DNA serves two important functions with respect to cellular homeostasis: the storage of genetic information and the transmission of genetic information. In order to fulfill both of these functions, the DNA molecule must serve as a template. The cellular DNA provides the source of information for the synthesis of all the proteins in the cell. In this respect, DNA serves as a template for the synthesis of RNA. In cell division, DNA serves as the source of information inherited by progeny cells. In this case, DNA serves as a template for the faithful replication of the genetic information that is ultimately passed into daughter cells.
7.1. TRANSCRIPTION OF RNA Contained within the linear nucleotide sequence of the cellular DNA is the information necessary for the synthesis of all the protein constituents of a cell (Table 1). Transcription is the process in which mRNA is synthesized with a sequence complementary to the DNA of a gene to be expressed. The correct start and end points for transcription of a specific gene are identified in the DNA by a promoter sequence upstream of the gene and a termination signal downstream (Fig. 4). In the case of RNA transcription, only one strand of the DNA molecule serves as a template. This strand is referred to as the "sense" strand. Transcription of the sense strand ultimately yields a mRNA molecule that encodes the proper amino acid sequence for a specific protein.
7.2. REPLICATION OF DNA The double-stranded model of the structure of DNA strongly suggests that replication of the DNA can be achieved in a semiconservative manner (69-72). In semiconservative replication, each strand of the
DNA helix serves as a template for the synthesis of complementary DNA strands. The result is the formation of two complete copies of the DNA molecule, each consisting of one strand derived from the parent DNA molecule and one newly synthesized complementary strand. The utilization of the DNA strands as the template for the synthesis of new DNA ensures the faithful reproduction of the genetic material for transmission into daughter cells (19).
7.3. GENETIC RECOMBINATION Genetic recombination represents one mechanism for the generation of genetic diversity through the exchange of genetic material between two homologous nucleotide sequences (73,74). Such an exchange of genetic material often results in alterations of the primary structure (nucleotide sequence) of a gene and, subsequently, alteration of the primary structure of the encoded protein product. In organisms that reproduce sexually, recombination is initiated by formation of a junction between similar nucleotide sequences carried on the same chromosome from the two different parents. The junction is able to move along the DNA helix through branch migration, resulting in an exchange of the DNA strands.
7.4. DNA REPAIR Maintenance of the integrity of the informational content of the cellular DNA is absolutely required for cellular and organismal homeostasis (75,76). The cellular DNA is continuously subjected to structural damage through the action of endogenous or environmental mutagens. In the absence of efficient repair mechanisms, stable mutations can be introduced into DNA during the process of replication at damaged sites within the DNA. Mammalian cells possess several distinct DNA repair mechanisms and pathways that serve to maintain DNA integrity, including enzymatic reversal repair,
The Universal Genetic Code
The Universal Genetic Code
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