Eukaryotic Transcription

Transcription in Eukaryotes

1. Eukaryotic DNA is packaged into chromatin, which in the condensed state (default state) results in a promoter that is inaccessible to RNA polymerase. Transcription can only occur when DNA is decondensed and dissociated from the histones around which it is wound.
2. Transcription and translation cannot occur at the same time because eukaryotes have a separate compartment for the DNA (nucleus).
3. Prokaryotic cells: genes with common function are arranged linearly and are transcribed together on a single mRNA (polycistronic).
4. Eukaryotic cells: genes with common functions can be scattered throughout the genome, on different chromosomes and can therefore be transcribed and regulated separately from one another.
5. Eukaryotic mRNA is always monocistronic: one mRNA = one protein.
6. Eukaryotes have three types of RNA polymerase:

• RNAP I: transcribes rRNA
• RNAP II: transcribes mRNA
• RNAP III: transcribes tRNA

1. Messenger RNA requires extensive modification before being translated into protein (maturing of RNA).

1. Promoters are more complex but also have a consensus sequence similar to that in prokaryotes. This is called the TATA box and is located about -30 nucleotides upstream of the transcriptional start site.
2. RNA polymerase needs the help of transcription factors to bind to DNA and start transcribing. Instead of a simple sigma factor, several transcription factors (TFs) bind to the promoter to recruit RNA Polymerase II. See below for more details on this.

Transcriptional Control

• The equivalent of sigma proteins in prokaryotic transcription are basal transcription factors in eukaryotic cells.
• Basal transcription factors include a TATA-binding protein (TBP) that binds to the TATA box in the promoter of eukaryotic genes. Following TBP binding, the RNA polymerase is recruited.
• Other transcription factors (such as enhancers) bind to different areas within the gene and interact with the polymerase to initiate transcription.
• Enhancers can even be located far away from the genes they affect.
• Transcription factors help to coordinate gene expression. Genes located on different chromosomes can be controlled simultaneously through the use of the same transcription factor.