Organization Of Genomic

The human genomic DNA is packaged into discreet structural units that vary in size and genetic composition. The structural unit of DNA is the chromosome, which is a large continuous segment of DNA (62). A chromosome represents a single genetically specific DNA molecule to which are attached a large number of protein molecules that are involved in the maintenance of chromosome structure and regulation of gene expression (63). Genomic DNA contains both "coding" and "noncoding" sequences. Noncoding sequences contain information that does not lead to the synthesis of an active RNA molecule or protein (54,64). This is not to suggest that noncoding DNA serves no function within the genome. On the contrary, noncoding DNA sequences have been suggested to function in DNA packaging, chromosome structure, chromatin organization within the nucleus, or in the regulation of gene expression (65,66). A portion of the noncoding sequences represent intervening sequences that split the coding regions of structural genes. However, the majority of noncoding DNA falls into several families of repetitive DNA whose exact functions have not been entirely elucidated (67,68).

Coding DNA sequences give rise to all of the transcribed RNAs of the cell, including mRNA. The organization of transcribed structural genes consists of coding regions that are interrupted by intervening noncoding regions of DNA (Fig. 4). Thus, the primary RNA transcripts contain both coding and noncoding sequences. The noncoding sequences must be removed from the primary RNA transcript during processing to produce a functional mRNA molecule appropriate for translation.

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