represented by 
three nucleotides from the DNA. That thi'ee- 
hase unit acted as a word in a DNA sentc'nce 
that spelled out each |)rotein— the genetic codcv 
(See figure 10.) 
Thiough the genetic ('ode, an entire* gene— a 
linear assemblage of nuclen)tides— could now he 
Figure 10.— The Genetic Code 
SECOND 
BASE 
THIRD BASE 
ser 
pro 
thr 
ala 
SECOND 
BASE 
T 
THIRD BASE 
A 
E 
his 
asn 
asp 
tyr 
his 
asn 
asp 
och’ 
gin 
lys 
glu 
I 1 
amb* 
I I 
gin 
lys 
glu 
SECOND 
BASE 
G 
cys 
arg 
ser 
giy 
ser 
pro 
thr 
ala 
ser 
pro 
thr 
ala 
ser 
pro 
thr 
ala 
THIRD BASE 
cys 
arg 
ser 
giy 
end* 
arg 
arg 
giy 
trp 
arg 
arg 
giy 
*och (ochre); amb (amber), and end are stop signal for translation, i.e., 
signal the end of synthesis of the protein chain. 
Amino acid 
Three-letter 
symbol 
alanine 
ala 
arginine 
arg 
asparagine 
asn 
aspartic acid 
asp 
asn and/or asp 
asx 
cysteine 
cys 
glutamine 
gin 
glutamic acid 
glu 
gin and/orglu 
glx 
glycine 
giy 
histidine 
his 
isoleucine 
ileu 
leucine 
leu 
lysine 
lys 
methionine 
met 
phenylalanine 
phe 
proline 
pro 
serine 
ser 
threonine 
thr 
tryptophan 
trp 
tyrosine 
tyr 
valine 
val 
Each amino acid is determined by a 
three letter code (A, G, T, or C) along 
the DNA. If the first letter in the code 
is A, the second is T. and third is A, 
the amino acid will be tyrosine (or tyr) 
in the complete protein molecule. For 
leucine (or leu), the code is GAT, and 
so forth. The dictionary above gives 
the entire code. 
ATA GAT AGA TAG ATAG 
V V V V V 
tyr - leu • ser • ileu • tyr 
SOURCE: Office of Technology Assessment. 
