Chapter 18 Flashcards
Prokaryotes and eukaryotes alter…
gene expression in response to their changing environment
in multicellular eukaryotes, gene expression regulates …
development and is responsible for differences in cell types
i.e. neuron vs. fibroblast
Gene regulation
a cell can regulate the production of enzymes by feedback inhibition
Operon Model
control gene expression (in bacteria)
E. coli, a type of bacteria that lives in human colon,
can tune its metabolism to the chaining environment and food sources at 2 levels:
1) adjusting the activity of metabolic enzymes already present
2) regulating the genes encoding the metabolic enzyme
i.e. enzymes that synthesize or metabolize tryptophan or lactose, respectively
2 levels of metabolic control
1) adjusting the activity of metabolic enzymes already present
(regulation of enzyme activity in metabolic pathway by feedback inhibition)
2) regulating the genes encoding the metabolic enzyme
(regulation of enzyme production stops the metabolic pathway)
in bacteria, genes are
often clustered into operons
composed of, an operator or “on-off” switch, a promoter, and genes for metabolic enzymes (lactose and tryptophan)
Repressor
a protein that switches an operon off since its inital
Corepressor
a molecule that cooperates with a repressor protein to switch an operon off
i.e. amino acid tryptophan
trp operon
regulated synthesis of repressible enzymes: prevent RNA polymerase from working
the presence of the amino acid tryptophan binds to the repressor protein to stop tryptophan synthesis
when top levels are low, the repressor protein is not active and tryptophan is synthesized for energy
in a repressible operon …
binding of a specific repressor protein to the operator shuts off transcription
i.e. trp operon
repressible operon on unless trp levels are high
Lac operon
an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose
by default, the lac repressor is active and
switches the lac operon off
when glucose is absent and lactose (allolactose) is present,
bacteria can utilize lactose for their energy
inducer (allolactose)
inactivates the repressor to turn the lac operon on for metabolism of lactose
in a repressive operon…
binding of a specific repressor protein to the operator shuts off transcription
usually anabolic
i.e. trp operon
in an inducible operon…
binding of an inducer to an innately inactive the repressor and turns on transcription
i.e. lac operon
regulation of both the trp and the lac operons involves the …
negative control of genes, because the operons are switched off by the active form of the repressor protein
all organisms must regulate
which genes are expressed at any given time and in any given cell type
*all genes ARE NOT expressed at the same time
almost all the cells in an organism are genetically identical however, differences between cell types result from …
differential gene expression, the expression of different genes by cells with the same genome
errors in gene expression …
can lead to diseases, such as cancer
how is gene expression controlled?
at the level of DNA, transcription (RNA synthesis) and translation
All organisms must regulate which genes re expressed at any given time but the key step is…
transcription
During development of a multicellular organism, its cells undergo a process of specialization in form and function called
cell differentiation
each cell of a multicellular eukaryote expresses
only a fraction of its gene in each type of differentiated cell
In eukaryotes, the DNA-protein complex is called
chromatin
and is ordered into higher structural levels than the DNA-protein complex in prokaryotes
Eukaryotic DNA is precisely combined with a large amount of protein because
eukaryotic chromosomes contain an enormous amount of DNA relative to their condensed length
Histones
proteins that associate with DNA and are responsible for the 1st level of DNA packing in chromatin and bind tightly to DNA throughout the cell cycle
Unfolded chromatin
has the appearance of beads on a string and each “bead” is a nucleosome and the “string” in linker DNA
Nucleosome
basic unit of DNA packing
consists of DNA wound around a protein core composed of 4 types of histone proteins
1st level of DNA packing
nucleosome
histone and DNA
2nd level of DNA packing
30nm chromatin fibers
due to the interactions between histone tails of one nucleosome and the linker DNA (coiled nucleosome)
3rd level of DNA packing
300nm chromatin fibers
form looped domains
attached to a scaffold of non-histone proteins
mitotic chromosome
the looped domains themselves coil and fold forming the characteristic metaphase which is maximally compacted
interphase cells
most chromatin is in the highly extended and uncompacted form called euchromatin but can also be found in the highly condensed areas called heterochromatin
euchromatin
uncompacted form of chromatin
heterochromatin
highly condensed areas of chromatin
genes not expressed
Chemical Modificaion
of histone tails affect chromatin structure and thus gene expression
Histone Acetylation
the addition of an acetyl group (COCH3) loosens chromatin structure and enhances transcription
relaxes the DNA, creating space
Methylation of histones
condenses chromatin and reduces transcription
the addition of phosphate groups (phosphorylation) next to a methylated amino acid can loosen chromatin to enhance transcription
Histone code hypothesis
proposes that the combinations of modifications determine chromatin configuration and influence transcription
Chromatin-modifying enzymes
provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery
Associated with most eukaryotic genes are multiple Control Elements
that are segments of noncoding DNA that help regulate transcription by binding certain proteins
to initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called
transcription factors that bind DNA near the promoter
i.e. ELK-1 that can modify chromatin
responds to ERK
Proximal control elements
are located close to the promoter
intrinsic means
Distal Control Elements
called enhancers that may be far away from a gene or even in an intron
intrinsic means
Activator
a protein that binds to an enhancer and stimulates transcription of a gene
CAAT region, an distal activator to increase gene expression
