Lecture 6 Flashcards
(23 cards)
What defines the differences between cells?
How many proteins are produced as all cells contains the same DNA
What is difference in morphologies dependent on?
difference in gene expression
What are the 3 types of RNA polymerase
I is rRNA, II is all protein coding genes, III is tRNA genes
What are the general principles of the initiation of transcription
- RNA polymerase interacts with other proteins (transcription factors) when it binds to the promoter
- The most basic promoter is required for RNA polymerase to bind and initiate transcription at the appropriate site
- Additional control sequences can determine when a gene is transcribed
What is the promoter?
very close to protein coding region and includes imitation site where transcription begins
What are enhancers?
proteins bind here
What do activators do?
allow transcription to begin
bind to enhancers up the DNA and interact with the basal transcription complex
Describe TATA box
TATA-box binding protein recognises TATA sequence and sits over the box, recruits TFIID and TFIIB bind to promoter and a basal transcription complex forms after further transcription factors bind to the promoter.
Describe yeast
- Gal4 system of yeast: Gal 4 attaches to UASG but Gal80 a repressor also binds so transcription cannot occur of GAL1
- Adding galatose this binds to Gal80 allowing Gal 4 to interact with RNA polII etc on the promoter region allowing transcription factors to be switched on, activator stabilises it
- Complex regulatory regions enable an organism to fine tune gene expression by balances on silences on repressors and enhancers
Describe histones
DNA is wrapped in chromatin and wrapped around histones in what’s known as nucleosomes
Each nucleosome is made up of 8 histone proteins, 2 turns of histone.
Describe the packaging of DNA
Packaging of DNA into chromatin provides a good way for the cell to stop preventing transcription of genes. Activators unpack DNA allowing transcription to begin.
How is DNA made accessible?
Chromatin remodelling complex interact with nucleosome to allow activator to bind, histone chaperons- can remove histones and can replace histones with variance, histone modifying enzymes interact with the activator proteins
What are N terminal tails?
beginning of them allows counting of amino acids from core of histone
How can histone be covalently modified at many sites?
methylation, phosphorylation, aceylation, ubiquitlylation – act as tags or the histones themselves
Where do common modifications of the histones occur?
Lysine or serine, lysine can be competitve
Describe lysine at position 9 of the H3 histone protein
– methyl reduces the likelihood of gene expression but an acyl group makes it more likely for gene expression to occur. Amino acid residues in N terminal tails of core histones can be modified by covalent addition of functional groups
How can DNA be covalently modified directly?
methylation of cytosine CG sequence, reduces gene expression – can be inherited from cells during division – example of epigenetics – maintain patterns of differentiation.
What contributes to stable gene repression?
multiple epigenetic mechanisms
Describe control of eukaryotic transcriptional initiation
• Activator proteins bind upstream enhancer sequences and activate the basal transcription complex
• Multiple epigenetic mechanisms control expression via local chromatin condesnation
o Histone modificaitons
o In vertebrates, direct DNA methylation on cytosine helps maintain patterns of gene repression in somatic cells
Describe prokaryotes
Bacteria less complicated no nucleus, one cytoplasmic compartment, no histones, coupled transcription and translation – occur at the same time, genes of related function are clustered into operons
Lac Operon
single promoter, all genes transcribed together, translated to give separate proteins, lac I gene producing a lac repressor which binds to lac O, when lactose is absent the RNA polymerase cannot bind to the lacO
What is negative regulation?
repressor
What is positive regulation?
activator
