DNA or deoxyribonucleic acid is the molecule that codes genetic information. However, DNA can't directly order a cell to make proteins. It has to be transcribed into RNA or ribonucleic acid. RNA, in turn, is translated by cellular machinery to make amino acids, which it joins together to form polypeptides and proteins
Overview of Transcription
Transcription is the first stage of the expression of genes into proteins. In transcription, an mRNA (messenger RNA) intermediate is transcribed from one of the strands of the DNA molecule. The RNA is called messenger RNA because it carries the "message," or genetic information, from the DNA to the ribosomes, where the information is used to make proteins. RNA and DNA use complementary coding where base pairs match up, similar to how the strands of DNA bind to form a double helix.
One difference between DNA and RNA is that RNA uses uracil in place of the thymine used in DNA. RNA polymerase mediates the manufacture of an RNA strand that complements the DNA strand. RNA is synthesized in the 5' -> 3' direction (as seen from the growing RNA transcript). There are some proofreading mechanisms for transcription, but not as many as for DNA replication. Sometimes coding errors occur.
Differences in Transcription
There are significant differences in the process of transcription in prokaryotes versus eukaryotes.
- In prokaryotes (bacteria), transcription occurs in the cytoplasm. Translation of the mRNA into proteins also occurs in the cytoplasm. In eukaryotes, transcription occurs in the cell's nucleus. mRNA then moves to the cytoplasm for translation.
- DNA in prokaryotes is much more accessible to RNA polymerase than DNA in eukaryotes. Eukaryotic DNA is wrapped around proteins called histones to form structures called nucleosomes. Eukaryotic DNA is packed to form chromatin. While RNA polymerase interacts directly with prokaryotic DNA, other proteins mediate the interaction between RNA polymerase and DNA in eukaryotes.
- mRNA produced as a result of transcription is not modified in prokaryotic cells. Eukaryotic cells modify mRNA by RNA splicing, 5' end capping, and addition of a polyA tail.
Key Takeaways: Steps of Transcription
- The two main steps in gene expression are transcription and translation.
- Transcription is the name given to the process in which DNA is copied to make a complementary strand of RNA. RNA then undergoes translation to make proteins.
- The major steps of transcription are initiation, promoter clearance, elongation, and termination.
Steps of Transcription
Transcription can be broken into five stages: pre-initiation, initiation, promoter clearance, elongation, and termination:01of 05
Pre-InitiationAtomic Imagery / Getty Images
The first step of transcription is called pre-initiation. RNA polymerase and cofactors (general transcription factors) bind to DNA and unwind it, creating an initiation bubble. This space grants RNA polymerase access to a single strand of the DNA molecule. Approximately 14 base pairs are exposed at a time.02of 05
Forluvoft / Wikimedia Commons / Public Domain
The initiation of transcription in bacteria begins with the binding of RNA polymerase to the promoter in DNA. Transcription initiation is more complex in eukaryotes, where a group of proteins called transcription factors mediates the binding of RNA polymerase and the initiation of transcription.03of 05
Ben Mills / Wikimedia Commons / Public Domain
The next step of transcription is called promoter clearance or promoter escape. RNA polymerase must clear the promoter once the first bond has been synthesized. Approximately 23 nucleotides must be synthesized before RNA polymerase loses its tendency to slip away and prematurely release the RNA transcript.04of 05
Forluvoft / Wikimedia Commons / Public Domain
One strand of DNA serves as the template for RNA synthesis, but multiple rounds of transcription may occur so that many copies of a gene can be produced.05of 05
Forluvoft / Wikipedia Commons / Public Domain
Termination is the final step of transcription. Termination results in the release of the newly synthesized mRNA from the elongation complex. In eukaryotes, the termination of transcription involves cleavage of the transcript, followed by a process called polyadenylation. In polyadenylation, a series of adenine residues or poly(A) tail is added to the new 3' end of the messenger RNA strand.
- Watson JD, Baker TA, Bell SP, Gann AA, Levine M, Losick RM (2013). Molecular Biology of the Gene (7th ed.). Pearson.
- Roeder, Robert G. (1991). "The complexities of eukaryotic transcription initiation: regulation of preinitiation complex assembly". Trends in Biochemical Sciences. 16: 402-408. doi:10.1016/0968-0004(91)90164-Q
- Yukihara; et al. (1985). "Eukaryotic transcription: a summary of research and experimental techniques". Journal of Molecular Biology. 14 (21): 56-79.