Control of Gene Express in Prokaryotes

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Transcript Control of Gene Express in Prokaryotes

Regulation of Gene Expression
• Bacterial Gene Regulation
• Eukaryotic Gene Regulation
Operons-the basic concept of
Prokaryotic Gene Regulation
• Regulated genes can be switched on and
off depending on the cell’s metabolic
needs
• Operon-a regulated cluster of adjacent
structural genes, operator site, promotor
site, and regulatory gene(s)
Operon
• Structural gene-gene that codes for a
polypeptide
• Promoter region-controls access to the
structural genes, located between the
promoter and structural genes, contains
the operator site.
• Operator Site -region where the repressor
attaches
• Regulatory genes-codes for repressor
proteins
• Polycistronic mRNA-transcript for several
polypeptides
Repressible vs. Inducible Operons
two types of negative gene regulation
Repressible
Operons
• Genes are initially
ON
• Anabolic pathways
• End product
switches off its
own production
Inducible Operons
• Genes are initially
OFF
• Catabolic
pathways
• Switched on by
nutrient that the
pathway uses
trp: a repressible operon
lac: an inducible operon
Videos and Websites
• http://www.dnatube.com/
• http://vcell.ndsu.nodak.edu/animatio
ns/lacOperon/index.htm
• http://www.youtube.com/watch?v=V
Nok-vF03aI&feature=related
• http://www.youtube.com/watch?v=x_
dve8YMtrM&feature=related
An example of positive gene
regulation-cAMP
• cAMP exerts
positive control
• Binds to promoter,
stimulating
transcription
• Dependent on
glucose
concentration
Eukaryotic Genomes:
Organization, Regulation, and
Evolution
 The structure of chromatin
 Genome organization at the
DNA level
 The control of gene
expression
Nucleosomes –basic unit of
packing, made of two sets of
four histones, may control
The Structure of Chromatin
• DNA complexed with
protein forms
chromatin
• diffuse during
interphase
• condensed during
mitosis, forms
chromosomes
• histones and
nucleosomes
The structure of Chromatin
• Based on
successive levels
of DNA packing
The structure of Chromatin (2)
• Six nucleosomes/turn,
forms a cylinder
• Higher level of DNA
packing: looped
domains (20,000 to
100,000 nucleotides)
• Heterochromatin
remains highly
condensed during
interphase (Barr
bodies)
• Euchromatin able to
be transcribed during
interphase
Types of Chromatin
• Heterochromatin:
highly condensed
during interphase,
not actively
transcribed
• Euchromatin: less
condensed during
interphase, able to
be transcribed
The Code Beyond Genetics in DNA
• The original code is that each codon
specifies a particular amino acid and
subsequent protein
• The second code is determined by
the placement of the nucleosomes.
• Nucleosomes protect and control
access to the DNA
Nucleosomes
• 30,000,000 nucleosomes in each
human cell
• DNA wraps 1.65 times around a
nucleosome
• The DNA twist is 147 base pairs
• The average DNA strand contains
225 million base pairs
• Made of proteins called histones
How do Nucleosomes Function?
• Bind to the DNA at specific
sequences
• Prevent transcription factors from
attaching and initiating transcription
• Nucleosomes can and do move,
letting DNA open to be transcribed.
How?
This has not yet been determined!
The Control of Gene Expression
• Only a few genes
are active at any
time-differential
gene expression
• Control can be
exerted at any step
in the pathway.
• Chromatin
modifications
affect availability
of genes for
Transcriptional regulation via
Chromatin modification
• DNA methylation-methyl groups
added to cytosine-inactivate genes
• Histone acetylation- -COCH3 added to
amino acids. Reduce binding
between DNA and histoneconsequence?
Websites and Videos
• http://www.youtube.com/watch?v=eY
rQ0EhVCYA&NR=1
• http://www.youtube.com/watch?v=O
EWOZS_JTgk&feature=related
• http://www.biostudio.com/c_%20edu
cation%20mac.htm
Transcriptional regulation at
Initiation
• Role of transcription factors- act as
activators and/or repressors
• Coordinately controlled genesspatially different than prokaryotes,
no operons
• Examples: heat shock response,
steroid hormone action, cellular
differentiation
Control at the transcriptional
level
• Transcription Factors-augment
transcription by binding to DNA or to
each other. Act as repressors and
activators.
• Coordinately controlled genesusually associated with a specific
regulatory sequence and activated or
repressed by the corresponding
transcription factor
Posttranscriptional Mechanisms
• Regulation of mRNA degradation:
several hours or even weeks
• protein processing and degradation:
activation may require addition of
phosphate groups or sugars; use of
signal sequences; marking for
destruction
• control of translation: inactivation of
initiation factors, use of repressor
proteins
Posttranscriptional Mechanisms
• May be stopped or enhanced at any
posttranscriptional step
• Role of the nuclear envelope
• Regulation of mRNA degradation- several
hours to several weeks
• Control of translation- inactivation of
initiation factors, use of repressor
proteins
• Protein processing and degradation-may
require addition of sugars or phosphates;
use of signal sequences; marking for
destruction
Posttranscriptional Mechanisms
• microRNA (miRNA)
• Function: complementary to mRNA
and binds to different regions:
animal cells3’untranslated region
plant cells3’UTR and coding
regions
The Genetic Basis of
Development
•From single cell to
multicellular organism
•Differential gene expression
•Genetic and cellular
mechanisms of pattern
formation
From single cell to multicellular
organism
• Involves cell division,
morphogenestis and cell
differentiation
cell division: increases cell numbers
morphogenesis: overall shape of the
organism is established
cell differentiation: cells become
specialized in structure and function
• development has been studied using
model organisms
Differential Gene Expression
• Different types of cells in an
organism have the same DNA
• Plants are totipotent, cells retain the
ability of the zygote to give rise to all
differentiated cells
• Animals are not as plastic,
alternative approaches used,
nuclear transplantations such as
“Dolly”
Determination
• Different cell types make different
proteins
• role of transcription regulation
• two sources of cellular instructions
for determination: cytoplasmic
determinants and neighboring cells
Genetic and Cellular Mechanism
of Pattern Formation
• Pattern Formation:
spatial
organization of
tissues and organs
characteristic of
the mature
organism
• Plants-continuous
process
throughout life
• Animals-restricted
Homologous genes that affect
pattern formation
How genes control development
(Genetic analysis of Drosophila)
• Revealed roles of specific molecules
that direct position and
differentiation
• Cytoplasmic determinants provide
postional information (unfertilized
eggs: orientation of anteriorposterior and dorsal-ventral already
determined)
• 1200 genes essential for
development, 120 in segmentation
Role of Gradients of Maternal
Molecules
• Hypothesized over 100 years ago
• Bicoid Gene essential for
development of the anterior of a fly,
produces mRNA that concentrates in
anterior half of unfertilized eggs.
• Female flies w/out this gene produce
embryos lacking front half of embryo
• Bicoid protein regulate other genes,
a domino like effect
Homeotic Genes: What are they?
• Master regulatory genes that identify
specific regions of the body and
appropriate placement of
appendages
• contain a sequence of 180
nucleotides called the homeobox
• identical or similar homeobox
sequences have been identified in
many other invertebrates, vertebrates,
fungi and prokaryotes.
Role of Neighboring CellsInduction
• Signaling help coordinate spatial and
temporal expression of genes
• sequential inductions control organ
formation
• results in selective activation and
inactivation of genes within target
cells
Apoptosis-programmed cell
death
• “suicide” genes- product present
continuously
• depends upon regulating protein
activity
• tadpole tail?
• Degenerative diseases, cancersfaulty apoptotic mechanisms?
The Molecular Biology of Cancer
• Genetic changes that
affect the cell cycle
(viruses, carcinogens)
• Oncogene-cancercausing gene
• Proto-oncogenesnormally code for
regulatory proteins
controlling cell
growth, division, and
adhesion
The Molecular Biology of Cancer
• Result of genetic changes
-can be random
-can be caused by viruses or
carcinogens
• Oncogenes: cancer causing genes
• -formed from proto-oncogenes by
DNA movement within the
genome; gene amplification, or point
mutations
• changes in tumor-suppressor genes
Proto-oncogenesOncogenes
• Movement of DNA within the genome
• Gene amplification
• Point mutation
Sometimes suppressor genes that
normally inhibit growth can be
responsible for cancer
Multiple mutations underlie the
development of cancer
• 15% due to viruses
• Somatic mutations ( 5-10% of breast
cancer)