Regulation of transcription in bacteria I Flashcards
Why not all genes are expressed at once? (prokayotes)
– Different genes express
different environments
• presence of a food substrate (lactose)
– Don’t waste energy expressing genes which
are of no use in a particular environment?>, MNB
What determines the different types of cells?
– All cells retain entire hereditary potential of the
organism
– BUT differentiate into different types (brain vs
muscle) so need to express different factors
Eukaryotic cell have off ground state why?
1- whole bunch of proteins beside RNA polymerase are needed to turn the gene on
2- Genes are not readily available for expression, Because they are wrapped around nucleosome in a chromosome
The structure of a eukaryotic chromosome
In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure
DNA wrapped around histone—> form nucleosome–> few nucleosome form chromatin
How gene expression is controlled in prokaryotic cells in presence of sugar?
Bacteria sense and respond to particular stimuli
often very rapidly (within minutes)
• Example
– Bacteria can utilise many different sugars for growth
– Only express the genes necessary for metabolism of
the sugar when it is present
• Must be able to
– Sense when the sugar is present
– Up-regulate necessary genes in response
– Down-regulate necessary genes when sugar is
absent
two levels of regulation in prokaryotes
Regulation at the level of transcription
– Most common form of regulation
– Transcription of DNA into mRNA
– Can regulate how much mRNA is made from a gene
– If no mRNA then final protein can’t be expressed
• Regulation at the level of translation
– Translation is the synthesis of protein from an mRNA
message
– Regulation of how much active protein is made from an mRNA transcript
what is negative regulation?
Negative regulation (repressor)
– A regulatory protein is used to stop transcription of a
gene (turns promoter OFF)
– When genes need to be turned on in response to a
stimulus the negative regulator must be inactivated
What is positive regulation?
• Positive regulation (activator)
– A regulatory protein is required before the gene is
efficiently transcribed (turn promoter ON)
– When genes need to be turned on in response to a
stimulus the positive activator must be activated
regulation of transcription mostly occurs at …..
the initiation point
what is the role of the promotor sequence?
Promoter sequences determine where RNA
polymerase starts transcription
– For eg. TTGACA-17bp-TATAAT
All genes have slightly different promoters
One RNA polymerase – thousands of genes.
Are they all transcribed simultaneously?
No– many alternative sigma factors recognising alternative promoter sequences.
different sigma factors are produced for different promotor
The contraction of sigma factor produced can vary based on the external signal
sigma factor 32
recognizes the promotor on the genes encoding for chaperons
level of sigma factor 32 increases when temperature increases , leading to sigma factor binding to the promotor increasing the expression of chaperons
what determine which gene is transcribed ?
the bacterial cell has many alternative sigma
factors, all competing for binding to the core RNA
polymerase.
What genes are transcribed is determined by how
abundant this or that sigma factor is at this particular
moment
when the promotor is off ?
– A promoter is “off” when RNA polymerase can’t bind and begin transcription
– The repressor usually binds over the promoter sequence so RNA pol can’t bind to the DNA
where does the repressor binds to?
The place where the repressor binds is called the
“operator site”
