Figure

Posttranslational splitting of a protein can result in several smaller proteins, each of which may perform a different function. All these proteins are derived from the same gene.

polypeptide chain is able to complete its folding. The chaperones thus provide an isolated environment where protein folding can occur without interference.

Once a polypeptide chain has been assembled, it may undergo posttranslational modifications to its amino acid sequence. For example, the amino acid me-thionine that is used to identify the start site of the assembly process is cleaved from the end of most proteins. In some cases, other specific peptide bonds within the polypeptide chain are broken, producing a number of smaller peptides, each of which may perform a different function. For example, as illustrated in Figure 5-8, five different proteins can be derived from the same mRNA as a result of posttranslational cleavage. The same initial polypeptide may be split at different points in different cells depending on the specificity of the hydrolyzing enzymes present.

Carbohydrates and lipid derivatives are often co-valently linked to particular amino acid side chains. These additions may protect the protein from rapid degradation by proteolytic enzymes or act as signals to direct the protein to those locations in the cell where it is to function. The addition of a fatty acid to a protein, for example, can lead to the anchoring of the protein to a membrane as the nonpolar portion of the fatty acid becomes inserted into the lipid bilayer.

The steps leading from DNA to a functional protein are summarized in Table 5-1.

Although 99 percent of eukaryotic DNA is located in the nucleus, a small amount is present in mitochondria. Mitochondrial DNA, like bacterial DNA, does not contain introns and is circular. These characteristics support the hypothesis that mitochondria arose during an early stage of evolution when an c b

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

Genetic Information and Protein Synthesis CHAPTER FIVE

Genetic Information and Protein Synthesis CHAPTER FIVE

TABLE 5-1 Events Leading from DNA to Protein Synthesis

Transcription

1. RNA polymerase binds to the promoter region of a gene and separates the two strands of the DNA double helix in the region of the gene to be transcribed.

2. Free ribonucleotide triphosphates base-pair with the deoxynucleotides in the template strand of DNA.

3. The ribonucleotides paired with this strand of DNA are linked by RNA polymerase to form a primary RNA transcript containing a sequence of bases complementary to the template strand of the DNA base sequence.

4. RNA splicing removes the intron-derived regions in the primary RNA transcript, which contain noncoding sequences, and splices together the exon-derived regions, which code for specific amino acids, producing a molecule of mRNA.

Translation

5. The mRNA passes from the nucleus to the cytoplasm, where one end of the mRNA binds to the small subunit of a ribosome.

6. Free amino acids are linked to their corresponding tRNAs by aminoacyl-tRNA synthetase.

7. The three-base anticodon in an amino acid-tRNA complex pairs with its corresponding codon in the region of the mRNA bound to the ribosome.

8. The amino acid on the tRNA is linked by a peptide bond to the end of the growing polypeptide chain (see Figure 5-6).

9. The tRNA that has been freed of its amino acid is released from the ribosome.

10. The ribosome moves one codon step along mRNA.

11. Steps 7 to 10 are repeated over and over until a termination sequence is reached, and the completed protein is released from the ribosome.

12. Chaperone proteins guide the folding of some proteins into their proper conformation.

13. In some cases, the protein undergoes posttranslational processing in which various chemical groups are attached to specific side chains and/or the protein is split into several smaller peptide chains.

anaerobic cell ingested an aerobic bacterium that ultimately became what we know today as mitochondria. Mitochondria also have the machinery, including ribo-somes, for protein synthesis. However, the mitochondrial DNA contains the genes for only 13 mitochondrial proteins and a few of the rRNA and tRNA genes. Therefore, additional components are required for protein synthesis by the mitochondria, and most of the mitochondrial proteins are coded by nuclear DNA

genes. These components are synthesized in the cytoplasm and then transported into the mitochondria.

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