composed of structural and catalytic rRNAs, and many distinct polypeptides. In eukaryotes, the nucleolus is completely
specialized for the synthesis and assembly of rRNAs.
Ribosomes are located in the cytoplasm in prokaryotes and in the cytoplasm and endoplasmic reticulum of eukaryotes.
Ribosomes are made up of a large and a small subunit that come together for translation. The small subunit is responsible
for binding the mRNA template, whereas the large subunit sequentially binds tRNAs, a type of RNA molecule that
brings amino acids to the growing chain of the polypeptide. Each mRNA molecule is simultaneously translated by many
ribosomes, all synthesizing protein in the same direction.
Depending on the species, 40 to 60 types of tRNA exist in the cytoplasm. Serving as adaptors, specific tRNAs bind to
sequences on the mRNA template and add the corresponding amino acid to the polypeptide chain. Therefore, tRNAs are
the molecules that actually “translate” the language of RNA into the language of proteins. For each tRNA to function, it
must have its specific amino acid bonded to it. In the process of tRNA “charging,” each tRNA molecule is bonded to its
correct amino acid.
The Genetic Code
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To summarize what we know to this point, the cellular process of transcription generates messenger RNA (mRNA), a
mobile molecular copy of one or more genes with an alphabet of A, C, G, and uracil (U). Translation of the mRNA
template converts nucleotide-based genetic information into a protein product. Protein sequences consist of
occurring amino acids
; therefore, it can be said that the
protein alphabet consists of 20 letters
. Each amino acid is
defined by a three-nucleotide sequence called the
. The relationship between a nucleotide codon and its
corresponding amino acid is called the genetic code.
Given the different numbers of “letters” in the mRNA and protein “alphabets,” combinations of nucleotides
corresponded to single amino acids. Using a three-nucleotide code means that there are a
total of 64
(4 × 4 × 4) possible
combinations; therefore, a given amino acid is encoded by more than one nucleotide triplet (Figure 9.20).
Three of the 64 codons terminate protein synthesis and release the polypeptide from the translation machinery. These
triplets are called stop codons. Another codon, AUG, also has a special function. In addition to specifying the amino acid
methionine, it also serves as the start codon to initiate translation. The reading frame for translation is set by the AUG start
codon near the 5′ end of the mRNA.
The genetic code is universal
. With a few exceptions, virtually all species use the
same genetic code for protein synthesis, which is powerful evidence that all life on Earth shares a common origin.