Overview of Transcription
Transcription
- The process of making a copy of the DNA in the form of RNA
- Part of the Central Dogma
- Three Major Components
- DNA Template
- Raw materials needed to build a new RNA molecule
- Transcription apparatus- proteins necessary for catalyzing synthesis of RNA
What is Transcription?
Transcription Unit
- Sequence of nucleotides in DNA that encodes a single RNA molecule and the sequences necessary for its transcription
- Contains a promoter, an RNA-coding sequence, and a terminator.
Transcription Unit
Step 1: Initiation
Initiation
- Initiation is the first step of the transcription process
- Comprises all the steps necessary for RNA synthesis...
1) Promoter recognition
2) Formation of transcription bubble
3) Creation of the first bonds between rNTPs
4) Escape of the transcription apparatus from the promoter
- Initiation is slightly different in Eukaryotic and Prokaryotic cells
Prokaryotic Initiation
1) A sigma factor binds to the core RNA polymerase to form a holoenzyme.
- This factor allows the enzyme complex to recognize consensus sequences in the promoter
- Different sigma factors target different promoters
2) The holoenzyme binds tightly to the promoter and unwinds the doubled-stranded DNA
3) The base at the start site on the DNA template strand is paired with its complementary base on an rNTP
4) The sigma factor is released as the RNA polymerase moves beyond the promoter
Eukaryotic Initiation
Eukarytoic Initiation
1) TFIID binds to the TATA box (initiation primer) in the core promoter
2) RNA polymerase binds with other transcription factors to the core promoter
3) Transcriptional activator proteins bind to sequences in enhancers
4) The template strand changes conformation and loops around to allow the enhancer to interact with the RNA polymerase complex (open complex formed)
5) RNA synthesis can begin
Elongation
- As RNA polymerase moves downstream, the (3' to 5'), nucleotides are joined to grow the growing RNA molecule
- The RNA molecule follows the sequence on the template strand
- The RNA polymerase rewinds the DNA strands at the trailing end.
Elongation
Termination
- Termination is the final step of the transcription process
- Transcription ends after the transcribing of a terminator
- Several events occur during termination...
- RNA polymerase must stop synthesizing RNA
- The newly made RNA molecule must be released from RNA polymerase
- The RNA molecule must dissociate fully from the DNA
- RNA polymerase must detach from the DNA template
Prokaryotic Termination
Prokaryotic cells have two major types of terminators...
- Rho-independent terminators:
- causes the termination of transcription only in the presence of an ancillary protein (rho factor)
- Rho-factor: a protein that binds to bacterial RNA polymerase and facilitates the termination of transcription in some genes
- Rho-Dependent terminators:
- able to cause the termination of transcription in the absence of a rho factor
- also known as intrinsic terminators
Rho-dependent termination
Termination with RNA Polymerase II
Eukaryote Termination
Eukaryotic Termination
- Termination in eukaryotic cells is dependent on the type of RNA polymerase used during transcription
- RNA Polymerase I - requires a termination factor similar to a rho factor, but it binds to a DNA sequence downstream instead of to the newly transcribed RNA molecule
- RNA Polymerase II - Rat1 endonuclease attaches to trailing end of RNA and works its way up to the RNA polymerase II in order to terminate its continuation of coding. The RNA strand is degraded by Rat1 in the process
- RNA Polymerase III - ends transcription after trasncribing a terminator sequence that produces a string of uracil nucleotides in the RNA.
A Note on Transcription in Archaea
Transcription in Archaea
- Transcription in archaea is more closely related to transcription in eukaryotes than in eubacteria
- This suggests a closer relationship between eukaryotes and archaea
- Similar RNA polymerases
- Archaea promoters similar to TATA box are found in eukaryotic promoters
- Archaea possess a TATA-binding protein
Final Product of Transcription
- In the end, the process of transcription produces a new strand of RNA
- This strand will go on to make a specific chain of amino acids known as a polypeptide chain (protein) in the next process of DNA replication- translation.