Regulation of gene expression is important for
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Responding to the environment
Cell specialization (cell differentiations)
Bacteria typically control gene expression at transcription
Eukaryotes control gene expression at transcription –OR- a bunch of other ways
1.
2.
3.
4.
5.
6.
By altering chromatin- either DNA or histones
Transcription
RNA processing
mRNA degradation
Translation
Protein processing and degradation
1. Access to DNA
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Most of the DNA in a eukaryotic cell is unavailable for transcription.
The addition/removal of acetyl groups from the histone proteins that DNA is wound around leads to tighter or
looser “packing” of the DNA.
2. Transcription
Remember: in eukaryotes, a cluster of proteins (transcription initiation complex) assembles on the promoter sequence
of DNA before transcription
Terms:
Regulatory sequence- stretches of DNA that interact with regulatory proteins to control transcription
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Promoter- The DNA sequence where RNA polymerase attaches and initiates transcription
Terminators- the sequence that signals the end of transcription
Enhancer- regulatory sequence of DNA that binds to activator proteins
o Activator proteins bind to other transcription factors to help form the transcription initiation complex
Regulatory gene- sequence of DNA that codes for regulatory protein or RNA
Controlling transcription factors present in the cell can help to control which genes are transcribed
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Transcription factors can include:
o Activators that increase expression
o Repressors that decrease expression
3. RNA Processing
 Eukaryote mRNA is processed after transcription (introns removed, 5’ cap, etc)
o Alternative RNA splicing- different mRNA transcripts are produced from the same primary transcript due
to intron-exon choice
 Controlled by regulatory proteins
RNA Interference (RNAi)- micro RNA’s are produced that bond with specific transcripts and block translation or degrade
mRNA
4. mRNA degradation- the lifespan of mRNA transcripts is important because a long-living mRNA can be
translated many, many times.
5. Translational- RNAi can block translation of mRNA
6. Protein processing
 Protein targeting- signal sequences on polypeptides target proteins to different parts of the cell for processing
 Proteins can be made inactive and be activated when specific signals are received
(green is signal sequence)
 Proteasomes- organelles that destroy proteins that have been “tagged” to be degraded.
Why do we regulate gene expression?
Gene Regulation  Phenotype: Phenotypes are the result of protein interactions.
Ex. X-inactivation http://www.hhmi.org/biointeractive/x-inactivation
Control of gene expression leads to control of proteins, which leads to control of
phenotypes.
 Cell Differentiation (in multicellular eukaryotes)- Different cellular phenotypes are
due to different genes being expressed in different cell types.
 Respond to the environment- Gene expression is due to both genome content
and environmental interactions. This accounts for some phenotype differences
among genetically identical individuals
Signal Control of Gene Expression
 One of the major ways that cells respond to changes is
by altering gene expression.
 Signals that affect gene expression can be internal or
external to the cell.
 External signals are transmitted between cells, or are
present in a cell’s environment.
o Ex. Mating in yeast- controlled by the presence
of mating pheromones.
 Internal signals are molecules present inside of cells that
control gene expression.
o Ex. Hox gene products: control the development
of animal body segments
http://www.hhmi.org/biointeractive/pitx1
-expression
Phenotypes arise from interactions between the genome and the environment of the organism.
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The environment directly controls certain phenotypes.
The genome can also respond to environmental changes by altering gene expression.
o Soil pH and hydrangea color
o Tanning in humans
o Sex determination in reptiles
Plasticity of the Genome- Different traits will interact with the environment in different ways
GENERATION OF VARIATION- Biological systems have multiple processes that increase genetic variation.
Life needs Variation
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The ability of living systems to evolve is dependent upon phenotypic variations being present
in a population.
 All lineages have modes of generating variation.
o Ex. Replication Mistakes.
 Horizontal Transfer: Refers to the exchange of genetic material among members of the same
generation.
a. Very common in modern prokaryotes and in ancestral development of all lineages.
Transduction: The transfer of DNA between bacteria
Transformation: The uptake of DNA directly from the
due to mistakes in viral reproduction.
environment.
Conjugation: Transfer of DNA via cell:cell contact. (such as a
plasmid to a cell)
Transposition: The replication and movement of
genetic elements separately from replication.
http://www.hhmi.org/biointeractive/bacterial- (chromosomal segment transferred to a new
conjugation
position)
Sexual Reproduction: The events of sexual reproduction create unique combinations
of existing genetic material.