7.2 Transcription and gene expression Flashcards
What is the function of the promoter?
is a sequence that is located near a gene
it is the binding site of RNA polymerase, the enzyme that catalyses the formation of the covalent bond between nucleotides during the synthesis of RNA
Promoter is not transcribed but plays a role in transcription
How is gene expression regulate in prokaryotes?
based on variations in the environmental factors
i.e.
* absorption and metabolism of lactose by E.coli are only expressed in the presence of lactose
* in this case, the breakdown of lactose results in regulation of gene expression by negative feedback loop
* in the presence of lactose a repressor protein is deactivated.
* Once the lactose has been broken down, the repressor protein is no longer deactivated and proceeds to block the expression of lactose metabolism genes
What proteins whose binding to DNA regulates transcription?
enhancers, silencers and promoter-proximal elements
are unique to the gene its linked to
What are control elements?
the DNA sequences that regulatory proteins bind to
What are elements close and distant to the promoter called?
Priximal elements and distal elements
What are the 3 control elements? what do they do respectively?
promoters - a sequence located near a gene where RNA polymerase binds
enhancers - regulatory sequences on the DNA which increase the rate of transcription when proteins bind to them
silencers - regulatory sequences on the DNA which decrease the rate of transcription when proteins bind to them
What is a environmental factor that can affect gene expression in humans?
production of skin pigmentation during exposure to sunlight
What are regulatory mechanisms?
mechanisms that involve the rapid turn-on and turn-off of gene expression in response to environmental changes
What are the 4 levels in which regulatory mechanisms can regulate genes in eukaryotes?
- transcriptional - genes can be switched on or off
- Post-transcriptional - mRNA can be modified which regulates translation and types of proteins produced
- Translational - translation can be stopped or started
- Post- translational - proteins can be modified after synthesis which changes their functions
Which level of regulatory mechanism does not occur in prokaryotes?
Post-transcriptional because they dont have a nucleus and sometime translation occur straight away or at the same time
What are housekeeping genes?
genes that code for enzymes that that catalyse the metabolic reactions in prokaryotes and are always on
What are the 3 transcriptional regulatory mechanisms that can occur in eurkaryotic or prokaryotic
Chromatin remodelling (eukaryotic)
Histone modification (eukaryotic)
Lac operon (prokaryotic)
What is chromatin remodelling?
- Active genes are packed in expanded form - euchromatin (accessible to transcriptional machinery)
- Inactive genes are condensed - heterochromatin (saves space, not transcribed, cannot access)
Is DNA more negative or positive?
negative
What is histone modification?
Addition of acetyl, phosphate, and methyl groups
Acetylation and phosphorylation - makes histones more negative, cause DNA to coil less tightly. Allows certain genes to be transcribed
Methylation - make the histones more hydrophobic so they bind more tightly to each other, cause DNA to coil more tightly and prevent transcription
What is epigenetics a study of?
changes in phenotype as a result of variation in gene expression levels
What are epigenetic tags?
the chemical modifications of chromatin that impact gene expression (acetylation, methylation and phosphorylation)
What is a epigenome?
the sum of all the epigenetic tags
You are born with certain number of epigenetic tags
epigenome of cells can be passed onto daughter cells
Why does only about 1% of the epigenome passed down from parents are present?
the epigenome is erased during fertilisation through a process called programming (from environment throughout life i.e. exposure to pathogen, lifestyle, food)
About 1% of the epigenome is not erased and survives yielding a result called imprinting (inherited)
What is imprinting?
when epigenetic tags make it through reporgramming unchanged and gets passed from parent to offspring
What evidence shows methylation patterns are influenced by environment?
Using twin studies
born with a few similar epigenetic tag
but very different when they are older cuz environment changes a lot
What is an operon?
a group of genes that are under the control of the same regulatory mechanism and are expressed at the same time
What is lac operon?
an operon that responds to lactose in E.coli
contains gened coding for proteins in charge of transporting lactose into the cytosol and digesting it into glucose
When is lac operon turned off? What happens?
Glucose is present
* RNA polumerase binds to the promoter of the regulatory gene (Lacl) - a repressive protein
* Lac repressive protein is produced
* no lactose binds with repressive protein
* repressive protein binds to the operator region and blocks RNA polymeras
* RNA polymerase cant bind to promoter for LacZ (β-galactosidase) + LacY (Lactose permease)
* No transcrition of lactase enzymes
When is lac operon turned on? What happens?
Glucose is not present but Lactose is
* RNA polymerase binds to the promoter of the regulatory gene (Lacl) - a repressive protein
* Lac repressor protein is produced
* Lactos binds with repressive protein
* Repressor protein undergoes conformational change
* Repressor protein cannot bind to operator site
* Genes for lactase enzymes are transcribed - LacZ (β-galactosidase) + LacY (Lactose permease)
What is LacZ and Lac Y?
LacZ (β-galactosidase) - hydrolyses lactose to glucose and galactose
LacY (Lactose permease) - allows lactose to enter the bacterial cell
What is the level of cAMP influenced by?
Level of cAMP decreases when glucose is present
What does cAMP do in the presence of glucose and in the presence of lactose and no glucose?
glucose present
* level of cAMP decrease
* reduce transcription of LacY and Z
* lactose isnot metabolised
glucose not present, lactose present
* cAMP binds to cAMP receptor protein
* Acts as an enhancer
What are two regulatory mechanisms post-transcriptional?
- RNA processing
- RNA editing
What is RNA processing?
- 5’ methyl cap and 3’ poly-A tail to aid stability to mRNA is added on the product of transcription (pre mRNA)
- Introns (the non-coding regions of DNA) are removed and exons (the coding, or expressed, region of DNA) are spliced together to form a mature mRNA
- Mature mRNA leaves the cytoplasm through nuclear pores to be translated
Alternative splicing can occur
* selective removal of specific exons will result in the formation of different polypeptides from a single gene sequence
What does alternative splicing do?
make different form of protein from same DNA
What is RNA editing?
Nucleotide bases substitution, deletion or addition to increase the range of proteins produced from a single gene
What are 3 translational regulatory mechanisms? Explain each
Degradation of mRNA - the more resistance the molecule the longer it will last in the cytoplasm and a greater quantity of protein will be synthesised
inhibitory proteins - binding of these to mRNA prevents it binding to ribosomes and the synthesis of proteins
Initiation factors - activation of these aid the binding of mRNA to ribosome
What are 4 post-translational regulatory mechanisms?
- additon of protein groups such as carbohydrate chains, lipids or phosphates
- modifying amino acids and the formation of bonds such as disulfide bridges
- Folding or shortening of proteins
- Modification by cAMP and protein kinases
How are proteins activated post-translational?
via phosphorylation (addition of phosphate groups)
What are protein kinases?
enzymes that catalyse the phosphorylation of proteins
What does the addition of a phosphate group do to a protein?
changes the tertiary structure and so the function of a protein changes
What are protein kinases often activated by?
secondary messenger cAMP
