book 4 part 2 (control of gene expression) Flashcards
explain how gene expression in eukaryotes can be increased at the chromatin level
via Histone acetylation and DNA demethylation.
1. Demethylation of DNA is the removal of the extra methyl groups from the DNA by DNA demethylases
- Acetylation of histones involves the addition of acetyl groups to lysine residues in histone tails by histone acetyltransferases
- this causes the positive charge of the lysine residues in histone tails to be neutralized
- nucleosomes no longer able to bind to neighbouring nucleosomes
- results in the loosening of histone complex from DNA ref. euchromatin
-gives transcription proteins and enzymes easier access to genes and increases transcription of
genes
explain how gene expression in eukaryotes can be reduced at the chromatin level
via Histone deacetylation and DNA methylation.
1. DNA methylation is the addition of methyl groups to
certain bases (usually cytosine) of DNA by DNA methyltransferases
2.Histone deacetylation involves the removal of acetyl groups from lysine residues of histone
tails by histone deacetylases
- ionic bonds between positive histone tail and negative DNA can form again
- makes the DNA more compact / ref. heterochromatin
- RNA polymerase cannot access the promoter sequence
- reduces transcription of genes
explain how gene expression in eukaryotes can be increased at the transcriptional level
via the binding of activator proteins to enhancer.
- DNA binding protein bends DNA
- activator proteins bind to enhancer via DNA-binding domain
- activator proteins then bind to co-activators via its activation domain
- co-activators binds to generalised transcription factors and RNA polymerase II via protein-protein interactions
- this facilitates the efficient positioning of RNA polymerase II on the promoter to increase the rate of transcription
explain how gene expression in eukaryotes can be reduced at the transcriptional level.
via the binding of repressor proteins to silencer.
- repressor binds to silencer and turns off transcription even in the presence of activator proteins, by blocking the binding of activator proteins to the control elements
- or to the components of the transcription initiation complex
- or repressor binds to control element within enhancer to turn off transcription
state the process and function of 5’ capping
- the addition of a methylated guanine nucleotide to the first nucleotide of pre-mRNA at the 5’ end via 5’-5’ triphosphate bridge catalysed by capping enzymes
- a cap-binding protein associates with the 5’ cap to
- protect mRNA from degradation by 5’ exonucleases & helps to increase mRNA stability
- cap-binding protein complex is recognised by nuclear pore complex to facilitate the export of mRNA from nucleus
state the process and function of 3’ polyadenylation
- after the mRNA polyadenylation signal (AAUAAA) is transcribed, cleavage takes place 10 to 35 nucleotides downstream from the signal
- poly(A) polymerase adds 30-200 adenine nucleotides to the 3’ end
- poly(A) binding protein binds to 3’ poly(A) tail
- to protect mRNA from degradation by 3’ exonucleases and helps to increase mRNA stability
- facilitates the export of mRNA from nucleus to the cytoplasm
- helps ribosome recognise mRNA as a molecule to be translated
state the process of splicing
- spliceosomes (made up of small nuclear ribonucleoprotein) recognise the splice sites & folds the pre-mRNA into correct orientation for splicing & catalyse the excision of the introns & ligation of exons
- spliceosomes cleave at the 5’ end of introns
- the cleaved end joins to branch point sequence to form a lariat (a loop)
- 3’ end of introns are excised
- introns are excised & exons are ligated
- spliceosomes dissociates
explain alternative splicing and state the functions
- some exons are excised together with the introns whereby not all exons are included in the final mature mRNA transcript
- alternative splicing allows a single gene to encode different polypeptides –> allows for protein variants from a single gene
explain the control of gene expression at translational level
- via cytoplasmic elongation of poly(A) tails on mRNA
- this helps to increase the half-life of RNA and
- signal for the initiaiton of translation - via RNA interference by microRNA & small interfering RNA
- prevents translation
- degrade mRNA - via phosphorylation of ribosomal translational initiation factors
- can prevent or increase translation - via the binding of regulatory proteins
- prevents ribosome attachment
- lowers rate of translation
explain how transport of protein to target destinations can help to regulate the levels of functional proteins.
- newly synthesised polypeptides are delivered to their specific intracellular location or exported from cell in order to function (protein targeting)
- transportation of proteins to target destinations in the cell is mediated by signal sequences at the N-terminus of some proteins
- once transported, signal sequence is enzymatically removed from proteins
explain how post-translational modifications (proteolysis & biochemical modifications) helps to regulate the levels of functional proteins
- proteolysis is the hydrolytic processing (cleaving) of eukaryotic polypeptide to yield smaller, functional proteins
- biochemical modifications involves the covalent addition of one or more groups to amino acids in a polypeptide to form a functional protein
- eg glycosylation, formation of disulfide bonds, phosphorylation, acetylation
explain how protein degradation via ubiquitination helps to regulate the levels of functional proteins
- selective degradation of proteins will regulate the length of time each protein can function in the cell
- the covalent attachment of ubiquitin molecules to the protein marks ubiquitinised proteins for the degradation by proteasome which involves hydrolytic reactions to break peptide bonds between amino acids in polypeptides
- this happens so that proteins do not stay too long in the cytoplasm or so it doesn’t remain active when not needed. this is to prevent aberrant activities
