Eukaryotic DNA Transcription
Eukaryotic RNA Polymerases
Unlike prokaryotic RNA polymerase that initiates the transcription of all different types of RNA, RNA polymerase in eukaryotes (including humans) comes in three variations, each encoding a different type of gene. RNA polymerase I is responsible for transcribing RNA that codes for genes that become structural components of the ribosome, a protein responsible for the translation of RNA into proteins. RNA polymerase II transcribes protein-encoding genes, or messenger RNAs, which are the RNAs that get translated into proteins. RNA polymerase III transcribes a different structural region of the ribosome, transfer RNAs, which are also involved the translation process, as well as non-protein encoding RNAs.
The eukaryotic transcription machinery is complex. It is a compact structure of different proteins that are bound at different positions on a DNA strand. Because the eukaryotic DNA template is bound in nucleosomes, it is not completely exposed, as is prokaryotic DNA. As a result, eukaryotic transcription machinery must be able to penetrate chromatin layers in order to contact the DNA. The activity of the transcription machinery can be influenced by proteins that are bound to specific promoters.