lecture 15 Flashcards
housekeeping genes
genes that are common to all cells
ex. ribosomal proteins, RNA polymerases..
if you inject cortisol into a liver cell, fat cell and another cell, what will happen?
they won’t respond in the same way
regulation of gene expression is important in step..
1!! very important for control over transcription at step 1
but regulation can occur at various steps
regulatory DNA sequences turns
DNA “on” or “off”
give an example (trp operon) of a regulatory DNA sequence
the operator trp operon is a regulatory DNA sequence that controls transcription of the trypothan production-related products by being bound to a repressor protein
transcription regulators bind to
regulatory DNA sequences
transcriptional regulator (tryptophan example)
the tryptophan repressor is a transcription regulator
it is a transcriptional REPRESSOR specifically because it inhibits transcription
Transcription regulators can also promote…
gene expression
example of a transcriptional activator
CAP is a transcriptional activator bc it binds to regulatory DNA sequences which promotes gene expression
in bacteria, genes that encode proteins that are involved in the same process are often clustered in
operons!!
controlled by a single promoter
Lac operon has multiple
transcription regulators
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
CAP is a
transcriptional activator
binds to regulatory sequences to promote transcription of the lac operon
explain positive regulation of Lac operon (CAP)
CAP helps the RNA polymerase bind
CAP only binds when there are high levels of cyclic AMP (cAMP)
levels of cAMP are inversely proportional to levels of
glucose
enhancer
binding site for activator proteins
promotes transcription
repressors do the opposite of
enhancers
mediator
complexes of proteins that are an intermediate between regulatory proteins and the transcription complex
eukaryotes need to deal with
higher levels of chromatin condensation
eukaryotes and chromatin-modifying proteins
- chromatin-remodeling complexes
- covalently modify the histone proteins
give an example of a chromatin-modifying protein
Histone acetyltransferases promote the acetylation of lysine, allows greater accessibility of the DNA
Histone deacetylases remove the acetyl groups to reverse this effect
cell memory
changes in gene expression are remembered by a cell
combinatorial control is
the way groups of transcription regulators work together
- many genes are controlled by dozens of regulators
do eukaryotes cluster genes into operons?
no!
only bacteria do this
eukaryotes use combinatorial control to make
a single transcription regulator to control multiple genes at the same time
combinatorial control– lock example
when the triangle is present (transcription activator), all three genes can be expressed at the same time
3 epigenetic mechanisms behind cell memory
- positive feedback loops
- DNA methylation
- histone modifications
how do the 3 epigenetic mechanisms work?
they alter gene expression WITHOUT altering the nucleotide sequence of the DNA
they are forms of epigenetic inheritance
positive feedback loops
transcription regulator that causes a cell to differentiate into a particular cell type activates transcription of itself,
this ensures that all future
progeny will also be of the same type
DNA methylation
can affect gene expression
patterns are passed down to progeny cells
Histone modifications
can affect gene expression (histone code!!)
the modifications can be inherited by daughter chromosomes
regulatory RNAs are
noncoding RNAs that can regulate gene expression
3 regulatory RNAs
miRNAs (micro)
siRNAs (small interfering)
long noncoding RNAs
long noncoding RNAs can work in 2 ways
- coat the chromosome, causes association of chromatin-remodeling complexes to form heterochromatin
- transribed from the “wrong” DNA strand which bind to the mRNA transcript
RISC
RNA-inducing silencing complex
miRNA– how does it cause degradation of mRNA?
miRNA binds to complementary sequences on target mRNA and causes degradation of that mRNA by nucleases in the RISC
siRNAs acts as a
defense against foreign RNA
- system known as RNA interference
how do siRNAs degrade mRNA?
siRNAs bind to RISC
the bound RNA binds to complementary RNA which causes its degradation by nucleases in the RISC
how are siRNAs formed?
foreign dsRNA is cleaved by a dicer (protein) which results in siRNAs
