Principles of recombinant DNA

The process by which the ribosomes convert the nucleic acid code into the amino acid sequence is appropriately called translation , and the protein synthesis machinery is called the translationa 1 apparatus of the cell. A single mRNA is translated by several ribosomes before it is degraded. Thus a single mRNA molecule gives rise to several molecules of the protein it encodes, causing an amplification of gene expression similar to the situation discussed earlier in the transcription of a gene into several mRNA copies. 

The genetic code is universal. All biological systems (or nearly all) use the same code. This principle has important consequences to the developments in rDNA. Thus, the code in messenger RNAs of a human cell can be translated by bacterial protein synthesis machinery into a protein of the same sequence as in the human cell. Production of human proteins in bacteria and other organisms would be immensely difficult, if not impossible, if each organism had a different genetic code. 

Once protein synthesis is initiated, amino acids are added to the peptide chain corresponding to each triplet in the mRNA until the ribosome encounters a termination or stop codon, whereupon the polypeptide chain is released from the ribosome, and assumes its final configuration. A ribosome covers about 50 bases of an mRNA, which is usually hundreds of bases long. Thus, several ribosomes translate an mRNA consecutively and simultaneously at any instant as shown in Fig. 2.4. A group of ribosomes translating a message is called a polyribosome . 

Incorporation of amino acids into a polypeptide chain occurs through the involvement of a class of small RNA molecules known as transfer RNA or tRNA. For each codon in mRNA, there is a corresponding tRNA to which the amino acid specified by the codon is covalently attached. Since multiple codons specify the same amino acid (degenerate code), a given amino acid may be linked to a set of tRNAs. 

All the information coding for every protein in an organism is contained in its DNA in units called genes. The DNA content of almost every cell in a multicellular organism is the same and contains all the information needed to create the entire organism. 

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Section IV describes the structures and functions of the nucleotides and nucleic acids, and covers many major topics such as DNA replication and repair, RNA synthesis and modification, and protein synthesis. It also discusses new findings on how genes are regulated and presents the principles of recombinant DNA technology. 

CH.2] PRINCIPLES OF RECOMBINANT DNA 29 Calcitonin exon CGRP exon... 

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