Transcription in Eukaryotes Flashcards
What are the differences between Prokaryotic and Eukaryotic transcription?
Chromatin Structure
Three distinct RNA Polymerases
Transcription Factors
Eukaryotic Promotors
Regulatory Elements and Activators
Enhancers
Post-Transcriptional Modifications of RNA Molecules
mRNA Splicing
mRNA Editing
Actively transcribed areas of DNA are called …
Euchromatin
Inactive Segments of DNA are called …
Heterochromatin
Conversion between forms of chromatin is called …
Forms of chromatin refers to euchromatin and heterochromatin
Chromatin Remodeling
Chromatin Remodeling
Mechanism of Chromatin Remodeling
Covalent modification of histone
Chromatin Remodeling
Acetylation is catalysed by …
Histone Acetyl Transferases (HATs)
Chromatin Remodeling
Acetylation causes …
elimination of positive charge of lysine which decreases interactions of chromatin
Chromatin Remodeling
Deacetylation is catalysed by …
Histone Deactylases (HDACs)
What are the three types of RNA Polymerases?
RNA Polymerase I
RNA Polymerase II
RNA Polymerase III
Polymerase Types
RNA Polymerase I Function
Synthesis of rRNA subunits (28S, 18S and 5.8S)
Polymerase Types
RNA Polymerase II Function
Synthesizes mRNA
Synthesizes snRNA
Polymerase Types
RNA Polymerase III Function
Synthesizes tRNA
Synthesizes 5S rRNA
Syntheizes snRNAs
How many types of Polymerases does Mitochondria contain?
1
Promoters of RNA Polymerase II
Names
TATA Box (-25bp from transcription start)
Initiator (Inr)
Downstream Promoter Elements (DPE)
TATA Boxes are less common than the other two
Promoters of RNA Polymerase II
Others names for promoters
Cis-acting elements
Because they are on the same molecule of DNA as the gene being transcribed
Promoters of RNA Polymerase II
What binds to cis-acting elements?
Proteins called General Transcription Factors (GTFs)
Interact with each other and RNA Polymerase II
GTFs
Function of GTFs
Assembly of Transcription Initiation Complex at Promoter
Determination of which gene is transcribed
Each RNA Polymerase has its own transcription factors
GTFs
When can GTFs bind to the DNA Sequence?
When it is decondensed
GTFs
What GTFs are required for initiation?
TFIID
TFIIF
TFIIH
GTFs
TFIID Function
Contains TATA-binding protein and TATA-associated factors
Recognizes and binds TATA boxes
GTFs
TFIIF Function
Bring Polymerase to Promoter
GTFs
TFIIH Function
Melts DNA
Regulatory Elements
What are Regulatory Elements?
Regulatory Sequences upstream of promoter
Regulatory Elements
What are the types of Regulatory Elements?
Proximal Regulatory Elements
Distal Regulatory Elements/Enhancers
Regulatory Elements
What proteins can bind to Regulatory Elements?
Specific Transcription Factors (STFs)/Transcriptional Activators
Regulatory Elements
When STFs bind to proximal promoter elements …
regulates frequency of transcription initiation
Regulatory Elements
When STFs bind to distal promoter elements …
mediates response to hormones
Regulatory Elements
What are the two binding domains of STFs?
DNA-Binding Domain
Transcription Activation Domain (recruits GTF and HAT)
Enhancers
Function
DNA sequences increasing rate of initiation of transcription
Enhancers
Location
Upstream or downstream of transcription start site
Can occur near or far from promoter
Can occur on either DNA strand
Enhancers
Which DNA sequences of Enhancers can bind STF?
Response Elements
RNA Polymerase II is inhibited by …
α-Amanitin
Toxin produced by mushrooms
What is the Transcription Unit?
DNA Segment between Initiation and Termination Sequences
rRNA synthesis and modification
Generated from long precursor pre-rRNA
28S, 18S and 5.8S synthesized from single pre-rRNA
5S synthesized by RNA Polymerase III
Cleaved by Ribonucleases
tRNA modification
-CCA sequences addition to 3’-OH end by nucleotidyl-transferase
mRNA modification
Addition of 5’ cap
Addition of 3’-poly-A tail
Splicing
mRNA modification: 5’-cap
Process
Terminal Guanine methylated in Cytosol by Guanine-7-Methyltransferase
mRNA modification: 5’-cap
Source of methyl-group
S-Adenosylmethionine
mRNA modification: 5’-cap
Function of 5’-cap
Stabilizes mRNA
Permits Efficient Initiation
Protects 5’-end
mRNA modification: Poly-A-tail
Which enzyme catalyses addition of Poly-A-tail?
Polyadenylate Polymerase
mRNA modification: Poly-A-tail
Function
Stabilize mRNA
Facilitate Exit from Nucleus
Aids in Translation
mRNA modification: Splicing
Complex responsible for splicing
Spliceosome
mRNA modification: Splicing
How is Splicing mediated?
Mediated by 5 snurps: U1, U2, U4, U5 and US
snurps = Small Nuclear Ribonucleoprotein Particles
mRNA modification: Splicing
What are U1, U2, U4, U5 and US?
Small Nuclear RNA associated with Proteins
mRNA modification: Splicing
Function of snurps
Removal of introns by base pair formation with consensus sequences at intron ends
mRNA modification: Splicing
Procedure
2’-OH group of adenine (branch site A) of intron attacks phosphate at 5’-end of intron (splice donor site)
Forms 2’-5’-phosphodiester bond called Lariat Structure
3’-OH of exon 1 attack 5’-phosphate at splice acceptor site forming phosphodiester bond between exon 1 and 2
mRNA modification: Splicing
Fate of the Lariat Structure
Degraded
Used as precursor for ncRNA such as snoRNA
mRNA modification: Splicing
What may splice site mutation cause?
Improper Splicing
Synthesis of Abnormal Proteins
mRNA modification: Splicing
Example of splice site mutation
β-thalassemia
mRNA modification: Splicing
Alternative Splicing produces …
protein isoforms
