[Print] |
The central dogma of biology states that DNA is used to make RNA. But most cells make proteins from only a small fraction of their DNA. Let’s look at how a cell can make RNA from a specific gene.
The stretch of DNA that is transcribed into RNA is called the transcription unit. In prokaryotes, a transcription unit may include several genes. In eukaryotes, a transcription unit contains only one gene. In both eukaryotes and prokaryotes, only one of the DNA strands in the double helix is used as the template to make RNA. The other strand is complementary to the template and is not used to make RNA.
In DNA replication, the enzyme DNA polymerase adds the bases to the growing DNA molecule. In transcription, RNA polymerase adds the bases to the growing RNA molecule. Just like DNA polymerase, RNA polymerase adds nucleotides to the 3-prime end of the growing molecule. Therefore, the RNA molecule grows in the 5-prime to 3-prime direction, which is 3-prime to 5-prime along the DNA strand.
Prokaryotes use one type of RNA polymerase to synthesize all three types of RNA. Eukaryotes use different types of RNA polymerase to synthesize different types of RNA. RNA polymerase II synthesizes mRNA.
Prior to transcription, RNA polymerase binds to a specific region of the DNA, called a promoter. The promoter includes the initiation site, the place in the DNA where RNA synthesis actually begins. The promoter is located about 100 nucleotides before, or "upstream" from, the initiation site. In eukaryotes, the promoter generally contains a sequence of DNA called a TATA box. The TATA box consists mostly of thymines, or Ts, and adenines, or A's, which is where it gets its name. The TATA box is located about 25 nucleotides upstream from the initiation site.
RNA polymerase doesn’t recognize the TATA box on its own. Other proteins, called transcription factors, bind to the promoter. RNA polymerase recognizes the complex formed by the transcription factors and the DNA, and binds the DNA at the initiation site. Once RNA polymerase is bound, transcription begins.
The upper strand of this piece of DNA is the template for transcription. Slide the TATA box label to the correct position along the DNA. Place the transcription factor and RNA polymerase in the correct positions along the DNA.
The TATA box is rich in adenine and thymine bases. The transcription factor will bind to the region of DNA containing the TATA box. RNA polymerase adds bases to the RNA molecule in the 5- prime to 3- prime direction, which is 3- prime to 5-prime along the DNA. Therefore, RNA polymerase will bind to the left of the transcription factor in this diagram.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education