Gene Expression Flashcards
DNA properties
antiparallel double-stranded helix
ATGC
less reactive - 2’ deoxyribose
RNA properties
single-stranded
AUGC
more reactive - 2’ OH in ribose
Types of RNA
- ribosomal (rRNA) 2/3 of ribosomes
- transfer (tRNA) molecules that deliver amino acids to ribosomes
- messenger (mRNA) nucleotide sequences that direct protein synthesis
How do all cellular RNAs arise?
transcription from DNA templates, using DNA-dependent RNA polymerases, which are directed to specific genes (unlike DNAPs)
Action of RNA Polymerase
couples ribonucleotide triphosphates on DNA template (eg. ATP) which is driven by the release and hydrolysis of PPi
Is a primer needed in prokaryotic transcription?
no and template is fully conserved
How is porkaryotic transcription intiated?
promoters - RNAP binds to initiation site through base sequences (promoter), which is recognised by RNAP sigma factor
lies upstream of RNA’s starting nucleotide
What causes the DNA strands to separate?
RNAP holoenzyme binds in its vicinity
What direction is RNA synthesis?
5’ to 3’
What kind of genome is produced from RNA synthesis in PT?
small, single strand genome portions
Is PT rapid?
yes - RNA synthesis initiated as soon as RNAP clears promoters
and protein synthesis can begin before RNA is completely synthesised (only in Ps)
RNAP is processive and does not dissociate from DNA template
How is transcription terminated in prokaryotes?
DNA contains specific termination sites:
1. series of 4-10 AT bases
2. A,G,C rich region with palindromic sequence
RNA transcript form self-complementary hairpin, causing RNAP to pause and changes it conformationally
How do some prokaryotes terminate transcription without the intrinsic terminator?
‘Rho factor’ - helicase that runs along RNA until caused RNAP is found. Pushes it, causing rewinding of dsDNA and release of RNA
How does ET differ to PT?
similar, but have multiple RNAPs, therefore more complicated control sequences
RNAPs have greater subunit complexity
RNAPs do not have sigma factor
Enhancers
What is used to identify promoters in ET?
accesory proteins which ID promotors and recruit the RNAPs
What are enhancers?
certain sequences that encourage transcription
recognised by TFs and can stimulate RNAP II binding and mediate selective gene expression (in Es)
What are general transcription factors?
Uses?
equivalent of sigma factor
- needed for RNAP II transcription
- permit basal expression
- targeted to TATA box by TBP
- switching transcription machinery to elongation mode after RNAP II initiates RNA synthesis
What is the preinitiation complex?
in basal expression, where TFs combine with an enzyme and DNA
How can transcription be inhibited?
antibiotics which target RNAP’s essential functions
What is RNA capping?
RNAp contains CTD, which, when phosphorylated, recruits capping enzyme complex. This modifies the 5’ end to a 7-methylguanosine (joined by 5’-5’-triphosphate bridge)
involves several enzymatic reactions
Use of RNA capping
provides resistane to degradation and ID of translation start site
What is polyadenylation?
part of transcription termination, part of the process for producing mRNA for translation
poly(A) added to mRNA
poly (A) tail protects the mRNA from degradation, aids in the export of the mature mRNA to the cytoplasm
Splicing
introns removed
splicesomes carry out
Why are introns removed?
source of vulnerability
not required to make polypeptide
What are spliceosomes?
small ribonucleoproteins (snRNAs + proteins)
What is alternative splicing?
where primary transcripts are spliced in different ways
leads to proteins with differing destinations in the cell with different catalytic/interactive properties
