Genetics transcription Flashcards
DNA polymerase vs RNA polymerase
DNA Polymerase: Synthesize DNA using a DNA template. Uses A, T, G and C. Polymerizes using 5’ to 3’ direction.
Requires a primer to start polymerization
RNA Polymerase: Synthesize RNA using a DNA template. Uses A, U, G and C. Polymerizes using 5’ to 3’ direction. Does not requires a primer to start polymerization.
Sigma (σ) acts as a factor, and there are many types
create variations of the holoenzyme.
Holoenzyme
is made up to 4 polypeptide chains: a, b, b’ and σ.
b, b’
are the catalytic subunits, the ones adding nucleotides and creating a RNA using DNA template.
RNA polymerase: Prokaryotes
RNA pol II is the RNA plymerase involved in gene transcription.
promoter
serves as a ‘switch’ for the gene. RNA polII can recognize the promoter at start transcription
How does RNA pol II starts transcription?
- The (s) sigma factor recognizes the Pribnow box and brings the RNA pol II complex to the promoter.
- DNA is unwound locally and RNA pol II starts to create a mRNA transcript.
- The first nucleotide transcribed is considered the position +1
- Once transcription starts, sigma factor is released from the holoenzyme and RNA pol II continues chain elongation.
TRANSCRIPTION IN EUKARYOTES
Eukaryotes display three distinct forms of RNA polymerase (I, II, III)
I: produces rRNA
II: transcribes all protein-coding genes (makes mRNA)
III: transcribes all tRNA genes and one component of rRNA.
TRANSCRIPTION IN EUKARYOTES: Differences with prokaryotes
- THE PRESENCE OF NUCLEOSOMES
- THE PRESENCE OF MULTIPLE TRANSCRIPTION FACTORS
- THE STRUCTURE OF THE PROMOTER
- THE PRESENCE OF OTHER REGULATORY SEQUENCES BEYOND PROMOTER REGIONS
Differences with prokaryotes: THE PRESENCE OF NUCLEOSOMES
- In eukaryotes, DNA is highly packed as nucleosomes into chromatin
- Therefore, a mechanism must exist to allow the transcription machinery to gain access to the promoter and the gene.
- This is achieved through the presence of ‘Nucleosome Remodeling Complexes’ and ‘Histone Modification Complexes’.
- These two types of complexes that locally ‘relax’ chromatin and make it accessible for the transcription and thus, they allow transcription of genes in that region
Differences with prokaryotes: THE PRESENCE OF MULTIPLE TRANSCRIPTION FACTORS
Some of the major roles that can play are:
Recruitment of RNA pol II.
Activators of Transcription
Repressors of Transcription
Differences with prokaryotes: THE PRESENCE OF OTHER REGULATORY SEQUENCES BEYOND PROMOTER REGIONS
These are DNA regions called enhancers or silencers and they are usually upstream the promoter.
Enhancers: promote transcription initiation
silencers: function is to inhibit transcription.
The way they influence transcription from so far away is by inducing loops that place them in contact to the transcription machinery assembled at the promoter
In eukaryotes, the RNA molecule created by RNA pol II is further processed in three distinct steps:
5’ Capping
Poly A tail formation
Splicing
Processing of RNA transcript: 5’ Capping
- it not the usual 3’ to 5’ phosphodiester bond but a 5’ to 5’ tri-phosphodiester bond
- The function of this cap is to protect the nasent RNA transcript from the action of 5’exonucleases present in cells.
Processing of RNA transcript: Poly A tail formation
- The process starts when a series of cleavage factors recognize specific sequences (with the AAUAAA being the most common) around the 3’end of the pre-mRNA.
Those cleavage factors recruit PolyA polymerase that cleave the later portion of the 3’end and starts adding multiple A to create a poly A tail.
Once created, PABP (polyA binding protein) will recognize and bind the polyA tail, further stabilizing that end.
