19.2 Control of Gene Expression Flashcards
What is transcriptional control?
Turning specific genes on & off
to inhibit or facilitate their transcription
What does transcriptional control involve?
CHROMATIN REMODELLING: consists of DNA + histones
-
Heterochromatin = tightly wound state of DNA, causes chromosomes to be visible (e.g during mitosis)
- RNA polymerase cannot bind to/access genes
- genes in heterochromatin cannot be transcribed
-
Euchromatin = loosely wound state of DNA (e.g during interphase)
- RNA polymerase can bind to/access genes
- genes in euchromatin can be transcribed
- Histone modification
- ACETYLATION/PHOSPHORYLATION
- addition of acetyl/phosphate groups
- reduces positive charge on histone
- decreases coiling of DNA
- METHYLATION
- addition of methyl groups
- makes histones more hydrophobic
- histones bind together more tightly
- increases coiling of DNA
- ACETYLATION/PHOSPHORYLATION
19.2 Control of Gene Expression
Describe the different states of DNA:
- HETEROCHROMATIN
- tightly wound state of DNA
- condensed chromosomes which are visible, e.g during mitosis
- cannot be transcribed ∵ RNA polymerase cannot access genes
- EUCHROMATIN
- loosely wound state of DNA
- present during interphase
- can be transcribed ∵ RNA polymerase can access genes
Describe the different forms of histone modification:
- ACETYLATION/PHOSPHORYLATION
- addition of acetyl/phosphate groups
- makes histones less positive
- decreases coiling of DNA
- METHYLATION
- addition of methyl groups
- makes histones more hydrophobic
- histones bind together more tightly
- increases coiling of DNA
What is epigenetics?
external control of genetic regulation
What is an operon?
group of genes
under control of same regulatory mechanism
& are ∴ expressed together
What is the lac operon?
group of structural genes
involved in metabolism of lactose
What is the role of the regulatory gene for the lac operon?
codes for repressor protein
What is the role of the repressor protein in the lac operon?
binds to promoter site on DNA
preventing transcription of the structural genes of the lac operon
What are structural genes?
genes which code for structural proteins or enzymes not involved in DNA regulation
Outline the process by which lactose is metabolised using the lac operon:
- When lactose absent, repressor protein remains bound to promoter
- prevents RNA polymerase from binding to operator
- ∴ also prevents transcription of lac operon
- When lactose present, binds to repressor protein
- causes conformational change in repressor protein
- repressor protein detaches from promoter
- RNA polymerase can now transcribe downstream structural genes for metabolising lactose
- Lactose is metabolised by these enzymes
Describe & explain the role of cyclic AMP in the lac operon:
-
Regulates rate of transcription of genes by RNA polymerase:
- cAMP Receptor Protein (CRP) + cAMP ⟶ complex
- Complex acts as transcription factor, increasing rate of transcription of lac operon
- increases number of enzymes for efficient metabolism of lactose
What does post-transcriptional/pre-translational control involve?
pre-mRNA ⟶ mature mRNA
- “cap” (modified nucleotide) & “tail” (long chain of adenine molecules) added to 5’ & 3’ end respectively
- stabilises mRNA & delays degradation. cap facilitates binding of mRNA to ribosome.
- Splicing removes introns & joins exons
-
Edited by adding/deleting/substituting nucleotides
- increases range of proteins proucible by single mRNA strand
What does post-transcriptional/pre-translational control involve?
pre-mRNA ⟶ mature mRNA
- “cap” (modified nucleotide) & “tail” (long chain of adenine molecules) added to 5’ & 3’ end respectively
- stabilises mRNA & delays degradation. cap facilitates binding of mRNA to ribosome.
- Splicing removes introns & joins exons
-
Edited by adding/deleting/substituting nucleotides
- increases range of proteins proucible by single mRNA strand
What is the purpose of the “cap” & “tail” added onto pre-mRNA?
delays degradation & stabilises molecule
cap facilitates binding to ribosome
