Transcription Flashcards
General definition of transcription
DNA –> RNA
Similarities between DNA and RNA
-Linear polymer of 4 nucleotides
-Subunits linked by phosphodiester bonds
Differences between RNA and DNA
-RNA nucleotides have ribose sugar
-RNA has uracil instead of thymine
-Usually single-stranded
What are the three stages of transcription
Initiation
Elongation
Termination
What happends during initiation
RNA polymerase binds to promoter region
DNA unwinds (template now available)
Promotor sequence
Contains TATA box
Recognized by general transcription factors that recruit RNA polymerase
What happens during elongation
Nucleotides added to mRNA strand by RNA polymerase
What triggers elongation
RNA polymerase is phosphorylated, disengages from promotor sequence and travels along DNA strand
What direction is elongation
5’-3’
What happens during termination
Polyadenylation signal appears in RNA transcript
polyA tail added
What are the key RNA processing steps
5’ cap
Removal of introns (splicing)
polyA tail
5’ cap
first modification
Added as the RNA emerges from polymerase
Tells this cells it’s mRNA
Splicing
Removal of introns
How does the cell know where the introns are
Specific nucleotide sequences at the beginning and end
5’ site - donor
Middle - branch
3’ site - acceptor
Steps of splicing
- Adenine in the branch site attacks 5’ splice site
- Sugar backbone at 5’ exon site is cut
- 5’ end of intron covalently links to A in intron (forming a loop)
- Free 5’ and 3’ ends of exons are joined
- Loop od intron released
Alternative splicing:
different mRNAs generated from same primary transcript
How is splicing controlled (general)
Protein factors bind to intronic and extronic sequences
Splicing activator (SR) proteins
Splicing repressor (hnRNP) proteins
RNA sequences that bind with splicing activator proteins are called…
Exonic splicing enhancer (ESE) or intronic splicing enhancer (ISE) sequences
What do ESE and ISE do
Bind with splicing activator proteins and promote use of splicing sites
RNA sequences that bind with splicing repressor proteins are called…
Exonic splicing silencer (ESS) or intronic splicing silencer (ISS) sequences
What do ESS and ISS do?
Repressor proteins bind to the sequences and prevent use of the splicing sites
polyA tail
Added to the end of the RNA after consensus RNA sequence facilitates termination
Why are mutations a problem
Base pair substitutions can cause alternate donor or acceptor sites or the wrong use (repressor v activator)
Premature stop codons
What is the signal for termination
polyadenylation signal
