Page 353 - Applied Probability

343
Appendix A: Molecular Genetics in Brief
A.2 From Gene to Protein
Proteins are constructed from the 20 amino acids shown in Table A.1. With
an alphabet of four letters, three base words could in principle code for
3
4 = 64 different amino acids. Nature has elected to forgo this opportunity
and instead opts for redundancy in the genetic code. This redundancy is
very evident in Table A.2. Several other features of Table A.2 are worth
noting. First, three codons serve as genetic stop signals for terminating a
growing protein polymer. Second, the codon AUG for methionine also plays
the role of a start signal provided it is preceded by purine-rich sequences
such as AGGA. Third, the base U is substituted for the base T. This
substitution occurs because the DNA specifying a gene is first transcribed
into single-stranded premessenger RNA. After appropriate processing,
premessenger RNA is turned into messenger RNA (mRNA), which is then
translated into protein by cell organelles known as ribosomes. On release
from a ribosome, a protein folds into its characteristic shape. Depending
on its ultimate function, a protein may undergo further processing such as
cleavage or the addition of lipid or carbohydrate groups.
TABLE A.2. The Genetic Code
2nd Position

1st Position U C A G 3rd Position

Phe Ser Tyr Cys U
U Phe Ser Tyr Cys C
Leu Ser Stop Stop A
Leu Ser Stop Trp G

Leu Pro His Arg U
C Leu Pro His Arg C
Leu Pro Gln Arg A
Leu Pro Gln Arg G

Ile Thr Asn Ser U
A Ile Thr Asn Ser C
Ile Thr Lys Arg A
Met Thr Lys Arg G

Val Ala Asp Gly U
G Val Ala Asp Gly C
Val Ala Glu Gly A
Val Ala Glu Gly G