Transcription Flashcards
What is the difference between DNA and RNA?
- DNA contains deoxyribose, RNA contains ribose
- in RNA, U (intead of T in DNA) base pairs with A
- DNA double stranded, RNA single stranded (in eukaryotes and prokaryotes)
What is RNA polymerase (RNAP)
multi-subunit complex that synthesizes RNA using a DNA template => transcription
- Catalyse the formation of phosphodiester bonds between ribonucleotides
- Moves stepwise along DNA, unwinding helix
overview of how RNAP perform using DNA as template?
- RNAP unwinds DNA with its active site
- one of the two DNA strands acts as a template for complimentary base pairing of RNA monomers (ribonucleotides)
- RNAP catalyses the formation of phosphodiester bonds between ribonucleotides
- RNA then dissociates from DNA, DNA double helix reforms
- single stranded RNA released
What provides the energy for RNA polymerisation?
The energy stored in the P-P bonds (phosphoanhydride bonds) of ribonucleoside triphosphates (ATP, GTP, CTP, UTP)
What are the base parings in RNA?
- Adenosine pairs uracil
- guanine pairs cytosine
in what direction is RNA synthesised?
5’ to 3’
RNA does not always enter translation (process where mRNA is translated into proteins), what is the function of those RNA?
because RNA is single stranded, they can be folded into precise structures:
- can have structural and catalytic functions like proteins
- e.g. rRNA: forms basic structure of ribosomes and catalyse protein synthesis
Error rate of RNAP?
10-4 per nt
Does RNAP need primers like DNAP does?
no. because transcription need not to be as accurate as replication
Why doesn’t transcription need to be as accurate as DNA replication?
- RNA molecules are temporary, they do not store genetic infromation as long as DNA does
proofreading mechanism of RNAP?
when an incorrect base is added, RNAP can back up, perform excision reaction: reverse polymerisation using water, releasing nucleoside monophosphate
(bacterial) What is a σ factor?
- detachable subunit of the bacterial RNAP
- adheres to consensus sequences on DNA that preceeds pomoter regions
- bacterial RNAP binds to DNA via σ factor
- σ factor together with core enzyme (apoemzyme) are known as the RNAP holoenzyme
(bacterial) What are consensus sequences?
- consensus sequences are optimal seuqences for binding of σ factor
- promoters with consensus sequences in their vicinity initiates transcription much more frequently
what are promoters?
- specific sequecnes that initiates transcription
- A-T rich
inititation of trasncription in both eukaryotes and prokaryotes is described as a stochastic process, what does that mean?
- RNAP sticks weakly to DNA when they randomly collide and slides along it.
- any sequence will occasionally be transcribed => cell has lots of spurious RNA
- RNAP attaches tightly when it encounters a promoter
summary of inititation of transcription in prokaryotes
- RNAP holoenzyme assembles (RNAP core enzyme plus σ factor)
- σ factor adheres to consensus sequences
- RNAP holoenzyme slides along DNA, encounters promoter => binds tightly => starts trancription
What are the three types of eukaryotic RNAP and their respective functions?
- RNAP-I => rRNA
- RNAP-II => mRNA
- RNAP-III => tRNA
(eukaryotes) What are basal transcription factors
- proteins responsible for trancription initiation
- binds to TATA sequences that preceeds promoters
- eukaryotic RNAP binds to DNA via basal transcirption factors
(eukaryotes) what are the main basal transcription factors?
- TFIID
- TFIIB
- TFIIH
(eukaryotes) function of TFIID?
- has TBP (TATA binding protein) which binds to TATA box
- causes large distortion in TATA box
- Brings DNA sequences on both sides of the distortion together to allow for subsequent protein assembly
What is TATA box?
- short DNA sequence primarily composed of T and A nucleotides
- a type of promoter sequence
(eukaryotes) function of TFIIB?
- binds to BRE (B response element)
- initiates binding of other trancription factors
- positions RNAP-II
function of TFIIH?
- unwinds and opens DNA strands (helicase activity)
- phosphorylates CTD (C terminal domain) of RNAP-II, this kick starts transcription
summary of eukaryotic initiation of transcription on purified DNA?
- TFIID binds TATA box via TBP
- TFIIB binds BRE
- RNAP-II recruited
- TFIIH opens DNA
- TFIIH phosphorylates CTD of RNAP-II, starts transcription
- basal transcription factors fall off
