The DNA double helix transmits genetic information from generation to generation and is the repository of information required to guide an organism's development and interaction with the environment. The role of DNA in storing and transferring hereditary information depends on the innate properties of its four constituent bases. There are two purine bases, adenine (A) and guanine (G), and two pyrimidine bases, cytosine (C) and thymine (T). Within the DNA double helix, A is complementary to T, and G is complementary to C. Each block of DNA that codes for a single RNA or protein is called a gene, and the entire set of genes in a cell, organelle, or virus forms its genome. Cells and organelles may contain more than one copy of their genome. There are 46 chromosomes in a typical human cell; when "unraveled," the total DNA of a single cell is approximately 1 m in length. The 46 human chromosomes consist of 22 pairs of autosomes and 2 sex chromosomes, either XX for females or XY for males. Such a large amount of genetic material is effectively packaged into a cell nucleus, which is also the site of DNA replication and transcription. Only a small percentage of chromosomal DNA in the human genome is responsible for encoding the genes that act as a template for RNA strands; there are approximately 20,000-25,000 genes total, of which about 10,000-15,000 genes are expressed in any individual cell.
Among RNA strands, only ribosomal RNA (rRNA), transfer RNA (tRNA), and small nuclear RNA (snRNA) have independent cellular functions. Most cellular RNA, mRNA, serves as a template for protein synthesis. RNA, like DNA, is also composed of four nucleotide building blocks. However, in RNA, the nucleotide uracil (U) takes the place of thymine (T), and RNA is a flexible single strand that is free to fold into a variety of conformations. Thus, the functional versatility of RNA greatly exceeds that of DNA.
Chromosomal DNA contains both genes and more extensive intergenic regions. Some regions of DNA in genes act as the template for RNA, but some regions are responsible for regulatory functions. The distribution of genes on chromosomes is not uniform: some chromosomal regions, and indeed whole chromosomes, are richly endowed with genes, whereas other regions are more amply supplied with noncoding DNA. Regulation of gene expression conferred by the nucleotide sequence of a DNA molecule is referred to as c/s-regulation, because the regulatory and transcribed regions occur on the same DNA molecule. c/s-Regulatory elements that determine the transcription start site of a gene are called basal (or core) promoters; other c/s-regulatory elements are responsible for tethering different activators and repressor proteins to DNA. There are specific regions of DNA that bind to regulatory proteins. These regulatory proteins may be encoded at any regions in the genome, and because they are not coded by the stretch of DNA to which they bind, they are sometimes called trans-acting factors. Trans-acting factors that regulate the transcription of DNA are also called transcript/on factors.
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