The ribosomes in eukaryotic cells are either found freely in the cytosol or closely associated with the endoplasmic reticulum (ER), which is then referred to as the rough ER (RER). RER-associated ribosomes are responsible for synthesizing proteins, including glycoproteins, that will be exported from the cell, incorporated into membranes, or imported into lysosomes. On the other hand, cytosolic ribosomes synthesize proteins needed in the cytosol itself or destined for the nucleus, mitochondria, or peroxisomes. It should be noted that mitochondria have their own ribosomes and unique circular DNA, but most mitochondrial proteins are encoded by nuclear DNA, synthesized in the cytosol, and then targeted to mitochondria.

Polypeptide synthesis requires initiation, elongation, and termination (or release) factors. Some of these factors have a catalytic function, while others stabilize the synthetic machinery. Additionally, there are small cytosolic G proteins that act as factors, which are active when bound to guanosine triphosphate (GTP) and inactive when bound to guanosine diphosphate (GDP).

Energy is required for adding an amino acid to the growing polypeptide chain. This energy comes from cleaving four high-energy bonds, two from ATP in the aminoacyl-tRNA synthetase reaction, one in the removal of PPi, and one in the subsequent hydrolysis of PPi to two Pi by pyrophosphatase. Two high-energy bonds from GTP are also used, one for binding the aminoacyl-tRNA to the A site and one for the translocation step. It is important to note that additional ATP and GTP molecules are needed for initiation in eukaryotes, while an additional GTP molecule is required for termination in both eukaryotes and prokaryotes. Translation, therefore, consumes a significant amount of energy.

Correct pairing of the codon in the mRNA with the anticodon of the tRNA is crucial for accurate translation. Most tRNA molecules (isoaccepting tRNA) can recognize more than one codon for a specific amino acid. The binding between the tRNA anticodon and mRNA codon follows the rules of complementary and antiparallel binding, where the mRNA codon is read in the 5′→3′ direction, while the anticodon pairs in the opposite (3′→5′) orientation.

The wobble hypothesis explains how a tRNA can recognize multiple codons for a specific amino acid. According to this hypothesis, codon-anticodon pairing follows the traditional Watson-Crick rules (G pairs with C and A pairs with U) for the first two bases of the codon, but can be less strict for the last base. The base at the 5′-end of the anticodon is not as spatially defined as the other two bases, allowing nontraditional base-pairing with the 3′-base of the codon. This flexibility, known as wobble, enables a single tRNA to recognize more than one codon. The wobble phenomenon reduces the number of required tRNA species to read the 61 codons that code for amino acids.

Now let's move on to the steps involved in translation.
Cellular location: In eukaryotic cells, the ribosomes either are free in the cytosol or are in close association with the endoplasmic reticulum (which is then known as the rough endoplasmic reticulum, or RER). RER-associated ribosomes are responsible for synthesizing proteins (including glycoproteins; see p. 166) that are to be exported from the cell, incorporated into membranes, or imported into lysosomes (see p. 169 for an overview of the latter process). Cytosolic ribosomes synthesize proteins required in the cytosol itself or destined for the nucleus, mitochondria, or peroxisomes. [Note: Mitochondria contain their own ribosomes (55S) and their own unique, circular DNA. Most mitochondrial proteins, however, are encoded by nuclear DNA, synthesized completely in the cytosol, and then targeted to mitochondria.]
F. Protein factors
Initiation, elongation, and termination (or, release) factors are required for polypeptide synthesis. Some of these protein factors perform a catalytic function, whereas others appear to stabilize the synthetic machinery. [Note: A number of the factors are small, cytosolic G proteins and thus are active when bound to guanosine triphosphate (GTP) and inactive when bound to guanosine diphosphate (GDP). See p. 95 for a discussion of the membrane-associated G proteins.]
G. Energy sources
Cleavage of four high-energy bonds (see p. 73) is required for the addition of one amino acid to the growing polypeptide chain: two from ATP in the aminoacyl-tRNA synthetase reaction, one in the removal of PPi and one in the subsequent hydrolysis of the PPi, to two Pi by pyrophosphatase, and two from GTP, one for binding the aminoacyl-tRNA to the A site and one for the translocation step (see Fig. 32.13, p. 457). [Note: Additional ATP and GTP molecules are required for initiation in eukaryotes, whereas an additional GTP molecule is required for termination in both eukaryotes and prokaryotes.] Translation, then, is a major consumer of energy.
IV. CODON RECOGNITION BY TRANSFER RNA
Correct pairing of the codon in the mRNA with the anticodon of the tRNA is essential for accurate translation (see Fig. 32.6). Most tRNA (isoaccepting tRNA) recognize more than one codon for a given amino acid.
A. Antiparallel binding between codon and anticodon
Binding of the tRNA anticodon to the mRNA codon follows the rules of complementary and antiparallel binding, that is, the mRNA codon is read 5′→3′ by an anticodon pairing in the opposite (3′→5′) orientation (Fig. 32.9). [Note: Nucleotide sequences are always written in the 5′ to 3′ direction unless otherwise noted. Two nucleotide sequences orient in an antiparallel manner.]
B. Wobble hypothesis
The mechanism by which a tRNA can recognize more than one codon for a specific amino acid is described by the wobble hypothesis, which states that codon–anticodon pairing follows the traditional Watson-Crick rules (G pairs with C and A pairs with U) for the first two bases of the codon but can be less stringent for the last base. The base at the 5′-end of the anticodon (the first base of the anticodon) is not as spatially defined as the other two bases. Movement of that first base allows nontraditional base-pairing with the 3′-base of the codon (the last base of the codon). This movement is called wobble and allows a single tRNA to recognize more than one codon. Examples of these flexible pairings are shown in Figure 32.9. The result of wobble is that 61 tRNA species are not required to read the 61 codons that code for amino acids.
V. STEPS IN TRANSLATION