Chp 11 – Notes Directly from Book
Chp 11 – Notes Directly from Book
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Gene regulation (on/off of genes) helps organisms respond to environment
E. Coli uses 3 enzymes to start metabolizing Lactose
o Control sequences next to the enzymes
 Promoter: RNA polymerase attaches here
 Operator: determines whether RNA poly. can attach to promoter and start
transcription
Operon = cluster of genes + control sequences
Repressor – protein that turns off transcription
o Comes from regulatory gene
o Active when alone (no lactose is in it)
Tryptophan binds to repressor of trp operon
o This activates the trp repressor, enabling it to switch off the operon
 This operon allows bacteria to quit making molecules when those molecules are
already present, thus saving energy and materials
Other operon controls = activators (proteins that turn ON operons by binding to DNA)
Difference btwn cell types is not due to different gens being present, but to selective gene
expression
DNA fit within nucleus due to coiling + folding
o Histones = small proteins that help DNA
o Nucleosome = DNA wound around protein cone of 8 histones
o DNA packing can block gene expression by preventing RNA poly. (and other
transcription proteins) from contacting DNA
 Cells use higher levels packing for long-term inactivation of genes
Cells have capability to establish and maintain chemical modifications to their chromosomes in
ways that help regulate gene expression
o Ex: addition or removal of chemical groups to some amino acids in histones can cause
proteins to bind DNA more tightly or loosely, altering the ability of transcription
machinery to reach those genes
Enzymes add Methyl group (CH3) to DNA bases without changing actual sequences of bases
o Individual genes more methylated in cells where they aren’t expressed
o Removal of methyl groups turn on some of these genes
o Once methylated, genes stay that way through cell divisions in individual
o Methylation patterns passed on
 Ex: at DNA sites where one strand = methylated, enzymes methylate the
daughter strand
Epigenetic Inheritance = inheritance of traits transmitted by mechanisms NOT directly involving
nucleotide sequence
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Chp 11 – Notes Directly from Book
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In the way above (methylation notes) modifications to DNA & histones can be passed on
to future generations
Alterations in normal patterns of DNA methylation are seen in some cancers, where they are
associated with inappropriate gene expression
Enzymes that modify chromatin structure are parts of eukaryotic cells’ regulation of
transcription
X – INACTIVATION
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Why don’t females (since they have XX) make twice as many proteins encoded by genes on X
chromosome compared to males?
o One chromosome is almost inactive
Inactivation of X chromosome involves modification of the DNA (ex: methylation) and the
histones that help compact it
One X chromosome in each somatic cell inactivated at random
o Once x chromosome = inactivated, all descendant cells have same copy turned off (ex of
epigenetic inheritance)
Females either have active X chromosome from mother or father
If she is heterozygous for a gene on a chromosome, then ½ her cells express 1 allele and the
others express the alternate allele
11.3
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Initiation of transcription = most important stage for regulating gene expression
Initiation of eukaryotic transcription features an array of regulatory proteins that interact with
DNA (and with each other) to turn genes on/off
o Activators are more important than repressors in eukaryotes
In order to function, eukaryotic RNA poly. needs assistance of proteins called transcription
factors
The transcription factors consist of:
o Activator proteins which bind to
o DNA control sequences called enhancers
 Binding of activators to enhancers bends the DNA
o Once DNA = bent, the activators interact with other transcription factor proteins, which
then bind as a complex (giant group of all these proteins) to gene’s promoter
This complex facilitates the correct attachment of RNA poly. to promoter and the initiation of
transcription
o Only when the complex is assembled can RNA poly. move along gen, producing RNA
strand
o Repressor proteins called silencers that may bind to DNA sequences and inhibit start of
transcription
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Chp 11 – Notes Directly from Book
11.4 – RNA MAY BE SPLICED IN MORE THAN ONE WAY
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Splicing process of mRNA (addition of cap and tail; removal of introns; exons spliced together)
help control the flow of mRNA from nucleus to cytoplasm
o Because until splicing is completed, the RNA is attached to the molecules of splicing
machinery and cannot go to cytoplasm
Cell can carry out splicing in many ways; generating different mRNA molecules from same RNA
transcript
o With alternative RNA splicing, organism can produce more than one type of polypeptide
from single gene
11.5 – SMALL RNAS CAN CONTROL GENE EXPRESSION
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microRNAs (miRNAs) – small RNAs that can bind to complementary sequences on mRNA
molecules
each miRNA
1) Forms a complex with protein. The miRNA protein complex can
2) bind to any mRNA molecule with the complementary sequence then the complex either
3) degrades the target mRNA or
4) blocks it from translation
11.6
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Even after mRNA is processed and transported to cytoplasm, there are several opportunities for
regulation
Control points include:
o mRNA Breakdown – enzymes in cytoplasm break down the molecules of mRNA; timing
= important for how much protein is produced
 long-lived mRNA translated into more proteins than short-lived ones
o Initiation of Translation – among molecules involved in translation are proteins that can
control start of polypeptide synthesis
 By controlling start of protein synthesis, cells avoid wasting energy if the needed
components are currently unavailable
o Protein Activation – after translation = complete, some polypeptides need alterations
before they can function
 post-translational control mechanisms involve
 Cleavage (cutting) of polypeptide to yield smaller final product that is
active protein, able to carry out its function
o Protein Breakdown – final control mechanism operating after translation
 Lifetimes of proteins regulated
 Some proteins that trigger metabolic changes in cells are broken down in
minutes or hours
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Chp 11 – Notes Directly from Book
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This regulation allows cell to adjust the kinds/amounts of proteins in response
to environment changes
Also enable cell to maintain proteins in prime working order
When proteins are damaged, they are broken down right away and replaced by
functioning new ones
11.7 REVIEW
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Most important control point (in eukaryotes and prokaryotes) is at start of transcription
RNA processing (in nucleus) adds nucleotides to ends of RNA (cap an tail) and splices out introns
Once mRNA reached cytoplasm regulation includes:
o mRNA translation
o eventual breakdown
o alteration
o breakdown of protein
in eukaryotes genes controlled by proteins encoded by regulatory genes on other chromosomes
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