A G

Figure 7.10 The Genetic Code The genetic code correlates each series of three nucleotides, a codon, with one amino acid.Three of the codons do not code for an amino acid and instead serve as a stop codon, terminating translation. AUG functions as a start codon.

enzyme can catalyze the formation of a peptide bond between them. The ribosome also helps to identify key punctuation sequences on the mRNA molecule, such as the point at which protein synthesis should be initiated. The ribosome moves along the mRNA in the 5' to 3' direction, "presenting" each codon in a sequential order for deciphering, while maintaining the correct reading frame.

A prokaryotic ribosome is composed of a 30S subunit and a 50S subunit, each of which is made up of protein and rRNA; the "S" stands for Svedberg unit, which is a unit of size (figure 7.12). Some of the ribosomal components are important in other aspects of microbiology as well. For example, comparison of the nucleo-tide sequences of rRNA molecules is playing an increasingly prominent role in the establishment of the genetic relatedness of various organisms. Medically, ribosomal proteins and rRNA are significant because they are the targets of several groups of antimicrobial drugs. ■ ribosomal subunits, p. 67 ■ rRNA sequencing, p. 255 ■ antimicrobial drugs, p. 511

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