Lecture 15 Flashcards
different cell types of a multicellular organism contain _______
the same DNA
is the DNA sequence in the neutron the same as the DNA sequence found within a liver cell?
yes, it is the expression of different collection of genes in a cell type that cause large variations in size, shape, behaviour and function
what are house keeping genes
- genes that are common to all cells of a multicellular organism
example of house keeping genes
- ribosomal proteins
- RNA polymerases
- cytoskeleton proteins
gene expression
- cells can alter their pattern of gene expression in response to extracellular signals
for example: cortisol
- in liver cells, tyrosine aminotransferase increases
- in fat cells, tyrosine aminotransferase decreases
- in other cells there is no affect
how do we regulate gene expression
- regulation can occur at various steps, but control over transcription (step 1) is very important
how can transcription be turned “of” or “on”
- transcription is control by regulatory DNA sequences
- the operator of the trp opera is a regulatory DNA sequence that controls transcription of tryptophan production and related products by being bound by a repressor protein
transcriptional regulators (give examples)
- bind to regulatory DNA sequences to promote or repress gene expression
- tryptophan repressor is a transcriptional regulator, but more specifically it is a transcriptional repressor because it inhibits transcription
- CAP is a regulatory activator because its binding to regulatory DNA sequences promotes gene expression
how to transcriptional regulators bind
- often insert into the major groove of DNA where specific amino acids can interact with specific base pairs
- or bind as dimers which increases the area of contact with DNA which increases strength and specificity of protein-DNA interaction
what are operons
- genes that encode proteins that are all involved in the same process
- bacteria usually cluster genes into operons
how are operons controlled
- they are controlled by a single promoter
what is the significance of the tryptophan operon?
- when tryptophan levels are low, RNA polymerase binds to the promoter and begins transcription of products involved in tryptophan production
- when tryptophan levels are high, tryptophan binds to the tryptophan repressor which allows it to bind to the operator. This binding prevents RNA polymerase from binding
give an example of a transcriptional regulator that is controlled by multiple transcriptional regulators
Lac operon:
- encodes for proteins that are involved in lactose metabolism
- E. coli can used glucose or lactose as a carbon source, but it prefers glucose
What is negative regulation of the lac operon
- the lac repressor can bind to the operator and prevent transcription
- when lactose is present, the lac repressor does not bind to the operator
- when lactose is absent, the lac repressor binds to the operator and prevents transcription
what is produced when lactose is present
- allolactose
what is the positive regulation if the lac operon
- the transcriptional activator, CAP, can bind to regulatory DNA sequences to promote transcription of Lac operon
- helps RNA polymerase bind
- CAP only binds with high level cAMP
- levels of cAMP are inversely proportional to levels of glucose
when will lac operon be highly expressed
- no glucose
- high lactose
what do the controls ensure
- the controls ensure that the lac operon is expresses when lactose is present and no glucose is present
- this makes sense for E. coli because it prefers glucose over lactose
how do transcriptional activators interact with the promoter
- by “looping” the DNA
What is the enhancer and mediator in eukaryotic transcriptional regulation
- enhancer: binding site for activator proteins –> promotes transcription
- repressors do the opposite
- mediator: a complex of proteins that serves as an intermediate between regulatory proteins and the transcription complex
how do eukaryotes deal with higher levels of chromatin condensation when regulating gene expression
- chromatin-remodelling complexes
- covalently modify histone proteins
ex. histone acetyltransferases promote the acetylation of lysine, allowing greater accessibility of DNA
- histone deacetylases remove acetyl groups and reverse this affect
what is cell memory
changes in gene expression are remembered by a cell
what is combinatorial control
the way groups of transcription regulators work together
instead of clustering genes into operons like bacteria, what do eukaryotes do to control multiple genes at the same time
- eukaryotes use combinatorial control to make single transcription regulator control multiple genes simultaneously
combinatorial control example
- imagine each lock represented a gene and you need to express the gene you need all transcription regulators
- all the genes have all the transcription activators except for one (the triangle)
- when the triangle is present all genes can be expressed at the same time
- same thing can happen with transcription repressors
list epigenetic mechanisms behind cell memory
- cells can ensure their daughters “remember” what kind of cell they were
- positive feedback loops
- DNA methylation
- histone modification
what do epigenetic mechanisms do?
- they alter gene expression without changing the nucleotide sequence of the DNA
- they are forms of epigenetic inheritance
Describe the effects that a single transcriptional regulator can have on development
- can trigger formation of entire organs
- for example Ey can give rise to an entire eye in the middle of a leg
How does positive feedback loops aid in cell memory
- it ensures that all future progeny will also be same cell type
How is DNA methylation a method behind cell memory
- because DNA methylation can affect gene expression and DNA methylation patterns are passed down to progeny cells
How is histone modification aid in cell memory
- histone modification can affect gene expression
- histone modification can be inherited by daughter chromosome
post transcriptional controls
- gene expression can be controlled by regulating translation initiation
examples: - represser protein binds and prevent mRNA from binding to the ribosome
- mRNA is usually found with AUG blocked until the temperature rises
What are regulatory RNAs
noncoding RNAs with the ability to regulate gene expression
- microRNAs (miRNAs)
- small interfering RNAs (siRNAs)
- long noncoding RNAs
How do long noncoding mRNAs regulate gene expression
- may coat the chromosome, causing the association of chromatin-remolding complexes to form heterochromatin
What are micro RNAs
- 22 nucleotides in length
- complexes with proteins to create the RNA-induced silencing complex (RISC) as a single strand
- the miRNA binds to the complementary sequences on target mRNA and causes the degradation of that mRNA by nucleases within RISC
What are small interfering RNAs
- defines mechanism against foreign RNA (this is know as RNA interference)
- the foreign dsRNA is cleaved by dicer resulting in siRNA
- siRNAs bind to the RISC, but only one strand remains bound
- the bound RNA binds to complementary RNA which causes its degradation by nucleases within RISC
