DNA transcription

Transcription is the process of RNA synthesis, controlled by the interaction of promoters and enhancers. Several different types of RNA are produced, including messenger RNA (mRNA), which specifies the sequence of amino acids in the protein product, plus transfer RNA (tRNA) and ribosomal RNA (rRNA), which play a role in the translation process. Transcription is an essential step in using the information from genes in our DNA to make proteins. Proteins are the key molecules that give cells structure and keep them running. Blocking transcription with mushroom toxin causes liver failure and death, because no new RNAs—and thus, no new proteins—can be made.

Steps of Transcription

1.    Initiation:  The DNA molecule unwinds and separates to form a small open complex. RNA polymerase binds to the promoter of the template strand.

·         Promoters in bacteria

v  To get a better sense of how a promoter works, let's look an example from bacteria. A typical bacterial promoter contains two important DNA sequences, the -$10$1010 and -$35$3535 elements.

v  RNA polymerase recognizes and binds directly to these sequences. The sequences position the polymerase in the right spot to start transcribing a target gene, and they also make sure it's pointing in the right direction. 

v  Once the RNA polymerase has bound, it can open up the DNA and get to work. DNA opening occurs at the 10 element, where the strands are easy to separate due to the many As and Ts (which bind to each other using just two hydrogen bonds, rather than the three hydrogen bonds of Gs and Cs). The -10 and the -35 elements get their names because they come 35 and 10 nucleotides before the initiation site ($+1$plus, in the DNA). The minus signs just mean that they are before, not after, the initiation site.

·         Promoters in humans

v  In eukaryotes like humans, the main RNA polymerase in your cells does not attach directly to promoters like bacterial RNA polymerase. Instead, helper proteins called basal (general) transcription factors bind to the promoter first, helping the RNA polymerase in your cells get a foothold on the DNA.

v  Many eukaryotic promoters have a sequence called a TATA box. The TATA box plays a role much like that of the -$10$1010 element in bacteria. It's recognized by one of the general transcription factors, allowing other transcription factors and eventually RNA polymerase to bind. It also contains lots of As and Ts, which make it easy to pull the strands of DNA apart.

2.    Elongation:   RNA polymerase moves along the template strand, synthesising an mRNA molecule. In prokaryotes RNA polymerase is a holoenzyme consisting of a number of subunits, including a sigma factor (transcription factor) that recognises the promoter. In eukaryotes there are three RNA polymerases: I, II and III. The process includes a proofreading mechanism.

3.    Termination:   In prokaryotes there are two ways in which transcription is terminated. In Rho-dependent termination, a protein factor called "Rho" is responsible for disrupting the complex involving the template strand, RNA polymerase and RNA molecule. In Rho-independent termination, a loop forms at the end of the RNA molecule, causing it to detach itself. Termination in eukaryotes is more complicated, involving the addition of additional adenine nucleotides at the 3' of the RNA transcript (a process referred to as polyadenylation).

4.    Processing:   After transcription the RNA molecule is processed in a number of ways: introns are removed and the exons are spliced together to form a mature mRNA molecule consisting of a single protein-coding sequence. RNA synthesis involves the normal base pairing rules, but the base thymine is replaced with the base uracil.

Overview

Key points:

·         Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule.

·         RNA polymerase is the main transcription enzyme.

·         Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins).

·         RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule.

·         Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished.