Gross Structure
June 1st, 2008 | by admin |In eukaryotes, the coding regions of most genes are not continuous. Rather, they consist of areas that are transcribed into mRNA, the “exons,” which are interrupted by stretches of DNA that do not appear in mature mRNA, the “introns”. The functions of introns are not known with certainty. A purpose of some sort is implied by their conservation in evolution. However, their overall physical structure might be more important than their specific nucleotide sequences, since the nucleotide sequences of introns diverge more rapidly in evolution than do the sequences of exons.
Overall, DNA that contains genes comprises a minority of total DNA. Between genes, there are vast stretches of untranscribed DNA that are assumed to play an important structural role. In the nucleus, DNA is not present as naked nucleic acid. Rather, DNA is found in close association with a number of accessory proteins, such as the histones, and in this form is called chromatin.6 Although many of DNA’s accessory proteins have no known specific function, they generally appear to be involved in the correct packaging
of DNA. For example, DNA’s double helix is ordinarily twisted on itself to form a supercoiled structure.7 This structure must unwind partially during DNA replication and transcription.8 Some of the accessory
proteins, for example, topoisomerases and histone acetylases, are involved in regulating this process.
SUMMARY Genes specify the structure of proteins that are responsible for the phenotype associated with a particular gene. While the nucleus of every human cell contains 30 to 40,000 genes, only a fraction of them are expressed in any given cell at any given time. The “promoter” (with or without an “enhancer”) is the part of the gene that determines when and where it will be expressed. The “coding region” is the part of the gene that dictates the amino acid sequence of the protein encoded by the gene. DNA is a linear polymer of nucleotides.
Ordinarily, the nucleotide bases of one strand of DNA interact with those of another strand (A with T, C with G) to make double-stranded DNA. In the cell’s nucleus, DNA is associated with accessory proteins to make the structure called chromatin.
