Unit I2 Flashcards
What are two pieces of evidence for DNA being the template for transcription?
- DNase removes the central branches
- RNase removes the smaller branches
What are the three steps to transcription?
- Initiation
- Elongation
- Termination
What is the central dogma of genetics?
- DNA (gene/template strand)
Transcription - mRNA (codons)
Translation - Protein (Polymer of AA)
What is transcription?
The synthesis of an mRNA molecule from a DNA template strand
What is a consensus sequence?
A consensus sequence is the set of the most encountered nucleotides among sequences that possess considerable similarity.
How do conserved sequences relate to initiation of transcription?
Consensus sequences are found upstream of the start site
More specifically there are two consensus sequences, one found -10 bp upstream and another -35 bp a little further upstream
What happens during Step 1: Initiation (in bacteria) of transcription?
First, RNA holoenzyme recognizes promoter via sigma factor
Next, holoenzyme unwinds DNA, RNA synthesis begins from template (no primer needed) and sigma releases
What forms the holoenzyme?
The sigma factor and core RNA polymerase enzyme
- What is the difference between the template and non-template strand in a gene?
The template strand is used for transcription while the non-template strand is not
What is the base sequence of the template strand at the -35 consensus sequence?
TTGACA
What is the base sequence of the template strand at the -10 consensus sequence?
TATAAT
If pyrimidines together are the consensus, what is written?
Y
If purines are the the consensus, what is written?
R
What is written if cytosine and guanine are equally common?
C/G
What happens during Step 2: Elongation in bacteria?
After sigma is released, RNA polymerase core enzyme builds a complementary RNA copy of the DNA template
What direction is new RNA built?
5’ to 3’ direction
What happens during Step 3: Termination in RHO-dependent gene sequences in bacteria?
First, RHO binds to the rut site and moves toward the 3’ end
Then, When RNA polymerase encounters a terminator sequence , it pauses and RHO catches up
Finally, using helicase activity, RHO unwinds the DNA-RNA hybrid and brings and end to transcription
What happens during Step 3: Termination in RHO-independent gene sequences in bacteria?
Rho-independent terminators contain an inverted repeat followed by approximately six adenine nucleotides
After the inverted repeats are transcribed into RNA, the U’s are transcribed and cause RNA polymerase to pause
Then the inverted repeats in RNA fold into a hairpin which destabilizes the DNA-RNA paring
Lastly, the RNA transcript separates from the template, terminating transcription.
When does transcription terminate for RHO-independent gene sequences?
Transcription terminates when inverted repeats form a hairpin followed by a string of uracils.
What is polycistronic RNA?
Greater than one gene on a transcript
So one promoter caused the transcribing of multiple genes onto a single mRNA molecule.
What is the difference between the RNA polymerases in eukaryotes and prokaryotes?
In eukaryotes, there are different RNA polymerases to transcribe different types of RNA
What is different about initiation between eukaryotes and prokaryotes?
The promoter contains different consensus sequences and in eukaryotes it is recognized by a transcription factor
What are promoter proximal elements and where are they found?
They contain regulatory elements such as GG and CAAT boxes
They are in a fixed position upstream of core-promoter and are necessary for basal levels
What is the TATA box and what is its role in initiation of transcription?
It is the core promoter element at a fixed position up stream
TFIID binds to the TATA box via the TATA-binding protein (TBP) and several TBP associate factors (TAF)s which RNA pol II binds to
What is different about elongation in eukaryotes?
The RNA polymerase continues after phosphorylation of the carboxyl terminal domain (CTD)
What is different about termination in eukaryotes?
In eukaryotes, termination is triggered by the PolyA signal consensus sequence (AAUAAA) endonuclease cuts downstream
What does Poly-A-Polymerase do?
It adds 150-200 Adenines post termination
What is different about RNA post transcription in eukaryotes?
It undergoes three processing steps
What are the three processing steps eukaryotic DNA undergoes?
- 5’ end is “capped” with 7-methylguanosine
- Introns are removed (splicing)
- Poly-A-tail added to 3’ end (polyadenylation)
What is a technical term for pre-mRNA (mRNA that hasn’t been processed?
hnRNA - heterogenous Nuclear RNA
What are noncoding regions of DNA seen as?
Loops = intervening sequences
What is alternative splicing?
Eukaryotic Mechanism of getting different proteins from the same RNA molecule by producing different mRNAs from a single pre-mRNA (not at the same time)
What unique bond occurs to form the lariat during splicing?
2’-5’ phosphodiester bond
Explain how spliceosomes splice introns out of pre-mRNA?
- The mRNA is cut at the 5’ splice site and then the 5’end of the intron attaches to the branch point forming a 2’-5’ phosphodiester bond.
- A cut is made at the 3’ splice site, the intron is released as a lariat, the bond holding the lariat together breaks and the linear intron is degraded.
- The two exons are spliced together and the spliced mRNA is exported to the cytoplasm and translated
What is ribosomal RNA (rRNA)
along with ribosomal protein subunits it comprises the ribosome where protein assembly takes place
What are transcription factors?
Protein that binds to a eukaryotic promoter near the transcription start site and are required for initiation of transcription
