Translation Polypeptide Synthesis

After splicing, the mRNA moves through the pores in the nuclear envelope into the cytoplasm. Although the nuclear pores allow the diffusion of small molecules and ions between the nucleus and cytoplasm, they have specific energy-dependent mechanisms for the selective transport of large molecules such as proteins and RNA.

In the cytoplasm, mRNA binds to a ribosome, the cell organelle that contains the enzymes and other components required for the translation of mRNA's coded message into protein. Before describing this assembly process, we will examine the structure of a ri-bosome and the characteristics of two additional classes of RNA involved in protein synthesis.

Ribosomes and rRNA As described in Chapter 3, ri-bosomes are small granules in the cytoplasm, either suspended in the cytosol (free ribosomes) or attached to the surface of the endoplasmic reticulum (bound ribosomes). A typical cell may contain 10 million ribosomes.

A ribosome is a complex particle composed of about 80 different proteins in association with a class of RNA molecules known as ribosomal RNA (rRNA). The genes for rRNA are transcribed from DNA in a process similar to that for mRNA except that a different RNA polymerase is used. Ribosomal RNA transcription occurs in the region of the nucleus known as the nucleolus. Ribosomal proteins, like other proteins, are synthesized in the cytoplasm from the mRNAs specific for them. These proteins then move back through nuclear pores to the nucleolus where they combine with newly synthesized rRNA to form two ribosomal subunits, one large and one small. These subunits are then individually transported to the cytoplasm where they combine to form a functional ribosome during protein translation.

Transfer RNA How do individual amino acids identify the appropriate codons in mRNA during the process of translation? By themselves, free amino acids do not have the ability to bind to the bases in mRNA codons. This process of identification involves the third major class of RNA, known as transfer RNA (tRNA). Transfer RNA molecules are the smallest (about 80 nucleotides long) of the major classes of RNA. The single chain of tRNA loops back upon itself, forming a structure resembling a cloverleaf with three loops (Figure 5-5).

Like mRNA and rRNA, tRNA molecules are synthesized in the nucleus by base-pairing with DNA nu-cleotides at specific tRNA genes and then move to the cytoplasm. The key to tRNA's role in protein synthesis is its ability to combine with both a specific amino acid and a codon in ribosome-bound mRNA specific for that amino acid. This permits tRNA to act as the link between an amino acid and the mRNA codon for that amino acid.

Tryptophan tRNA

Tryptophan tRNA

Anticodon Codon Pairing

Tryptophan codon

FIGURE 5-5

Base-pairing between the anticodon region of a tRNA molecule and the corresponding codon region of an mRNA molecule.

Tryptophan codon

FIGURE 5-5

Base-pairing between the anticodon region of a tRNA molecule and the corresponding codon region of an mRNA molecule.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Genetic Information and Protein Synthesis CHAPTER FIVE

A tRNA molecule is covalently linked to a specific amino acid by an enzyme known as aminoacyl-tRNA synthetase. There are 20 different aminoacyl-tRNA synthetases, each of which catalyzes the linkage of a specific amino acid to a specific type of tRNA. The next step is to link the tRNA, bearing its attached amino acid, to the mRNA codon for that amino acid. As one might predict, this is achieved by base-pairing between tRNA and mRNA. A three-nucleotide sequence at the end of one of the loops of tRNA can base-pair with a complementary codon in mRNA. This tRNA three-letter code sequence is appropriately termed an anti-codon. Figure 5-5 illustrates the binding between mRNA and a tRNA specific for the amino acid tryp-tophan. Note that tryptophan is covalently linked to one end of tRNA and does not bind to either the anti-codon region of tRNA or the codon region of mRNA.

Protein Assembly The process of assembling a polypeptide chain based on an mRNA message involves three stages—initiation, elongation, and termination. Synthesis is initiated by the binding of a tRNA containing the amino acid methionine to the small ri-bosomal subunit. A number of proteins known as initiation factors are required to establish an initiation complex, which positions the methionine-containing tRNA opposite the mRNA codon that signals the start site at which assembly is to begin. The large ribosomal subunit then binds, enclosing the mRNA between the two subunits. This initiation phase is the slowest step in protein assembly, and the rate of protein synthesis can be regulated by factors that influence the activity of initiation factors.

Following the initiation process, the protein chain is elongated by the successive addition of amino acids (Figure 5-6). A ribosome has two binding sites for tRNA. Site 1 holds the tRNA linked to the portion of the protein chain that has been assembled up to this point, and site 2 holds the tRNA containing the next amino acid to be added to the chain. Ribosomal enzymes catalyze the linkage of the protein chain to the newly arrived amino acid. Following the formation of the peptide bond, the tRNA at site 1 is released from the ribosome, and the tRNA at site 2—now linked to the peptide chain—is transferred to site 1. The ribo-some moves down one codon along the mRNA, making room for the binding of the next amino acid-tRNA molecule. This process is repeated over and over as amino acids are added to the growing peptide chain (at an average rate of two to three per second). When

Ribosome

Ribosome

Events During Translation

FIGURE 5-6

Sequence of events during the synthesis of a protein by a ribosome. [Q] [cQ>]

FIGURE 5-6

Sequence of events during the synthesis of a protein by a ribosome. [Q] [cQ>]

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions the ribosome reaches a termination sequence in mRNA specifying the end of the protein, the link between the polypeptide chain and the last tRNA is broken, and the completed protein is released from the ribosome.

Messenger-RNA molecules are not destroyed during protein synthesis, so they may be used to synthesize many protein molecules. Moreover, while one ri-bosome is moving along a particular strand of mRNA, a second ribosome may become attached to the start site on that same mRNA and begin the synthesis of a second identical protein molecule. Thus, a number of ribosomes, as many as 70, may be moving along a single strand of mRNA, each at a different stage of the translation process (Figure 5-7).

Molecules of mRNA do not, however, remain in the cytoplasm indefinitely. Eventually they are broken down into nucleotides by cytoplasmic enzymes. Therefore, if a gene corresponding to a particular protein ceases to be transcribed into mRNA, the protein will no longer be formed after its cytoplasmic mRNA molecules are broken down.

For small proteins, the folding that gives the protein its characteristic three-dimensional shape occurs spontaneously as the polypeptide chain emerges from the ribosome. Large proteins have a folding problem because their final conformation may depend upon interactions with portions of the molecule that have not yet emerged from the ribosome. In addition, a large segment of unfolded protein tends to aggregate with other proteins, which inhibits its proper folding. These problems are overcome by a complex of proteins known as chaperones, which form a small, hollow chamber into which the emerging protein chain is inserted. Within the confines of the chaperone, the

Growing polypeptide chains

Completed protein

Ribosome I

mRNA

Growing polypeptide chains mRNA

Polypeptide Chain

Completed protein

Free ribosome

FIGURE 5-7

Several ribosomes can simultaneously move along a strand of mRNA, producing the same protein in different states of assembly.

Free ribosome

FIGURE 5-7

Several ribosomes can simultaneously move along a strand of mRNA, producing the same protein in different states of assembly.

mRNA

Protein 1

Translation of mRNA into single protein

Protein 2

Posttranslational splitting of protein 1

Protein 3

Posttranslational splitting of protein 3

Protein 4

Protein 5

Essentials of Human Physiology

Essentials of Human Physiology

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Responses

  • celio
    What is an anti codon?
    7 years ago
  • amber
    What is the difference between codon and an anticodon?
    6 years ago
  • myley
    Which rna is the smallest between mrna trna and rrna?
    6 years ago
  • olga
    Which end of a polypeptide links to the tRNA?
    6 years ago
  • abeba
    What is the function of ribosome in polypeptide synthesis?
    5 years ago
  • Linda
    What is the function of translation in the body?
    3 years ago
  • Philipp
    What is the function of growing polypeptide chain?
    11 months ago
  • Roma Asmara
    Where is protein assembled from the message on the rna?
    6 months ago

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