Chapter 18 Transcription Flashcards
What is transcription?
The synthesis of RNA (mRNA) from DNA (Transcribing the DNA, protecting the DNA from mutations & for easier handle with regulation), it is a complex process involving a variety of enzymes transcribing a RNA from 5’ to 3’ via the enzyme RNA polymerase (which synthesizes all RNA class)
What are the terms that describes the DNA during the transcription process?
1) Template stand “non-coding strand” (-): the strand that is antiparallel to the RNA from which the RNA is transcribed
2) Non-template strand “coding strand” (+): the strand that is the same as the RNA sequence, except for the fact that uracil is substituted with thymine
What is a promoter and terminator?
Promoter: The site in the gene where transcription starts
Terminator: part of the gene that terminates the transcription process
What is the main enzyme that transcribes DNA in prokaryotes?
RNA polymerase (DNA DEPENDENT RNA POLYMERASE)
Composed of core enzymes (4 subunits 2-alpha, 1-beta, 1-beta prime “the core is assembled by the omega subunit”) and subunits (sigma subunit which attaches to the polymerase for the initiation of transcription), with its active site being between the beta and beta prime
What are the stages of transcription?
1) Initiation
2) Elongation
3) Termination
What is the process of the initiation stage in prokaryotes?
1) RNA Polymerase slides down the DNA until it encounters a promoter sequence
2) once it is bound to a promoter region, sigma and beta prime break the hydrogen bonds between the two strands of the DNA segments
3) Transcription can begin
4) RNA polymerase catalyzes the addition of the first nucleoside tri-phosphate, once an RNA chain of 10 nucleotides has been synthesized it has cleared the promoter region and the sigma subunit is released, decreasing the affinity of RNA polymerase to the promoter site starting the elongation process
What is elongation?
Once a chain of 10 nucleotides is transcribed elongation takes place as the enzyme keep reading the DNA template strand synthesizing more nucleotides, the DNA unwinds ahead of the transcription bubble (12-14bp), in addition that the non-template strand loops away from the active site into its own channel, forming a double helix again as both strands emerge
What is the process of the termination stage in prokaryotes?
There are two processes in bacteria:
1) Intrinsic termination: the termination due to the transcription of inverted repeating sequence forming a hairpin that causes the RNA polymerase to slow or stop, releasing the transcript due to weak base-pair interaction
2) rho-dependent termination: the termination of the RNA synthesis based with the AID of the ATP-dependent helices Rho-factor, which binds to a specific recognition sequence on the RNA chain ahead of the termination site, unwinding the RNA-DNA helix to release the transcript
The growing chain of RNA exists from which portion of the enzyme?
Through a channel formed by the beta and beta prime subunits
What is the function of topisomerase enzyme?
It relives the supercoiled created by the RNA polymerase
What are the similarities & differences between the transcription in prokaryotic and eukaryotics?
Similarities:
1) the polymerases are similar in structure an function (but they are more diverse in eukaryotic)
2) Initiation factors perform a similar function
Differences:
1) The regulatory mechanism (eukaryotic have a limited access to the DNA to start the transcription)
What are the different types of RNA polymerases in eukaryotic?
There are three RNA polymerases in eukaryotics:
1) RNA polymerase I: Transcribes large rRNA (28s, 18s, 5.8s, in the nucleosome)
2) RNA polymerase II: produces the precursor of mRNA, miRNA & most snRNA
3) RNA polymerase III: Transcribes the precursor of tRNA, 5S rRNA, U6 snRNA (small nuclear RNA) & snoRNA (small nucleolar RNA)
How is the promoter sequence in eukaryotes different than those in prokaryote?
- Larger, more complex, and more variable
- Each promoter has a core promoter that can be focused (contains the transcription start site “TSS” & the core promoter elements “CPE”) other elements may include (INR “initiator”, BRE “B recognition element”, MTE “Motif 10 element” & DPE “downstream promoter”) or dispersed contains multiple TSS distributed over a broad region of 50-100 base pairs
What is a CpG island?
Found in the promoter region, a sequence of lots of C & G (by regulating it we can regulate the whole transcription process)
How man subunits are found in the core of RNAP II?
12 Subunits:
- RBP1 is the largest forming art o the enzymes active site involved in DNA binding, contains a C-terminal domain which is important for regulation as its phosphorylated it prevents the binding of RNAPII to the promoter
- It also contains General transcription factors (GTFs, TFIIB, TFIID, TFIIE, TFIIF, TFIIH), which facilitate the recognition of the promoter sequence, pre-initiation of the complex, & ATP-Dependent DNA unwinding
