Evolution__3rd_Edition

26 PART 1 / Introduction
Table 2.1
The genetic code. The code is here expressed for mRNA. Each triplet encodes one amino acid, except for the three “stop” codons,
which signal the end of a gene.
Second base in the codon
First base in Third base in
the codon U C A G the codon
U Phenylalanine Serine Tyrosine Cysteine U
Phenylalanine Serine Tyrosine Cysteine C
Leucine Serine Stop Stop A
Leucine Serine Stop Tryptophan G
C Leucine Proline Histidine Arginine U
Leucine Proline Histidine Arginine C
Leucine Proline Glutamine Arginine A
Leucine Proline Glutamine Arginine G
A Isoleucine Threonine Asparagine Serine U
Isoleucine Threonine Asparagine Serine C
Isoleucine Threonine Lysine Arginine A
Methionine Threonine Lysine Arginine G
G Valine Alanine Aspartic acid Glycine U
Valine Alanine Aspartic acid Glycine C
Valine Alanine Glutamic acid Glycine A
Valine Alanine Glutamic acid Glycine G
The tRNA molecule has a base triplet recognition site, which binds to the complementary
triplet in the mRNA, and has the appropriate amino acid attached at the other
end (Figure 3.7, p. 58, shows the structure of tRNA). Cells use less than the theoretical
maximum of 61 different kinds of tRNA. A single tRNA can be used for more than one
codon, for example, in some cases where the same amino acid is coded for by two
closely related codons. The ability of a single tRNA to bind to more than one codon is
called “wobble.” Cells in fact use about 45 kinds of tRNA. In summary, protein assembly
consists of tRNA molecules lining up on the mRNA at a ribosome. Other molecules are
also needed to supply energy and attach the RNAs correctly. Figure 2.3 summarizes the
transfer of information in the cell.
In addition to the DNA on the chromosomes in the nucleus, there are much smaller
quantities of DNA in certain organelles in the cytoplasm (see Figure 2.1). Mitochondria
a the organelles that control respiration a have some DNA, and in plants the
organelles called chloroplasts that control photosynthesis also have their own DNA.
Mitochondrial DNA is inherited maternally: mitochondria are passed on through eggs
but not through sperms.
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