Mechanism of Transcription

This lecture doesn't have a summary or an outcomes slide, so I'm just going to go through and talk about the stuff that looks important. (Given that it's me, though, everything looks important. I simply like those HDs far too much.)

The Central Dogma

I'm sure you probably know this by now- DNA gets transcribed to mRNA, mRNA gets translated into protein, yada yada yada.

Structure of RNA

See my earlier post on the properties of RNA.

Transcription

Transcription always takes place from 5' to 3', as with DNA replication. In fact you're probably going to be hearing "5' to 3'" so much that you'll be practically saying it in your sleep. Transcription is carried out by RNA polymerase, which does not require a primer (unlike DNA polymerase). It creates a strand of RNA that is complementary to the template strand of the DNA, and similar to the coding strand. In fact, the only difference between the coding strand and the initial transcript is that the coding strand has thymine while the initial RNA transcript has uracil.

RNA Polymerase

RNA polymerase is similar to DNA polymerase, but it doesn't require a primer and has a higher error rate. It uses the energy stored in ribonucleoside triphosphates in order to carry out its job.

In E. coli, RNA polymerase has five subunits- two alpha units, a beta unit, a beta prime unit and an omega unit. These five subunits are collectively known as the core enzyme of E. coli RNA polymerase. Another special subunit, sigma, is responsible for recognition of the promoter region of the DNA. It associates with the core enzyme to form a holoenzyme. After transcription initiation has taken place, the sigma subunit dissociates, leaving the core enzyme to continue elongation of the transcript.

Initiation and Termination Signals

Of course, for RNA polymerase to do its job properly, it has to know when to start and stop. Upstream of the gene, there are promoter regions. These are located at around 35 bases before and 10 bases before the start of the gene (a.k.a. -35 and -10. +1 is the start of transcription). The region between roughly -10 and -5 is known as the Pribnow box. These areas contain consensus sequences which are similar between genes and thus serve as a good guide for the RNA polymerase. If I remember correctly, promoters that have sequences more similar to the consensus sequence are considered to be "stronger" promoters than those that have sequences that contain more deviations from the consensus sequence.

Downstream of the gene, there are transcription termination signals. There are two main types of these. The first is a special DNA sequence in which there is an inverted repeat (i.e. the sequence is repeated but backwards the second time) followed by several thymine bases in a row. These are often translated to form "hairpin" structures in the mRNA. The second type of termination signal is a protein that binds to the DNA, physically blocking the movement of RNA polymerase.

Differences between Prokaryotic and Eukaryotic mRNA

There are several differences between prokaryotic and eukaryotic mRNA. Firstly, prokaryotic mRNA is often polycistronic- that is, it codes for multiple proteins at the same time- whereas eukaryotic mRNA is often monocistronic (one protein at a time). Prokaryotic mRNA may also have intercistronic regions, or spacers, located between the coding regions. Another important difference between the two types of mRNA is that prokaryotic mRNA is not processed, whereas eukaryotic mRNA is. Eukaryotic mRNA has a 7-methylguanosine cap added on its 5' end and a poly-A tail located on its 3' end. Also, eukaryotic mRNA has non-coding sequences called introns (prokaryotic mRNA does not have these) that are spliced out during mRNA processing.