Transcription Flashcards
Where does transcription occur?
In the Nucleus of the cell
How many RNA polymerases do Eukaryotes and Prokaryotes have
Eukaryotes have 3 RNA polymerases, while prokaryotes have one.
What are the three stages of transcription?
- Initiation: Promoter binding occurs, RNA polymerase lands. The DNA helix unwinds and the enzyme initiates transcription at the start point of the template strand.
- Elongation:
- Termination:
What is a promoter
The region of DNA where the RNA polymerase binds and initiates transcription
In Eukaryotes, the promoter contains a region that is rich in Thymine and Adenine about 25 nucleotides upstream from the transcriptional start point called a TATA box.
I prokaryotes they have a pribnow box. This is typically smaller but is also rich in T’s and A’s
The richness in Thymine and Adenine is important because weaker T-A hydrogen bonds and weaker base stacking effects (π − π stacking) with T-A allow for easier separation of the DNA strand to start transcription.
Initiation
Proteins called transcription factors recognize the promoter and bind to it and RNA polymerase. The bound proteins make up the Transcription Initiation Complex. (TIC)
The DNA helix unwinds and the enzyme initiates transcription at the start point of the template strand.
What is the TIC
It is the transcription initiation complex. It’s made up of proteins called transcription factors and RNA polymerase. These bind to the DNA and initiate transcription
Elongation
RNA polymerase moves down the DNA adding RNA subunits on the 3’ growing chain. A-U and G-C.
As the RNA is formed, it moves away from the DNA allowing the DNA to reform.
Only one strand of DNA is used for transcription. This strand is called the template strand or the anti-sense strand. The strand that is not coded is called the sense strand.
RNA polymerase doesn’t have proofreading functionality like DNA polymerase does. This means that there are more errors, but the errors only result in bad proteins and not issues passed on to progeny.
Codon
Three consecutive bases of RNA is called a codon
Template Strand
The DNA strand used for transcription
Anti-Sense Strand
The DNA strand used for transcription
Sense Strand
The DNA strand that is not used for transcription
Why is it important for DNA polymerase to have extremely good proofreading and why is it not nearly as important for RNA polymerase to have good proofreading?
DNA polymerase proofreads during the replication of DNA. That DNA is passed on to progeny and if errors occur, the progeny could not survive.
RNA polymerase does not proofread, but if errors occur, it will only result in a poorly formed protein. The results are not passed on to progeny.
Termination
RNA polymerase transcribes a terminator sequence in the DNA.
The sequence and process differs in Eukaryotes and Prokaryotes. The bottom line is that when it reaches the termination sequence, the mRNA is released from the RNA polymerase.
Post-transcriptional modification
- 5’ end: The 5’ end of the RNA sequence has three phosphate groups on it that formed when the first nucleotide of RNA was added by RNA polymerase. The phosphates are surrounded by a cap of molecules that protect the end from degradation. This end helps start translation at the ribosome through binding to the ribosome and it helps aid the mRNA out of the nucleus.
- 3’ end: A poly A tail is added at the 3’ end of the RNA. This is a long repeating sequence of Adenine that has proteins bound to it. This tail protects the 3’ end from nucleases and phosphatases
Spliceosome
A complex of snRNP’s and other proteins. Cutting occurs that removes the introns and leave only the exons of mRNA before it finally leaves the nucleus.
