Download PROKARYOTIC GENE REGULATION

PROKARYOTIC GENE REGULATION
 Cell must be able to respond to changes in environment
OPERON model Related genes grouped together with single “on-off” switch called an OPERATOR
 PROMOTOR- region where RNA polymerase binds to start transcription
 REGULATORY GENE farther away makes REPRESSOR that binds to operator to control binding of
RNA polymerase
INDUCIBLE OPERON- Example: Lactose (lac) operon
 Operon is TURNED OFF
 REPRESSOR is in ACTIVE FORM unless INDUCER (lactose) is present
– usually functions in catabolic pathways to digest nutrients into simpler
– cell avoids making proteins that have nothing to do
– cell allocates resources to other uses
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REPRESSIBLE OPERON- Example: Tryptophan (trp) operon
 Operon is TURNED ON
 REPRESSOR is in INACTIVE FORM unless COREPRESSOR (tryptophan) is present
– usually functions in anabolic pathways synthesizing end products
– when end product is present in excess, cell allocates resources to other uses
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EUKARYOTIC DNA STRUCTURE
LEVELS OF DNA PACKING REGULATE GENE EXPRESSION
DNA packaged in chromatin fibers- regulates access to DNA by RNA
polymerase
NUCLEOSOMES- made of DNA wrapped around 8 HISTONE proteins
HETEROCHROMATIN-tightly packed/no transcription
EUCHROMATIN- loosely packed/transcription
Acetylation (-COCH3) of histones unwraps DNA
NUCLEOSOMES→ FIBERS→LOOPED DOMAINS→ CHROMOSOMES
EPIGENETIC INHERITANCE –
inheritance not carried by nucleotide sequence
Genomic imprinting-methylation of DNA bases inactivates genes
EX: Angelman’s (maternal) & Prader Willi (paternal) syndromes
All cells have ALL GENESCell specialization (differentiation)= result of different cells turning on/off
different genes (ACTIVATORS/REPRESSORS)
POST TRANSCRIPTIONAL REGULATION OF GENE EXPRESSION

ALTERNATIVE RNA PROCESSING- different mRNA’s can be made by editing same transcript
depending on which parts are treated as introns/exons

mRNA DEGRADATION- siRNA’s (small interfering)- bind to mRNA and degrade it

INITIATION OF TRANSLATION-regulatory proteins can prevent attachment of ribosomes

PROTEIN PROCESSING/DEGRADATION-control activation and degradation of proteins
UBIQUITIN protein- tags proteins for destruction
PROTEASOMES-recognize ubiquitin tags and digest tagged proteins
Genes= only ~3% of human genome
 protein-coding sequences (1%)
 non-protein coding genes (2%) -tRNA’s, rRNA’s, siRNA’s
Most of DNA (97%) does not code for protein
regulatory sequences- promoters, enhancers, terminators
 “junk” DNA
o Introns
o repetitive DNA (centromeres, telomeres, tandem & interspersed repeats)
Ex: Fragile X syndrome – CGG repeats in promoter region on X chromosome
most common cause of inherited mental retardation
Ex: Huntington’s- CAG repeats produces mutant Huntingtin protein that causes
neurological damage
o transposons – 1st discovered by Barbara McClintock in corn
segments of DNA that move within genome; function unknown
Ex: Alu in humans (10% of human genome)
o retrotransposons- segments move with help of RNA intermediates and reverse
transcriptase coded for by retrotransposon (may be source of retroviruses)
Many genes exist in MULTIGENE FAMILIESCan be collections of identical or similar genes
EX: genes for rRNA subunits
Can be differentEX: HEMOGLOBIN-multiple chains coded for on different chromosomes; used at different times
(Fetal hemoglobin is different from adult hemoglobin)
PSEUDOGENES-nonfunctional sequences similar to functional genes
Can be used to determine evolutionary relationships
EX: Humans and other primates have nonfunctional pseudogene for vitamin C production
70% of the almost 100 different odor receptor genes are nonfunctional in humans
HOMEOTIC GENES- control overall body plan in plants and animals by regulating development
1st discovered as fruit fly mutants with body parts in wrong places
Highly conserved (few mutational changes)
Called HOX genes in animals
EUKARYOTIC GENE REGULATION
No OPERONS-individual genes PROMOTOER still binds RNA POLYMERASE
Regulatory sequences near or far from gene control its expression
 TATA BOXES within PROMOTER region help RNA polymerase attach
 ENHANCER REGION upstream bends; ACTIVATORS help RNA Polymerase attach
 Numerous other proteins TRANSCRIPTION FACTORS/MEDIATORS also involved
 SPECIFIC TRANSCRIPTION FACTORS (activators and repressors) in each kind of cell determine
which genes are expressed (Ex: ALBUMIN made in liver cell; CRYSTALLIN made in eye lens cell)