Why transcription occurs

It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.

Elongation is the addition of nucleotides to the mRNA strand. Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop termination sequence in the gene.

This video provides a review of these steps. You can stop watching the video at This page has been archived and is no longer updated. DNA safely and stably stores genetic material in the nuclei of cells as a reference, or template. Meanwhile, mRNA is comparable to a copy from a reference book because it carries the same information as DNA but is not used for long-term storage and can freely exit the nucleus. Transcription is carried out by an enzyme called RNA polymerase and a number of accessory proteins called transcription factors.

Eukaryotic Genome Complexity. RNA Functions. Citation: Clancy, S. Nature Education 1 1 If DNA is a book, then how is it read? Aa Aa Aa. The genetic code is frequently referred to as a "blueprint" because it contains the instructions a cell requires in order to sustain itself.

We now know that there is more to these instructions than simply the sequence of letters in the nucleotide code, however. For example, vast amounts of evidence demonstrate that this code is the basis for the production of various molecules, including RNA and protein. Research has also shown that the instructions stored within DNA are "read" in two steps: transcription and translation.

In some cases, the RNA molecule itself is a "finished product" that serves some important function within the cell. Often, however, transcription of an RNA molecule is followed by a translation step, which ultimately results in the production of a protein molecule. Visualizing Transcription.

Figure 1. The Transcription Process. Transcription Initiation. Figure 3. Figure Detail. Figure 2. Figure 4: Eukaryotic core promoter region. Genetics: A Conceptual Approach , 2nd ed. All rights reserved. Strand Elongation. Transcription Termination. Figure 5: Rho-independent termination in bacteria. Inverted repeat sequences at the end of a gene allow folding of the newly transcribed RNA sequence into a hairpin loop.

This terminates transcription and stimulates release of the mRNA strand from the transcription machinery. References and Recommended Reading Connelly, S. Genes and Development 4 , — Dennis, P. Journal of Molecular Biology 84 , — Dragon. Journal of Biological Chemistry , — Kritikou, E. Methods in Molecular Biology , 23—37 Logan, J.

Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. The prokaryotes, which include bacteria and archaea, lack membrane-bound nuclei and other organelles, and transcription occurs in the cytoplasm of the cell. In both prokaryotes and eukaryotes, transcription occurs in three main stages: initiation, elongation, and termination.

The region of unwinding is called a transcription bubble. The DNA sequence onto which the proteins and enzymes involved in transcription bind to initiate the process is called a promoter. In most cases, promoters exist upstream of the genes they regulate.

The specific sequence of a promoter is very important because it determines whether the corresponding gene is transcribed all of the time, some of the time, or hardly at all Figure 9. Transcription always proceeds from one of the two DNA strands, which is called the template strand. As elongation proceeds, the DNA is continuously unwound ahead of the core enzyme and rewound behind it Figure 9. Termination Once a gene is transcribed, the prokaryotic polymerase needs to be instructed to dissociate from the DNA template and liberate the newly made mRNA.

Depending on the gene being transcribed, there are two kinds of termination signals, but both involve repeated nucleotide sequences in the DNA template that result in RNA polymerase stalling, leaving the DNA template, and freeing the mRNA transcript.