What are promoters?
The region of DNA that contains sequences that are signals for transcribing a gene.
The promoter functions as a recognition site for transcription factors. The transcription factor (s) enable RNA polymerase to bind to the promoter. Following binding, the DNA is denatured into a bubble known as the open complex.
What stage of transcription is this?
Initiation
RNA polymerase slides along the DNA in an open complex to synthesize RNA.
What stage of transcription is this?
Elongation/Synthesis of the RNA transcript
A terminator is reached that causes RNA polymerase and the RNA transcript to dissociate from the DNA.
What stage of transcription is this?
Termination
Name the 4 factors required for transcription in prokaryotes
- RNA polymerase
- Accessory transcription activator proteins
- Template DNA containing gene or genes to be transcribed
- Promoter
What constitutes a core enzyme?
What are their functions?
3 different type of subunits:
☆B - binds incoming nucleotides
☆B’ - binds DNA
☆alpha- helps with enzyme assembly; interacts with other activator proteins; also interacts with some DNA sequences.
What is the structure of a holoenzyme?
Core enzyme (2alpha units; B and B’) + sigma factor
What is the function of a holoenzyme?
To recognize the promoter.
What are the functions of accessory transcription activator proteins?
☆They bind to specific DNA sequences and help RNA polymerase initiate transcription via protein- protein interactions or by altering the structure of the DNA.
☆Some just increase the rate of transcription.
What is the function of a promoter & where is it located?
☆It is the regulatory element that determines when a gene is “turned on” (transcribed) or “turned off”.
☆It is located upstream (towards the 5’ end) of the gene & contains a sequence which sigma factor of RNAP and other transcription factors can bind.
What is a consensus sequence?
It is the sequence of nucleotides in DNA or amino acids in proteins most often present in a particular gene.
What are weak promoters?
Promoters that have poor sigma recognition sequences & have additional sequences to which transcriptional activators can bind.
How does transcription in eukaryotes differ from that of prokaryotes in terms of:
- RNA polymerase
- Arrangement of genes
- mRNA being cistronic
Prokaryotes
- RNA polymerase
- RNA is synthesized by a single RNA polymerase & mRNA is translated during regulation. - Arrangement of genes
- genes are contiguous (adjacent) segments of DNA, that are colinear with the mRNA that is translated into a protein. - mRNA being cistronic
- is ployacistronic
Eukaryotes
- RNA polymerase
- 3 different polymerase are responsible for the different classes of RNA molecule. - Arrangement of genes
- genes are often split, they are not contiguous segments of coding sequences, rather the coding sequences are interrupted by intervening sequences. Introns & Exons. - mRNA being cistronic
- is monocistronic
Discuss the functions of the three eukaryotic RNA polymerases.
RNA polymerase I :
- synthesizes rRna
RNA polymerase II :
- plays a major role in cellular transcription, because it transcribes all structural genes.
- synthesizes all mRNAs
- transcribes certain snRNA genes which are needed for pre-mRNA splicing.
RNA polymerase III
- trabscribes all tRNA genes and the 5S rRNA
Where does transcription & translation in eukaryotes occur?
Transcription occurs in the nucleus.
Translation of the mRNA occurs in the cytoplasm.
The initiation of RNA synthesis by RNA polymerase is directed by ….
The promoter site on the 5’side of the transcriptional start site.
What is the name of the strand that is transcribed by RNA polymerase?
Antisense strand.
Does RNA synthesis require a primer?
No.
Where is the promoter site for RNA polymerase II located & what is the consensus sequence?
- It is located approximately 25-35 bp upstream of the start site.
- The start site is denoted as position +1, the TATA box is located at approximately position -25.
- TATA (A/T) A (A/T) , it is known as the TATA box.
The TATA box sequence resembles the …
-10 sequence in prokaryotes TATAAT, except that it’s located further upstream.
How is transcription regulated?
By upstream control elements that lie 5’ to the TATA box .
What does RNA polymerase II require, in order to initiate transcription?
The help of several other proteins/protein complexes, known as general (basal) transcription factors.
TRANSCRIPTION FACTOR for RNA POLYMERASE II (TF II)
What is the first event in initiation?
The binding of the transcription factor IID (TFIID) protein complex to the TATA box via one of its subunits called TBP (TATA box binding protein).
What is the transcription initiation complex?
It is the final protein complex, that contains at least 40 polypeptides.
It is composed of:
TFIID bound to TBP + TFIIA which stabilizes the TFIID complex
TFIIB binds to TFIID, but TFIIB can also bind to RNA polymerase II
RNA polymerase II, is already complexed with TFIIF, also binds
It is followed by the binding of TFIIE & H.
