Like so many other genetic events, transcription is a marvel of efficiency. It is highly organized and guided by special codes on DNA itself. To describe it in more detail, we use the synthesis of mRNA in bacteria as a model. It is important to keep in mind that all types of RNA are synthesized by a similar process.
During transcription, an RNA molecule is synthesized using the codes on DNA as a guide or template. A large enzyme complex, RNA polymerase, is responsible for this process. This polymerase is more multipurpose than DNA polymerase, because the RNA enzyme works alone and does not require a helicase. It can bind to DNA and unwind it, as well as synthesize RNA (process figure 1).
Fig1. The major events in transcription.
Transcription proceeds in three stages: initiation, elongation, and termination. Initiation requires the RNA polymerase to recognize a region on a gene called the promoter region (process figure 1, step 1). This region consists of two sets of DNA sequences located just before the initiation site. The first sequence is about 35 nucleotides from the initiation site, and the second is about 10 nucleotides from it. The primary function of the promoter is to provide a position for initial binding of the RNA polymerase. A special protein molecule, the sigma factor, guides the RNA polymerase to the correct position on the promotor. These promoter sequences are not highly varied, and they tend to be rich in adenine and thymine base pairs, which have one less hydrogen bond than guanine-cytosine pairs, which facilitates the separation of the DNA strands.
Prior to the first synthesis step of transcription, the RNA polymerase begins to separate the two strands of the DNA helix and forms an open “bubble” for transcription (process figure 1, steps 2 and 3). This bubble serves as the space where the nucleotides of mRNA will actually be assembled. Only one strand of DNA—the template strand—is transcribed. This strand will have a reading frame oriented in the 3′ to 5′ direction recognized by the RNA polymerase, similar to replication. The mRNA made by transcription of the template strand is a message for the correct sequence of amino acids that will be linked together during translation (protein synthesis). The other strand—the nontemplate strand—is sometimes called the coding strand because its sequence is the same order as the mRNA (although it will have thymine instead of uracil). In some cases, this strand may serve a genetic function, but it is not transcribed. Which strand acts as the template varies from one gene to another. One important triplet appearing early in a DNA template is TAC. This will be transcribed into AUG on the mRNA, which is the start codon, as it signals the location on the mRNA where translation starts. Note that the promotor sequences are not transcribed as part of the final mRNA molecule.
As elongation proceeds, the polymerase moves the transcription bubble forward, exposing subsequent sections of DNA. It simultaneously brings in nucleotides that are complementary to the DNA template and continues to assemble the mRNA strand. As with replication, this growing mRNA strand runs in the 5′ to 3′ direction. DNA that has already been transcribed rewinds back into its double helix structure (process figure 2, step 4). The part of the mRNA strand that is already assembled remains attached to the enzyme complex but is kept out of the way of the processing machinery.
Fig2. The events in protein synthesis.
At termination, the polymerase recognizes a site on DNA near the end of the gene that signals the separation and release of the completed mRNA, which will next enter the ribosome for translation. this mRNA delivers its message in co dons that are the master code for protein synthesis. The length of the final mRNA molecule depends upon the polypeptide or protein that it encodes. In bacteria, the shortest known gene is just 21 nucleotides in length, while the longest is over 110,000 nucleotides. These of course represent the extremes; most genes are a few thousand nucleotides in length.
