Trsnscription Flashcards
In 1955 Crick proposed the:
‘The central dogma of molecular biology’
The central dogma of molecular DNA
- DNA tells to cell what to do, when to do it, how much to do it, when to stop and which cell type to do it in
- mRNA interprets the information on the information
- Protien is thus the worker
Gene expression definition
The process by which information from a gene is used in the synthesis of a functional gene product.
Functional gene product: a protien of non-coding RNA (an RNA molecule will not give rise to a functional protien and has some other function
Definition of a gene:
A defined region (sequence) of DNA that produces a type of RNA molecules that has some function
The ‘gene’ (DNA) may contain sequences that:
- are responsible for the regulation of the synthesis of RNA
- produce the RNA
- are responsible for the further processing of RNA
What is transcription
- when we use a double stranded DNA molecule to produce a single stranded RNA molecule
DNA-DEPENDENT RNA SYNTHESIS
What is transcription catalysed by and how does it synthesise the mRNA
- the enzyme RNA polymerase
- RNA polymerase synthesises mRNA by catalysing the formation of PHOSPHODIESTER bonds between (RNA nucleotides) ribonucleotides (between the 3’ of one ribonucleotide and 5’ of another)
- RNA polymerase selects the correct nucleotides to incorporate into mRNA based on the sequence of the DNA which is being transcribed
Which strand is mRNA trasnscribed (made) from?
Template strand
Directions of the coding and template strands
Coding strand (5’ —-> 3’)
Template strand (3’ —-> 5’)
3 stages of transcription
- Intitiation
- Elongation
- Termination
Process of initiation (3 points)
- Transcription factors bind to the TATA box and other regions of the promotor (the transciption factors the recruit the RNA pol 2) -AT rich - easier to pull apart
- RNA pol 2 binds forming transcriptional initiation complex together with the trnascription factors
- The two DNA strands separate and RNA pol 2 starts mRNA synthesis without the need of a primer
Elongation
- RNA pol 2 uses the template strand, which tuns in the 3’ —> 5’ direction, as a template, and inserts complementary RNA nucleotides in the 5’ —-> 3’ direction
- RNA floats off the strand
- More RNA pol 2 come along and start trnascription so there are multiple transcriptions of one DNA strand at once
Where does transcription vs translation take place
TRanscription - nuclues
TRanslatiuon - cytoplasm
What does / doesn’t get transcribed ANATOTOMY OF EUKARYOTIC GENES
Doesn’t:
- Promotor
Does:
- 5’UTR
- 3’ UTR
- Coding sequence (not one continuos sequence - broken up into little bits)
Splicing process
- The transcribed region that will make up mRNA consists of ultinating exons and introns (introns are the intervening sequence)
- different genes have different numbers of introns and extrons
- To make a protein - coding sequence needs to be continuous
- splicing (pre-mRNA process removes the introns
- there are specific bases at the start and end of the introns for removal
Thus the coding sequence can now be translated
Purpose of an intron
- a part of regulating gene expression
- the cell can undergo transcription ahead of time and sit in a pre - mRNA stage with the 5’g cap and a poly A tail at the end, holding it in the cell
- when the cell needs a protein - all is required is splicing and translation
What is a coding sequence
- portion of a genes DNA that is translated into a protein
What is a promotor
- DNA segment recognised by RNA polymerase to initiate transcription
UTRs (UnTranslated Regioins)
- transcribed but (usually) not translated
- contain regulatory elements (sequences) that influence on gene expression at the transcriptional and / or translational level
- 5’ UTR facilitates the addition of the 5’G cap
- 3’ UTR facilitates the addition of the polyA tail
What does the 5’G cap do
- prevents mRNA degradation, promote intron excision and provides a binding site for the small ribosomal subunit
What does the Poly -A tail do
- prevents mRNA degration and facilitates export of the mRNA from the nucleus to the cytoplasm
Eukaryotic gene structure Elements
- contains non-coding DNA regions upstream and downstream of the coding sequence as well as within the coding sequence
- these non coding DNA regions can be transcribed but not translated
- these non-coding DNA elements are involved in regulating gene expression
- changes (mutations) in these non-coding gene sequences may ‘disrupt’ normal gene expression
Cellular location of eukaryotic transcription and translation
Prokaryote:
- transcription and translation in cytoplasm
- transcription ciption and translation are coupled
Eukaryote:
- transcription in nuclues
- translation in cytoplasm
- transcription and translation are not coupled
