Protein Synthesis Translation of the Genetic Message

Chapter 12 Protein Synthesis Translation of the Genetic Message... 

An overview of protein synthesis is illustrated in Figure 19.3. Despite its complexity and the variations among species, the translation of a genetic message into the primary sequence of a polypeptide can be divided into three phases initiation,... 

In addition to the different translational factors and mechanisms used by eukaryotic organisms as described earlier, an organizational difference exists in eukaryotes that contrasts with prokaryotic protein synthesis. In organisms lacking a nucleus, transcription of the genetic message from DNA to RNA occurs in the same location as translation. In fact, bacterial ribosomes typically be-... 

Protein synthesis means translation into a peptide chain of a genetic message first copied (transcribed) into m-RNA (p. 274). Amino acid (AA) assembly occurs at the ribosome. Delivery of amino acids to m-RNA involves different transfer RNA molecules (t-RNA), each of which binds a specific AA. Each t-RNA bears an anticodon nucleobase triplet that is complementary to a particular m-RNA coding unit (codon, consisting of 3 nucleobases. 

Although DNA replaced RNA in the role of storing the genetic information, RNA maintained many of its other functions. RNA still provides the template that directs polypeptide synthesis, the adaptor molecules, the catalytic activity of the ribosomes, and other functions. Thus, the genetic message is transcribed from DNA into RNA and then translated into protein. 

Recent work on the molecular organization of eukaryotic cells has shown that the classic notion of information transfer must be extended to include extranuclear cell components since the genetic information of a eukaryotic cell is not confined to the cell nucleus but is also present in the mitochondria and in the chloroplasts of photosynthetic cells. Replication of these extranuclear DNA species alra takes place outside the nucleus. Furthermore, protein synthesis is not confined to ribosomes of the endoplasmic reticulum or the nucleus but may also take place at ribosomes in mitochondria and in chloroplasts. Mitochondria, hke chloroplasts, contain a unique DNA species and the means of repheating it, as well as the machinery for transcription and for translation of the message. They also contain some unique tRNA species. 

PROTEINS are polymers of amino acids joined by peptide bonds (proteins are therefore also known as polypeptides). The number and order of the amino acids contained in a particular protein are prescribed by the DNA sequence of that protein s gene. The mechanism by which a genetic message is translated from its nucleic acid form to its polypeptide product is protein synthesis. Translation occurs at the ribosome, an RNA-protein complex often called the protein factory of the cell. 

Messenger RNA (mRNA) serves to carry the information or message that is encoded in genes to the sites of protein synthesis in the cell, where this information is translated into a polypeptide sequence. Because mRNA molecules are transcribed copies of the protein-coding genetic units that comprise most of DNA, mRNA is said to be the DNA-like RNA. ... 

The ultimate purpose of DNA expression is protein synthesis. mRNAs serve as the intermediate carrier of the DNA genetic information for protein synthesis. The DNA message is carried in the form of base sequences that are transferred to RNA, also in terms of base sequences, and finally these are transferred into amino acid sequences through a translation process based on the genetic code. This process of information from the RNA to the protein is called translation. 

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