| MadSci Network: Molecular Biology |
DNA (deoxyribo-nucleic acid) is the material in the nucleus of the cell which makes up chromosomes and carries all of the genetic information of the organism. Normally, DNA occurs as a double helix - two complementary strands winding around one another, in which the two strands run in opposite directions:
Transcription is the process of making RNA (ribo-nucleic acid) copies of a DNA template. Transcription is accomplished through RNA polymerase, the enzyme that binds to the double-stranded DNA, opens a portion to expose the two strands, and makes a copy of the template strand using free nucleotides (the building blocks of RNA). RNA polymerase can only read template DNA in the 3' to 5' direction, so the direction that the enzyme is pointed will determine which strand it transcribes.
Transcription of a specific gene is controled by transcription factors: proteins which bind to DNA and recruit RNA polymerase to the transcription start site. The location and direction of RNA polymerase's contact with the DNA is completely under the control of the many transcription factors which bind to the promoter region upstream of the gene. All genes have promoters, or else they are never transcribed.
However, this is not to say that only one of the chromosomal strands of DNA can act as a template for transcription. Along any stretch of chromosome, genes can be oriented in either direction (toward the telomere vs. toward the centromere), since each gene has its own promoter:
