36 3 Aspects of the Molecular Biology of Microorganisms of Relevance to the Aquatic Environment
triplet bases may code for the same amino acid. Thus
the amino acid glycine is coded by four different
codons: GGU, GGC, GGA, and GGG. However, a
codon is always for one amino acid. There are 64 dif-
ferent codons; three of these UAA, UAG, and UGA are
stop codons and stop the process of translation. The
remaining 61 code for the amino acids in proteins (see
Table 3.1). Translation of the message generally begins
at AUG, which also codes for methionine. For AUG to
act as a start codon, it must be preceded by a ribosome
binding site. If that is not the case, it simply codes for
methionine (Purvees et al. 1995 ).
Promoters are sequences of DNA that are the start
signals for the transcription of mRNA. Terminators
are the stop signals. mRNA molecules are long
(500–10,000 nucleotides).
Ribosomes are the sites of translation. The ribo-
somes move along the mRNA and bring together the
amino acids for joining into proteins by enzymes.
Transfer RNAs (tRNAs) carry amino acids to mRNA
for linking and elongation into proteins. Transfer RNA
is basically cloverleaf-shaped. tRNA carries the proper
amino acid to the ribosome when the codons call for
them. At the top of the large loop are three bases, the
anticodon, which is the complement of the codon.
There are 61 different tRNAs, each having a different
binding site for the amino acid and a different anti-
codon. For the codon UUU, the comple mentary anti-
codon is AAA. Amino acid linkage to the proper tRNA
is controlled by the aminoacyl-tRNA synthetases.
Energy for binding the amino acid to tRNA comes from
ATP conversion to adenosine monophosphate (AMP).Fig. 3.3 Summary of protein synthesis activities (Copyright Terese
Winslow; http://www.teresewinslow.com/. With permission)
Note: There are two phases in protein synthesis: Transcription and
Translation. In transcription, one strand of the DNA is used as a
template for synthesizing messenger RNA (mRNA). In translation,
the ribosome binds to this mRNA, and moves along it three
nucleotides at a time,each triplet (known as a codon) being respon-
sible for the synthesis of an amino acid. The polypeptide chain is
increased one amino acid at a time, until a codon which does not
code for any amino acid (a stop or non-sense codon) is reached.