Capter 8: Transcription Flashcards
What is the purpose of transcription?
- DNA to mRNA
- Copy in the same language (nucleotides)
How is mRNA synthesized?
using a DNA template
Which enzyme is responsible for Transcription?
RNA polymerase
How does RNA polymerase read and synthesize?
reads 3’ to 5’
synthesizes 5’ to 3’
Does transcription require helicase to separate the strands of DNA?
No
Is a free 3’ end needed?
No
What is the complementarity of mRNA?
A-U and G-C
What is the template strand?
Template strand is the DNA strand complementary to the newly made RNA (RNA polymerase
reads this strand and adds nucleotides through complementary base pairing)
Will the newly made RNA have the
same sequence as the template
strand or the coding strand?
The coding strand (replaces T with U)
What does RNA polymerase do?
- Enzyme that synthesizes RNA from DNA
- Catalyzes phosphodiester bond formation between ribonucleotides
- Synthesizes RNA from 5’ to 3’ (reads DNA template 3’ to 5’)
- Has helicase activity that unwinds DNA at the site of transcription (does not need helicase)
What are the 3 steps of Transcription?
- Initiation
- Elongation
- Termination
How does initiation work?
- RNApol and transcription factors(proteins) bind to the promoter
What is the promoter?
Promoter: region of DNA that includes
start point and TATA box. Found on the coding strand.
What are transcription factors?
Transcription factors: proteins that help recruit RNA pol to gene
How many base pairs does RNA polymerase unwind?
10-20
Can initiation happen on either strand?
yes
What is Elongation?
- RNA pol synthesizes mRNA from 5’ to 3’
- Reads template strand from 3’ to 5’
- Ribonucleotides are added to the 3’ end
5’-phosphate group of incoming nucleotide forms
phosphodiester bond with free 3’-OH end of growing
strand
Is a primer needed for Elongation?
No
How does termination work in prokaryotes?
- RNA pol transcribes a terminator sequence, causing it to detach from the mRNA
- mRNA is released and is immediately ready for translation!
How does termination work in Eukaryotes?
- RNA pol reaches termination sequence on template strand of DNA
- mRNA is released as pre-mRNA
- Pre-mRNA must be further processed before becoming mRNA by:
- the addition of a 5’-cap
- the addition of a poly A tail
- Splicing
- Processing is important for transport, stability and recognition
What is the purpose of adding a 5’ cap to pre-mRNA?
- Protects against degradation
- Attachment of ribosomes
- Facilitates transport out of nucleus
- Stops RNAase from breaking it down
What is a 5’ cap?
Modified guanine nucleotide is added to the 5’ end of the mRNA
What is a poly-A tail?
50-200 adenine nucleotides are added to the 3’ end of the mRNA
Why is a poly-A tail added to pre-mRNA?
- Protects against degradation
- Attachment of ribosomes
- Facilitates transport out of the nucleus
