Biology 1202B Lecture Notes - Lecture 15: Histone Deacetylase, Rna Interference, Xist (Gene)

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5 Apr 2013
Lecture 15: Control of Gene Expression
Human Development
o Human egg when released from the ovary is almost completely metabolically inactive
o Within seconds of the egg & the sperm meeting, rapid cell division (mitosis)
o Mitosis (growth) produce cells of the body
o Cells differentiate into specialized cells with different functions from the same
o Every nucleated cell of the body contains the same DNA template and genes i.e. liver
cells do not contain different DNA than skin cells
Then how do cells of a developing embryo, with the same set of DNA/genes create vastly
different specialized?
o Structural and functional differences in cell types result from the presence or absence
of the products resulting from expressed (transcribed or transcribed and translated)
genes rather than the actual presence of genes themselves on DNA.
o In order for cells to become specialized it is not just enough to have a gene present in
DNA. Gene must be expressed or not expressed in the correct tissue (special) and at
the correct time (temporal). This is a very complex system
Gene Expression is like music played by an orchestra
o Gene is present in DNA but is it “On” or “Off”
o On gene is expressed and actively transcribed
o Off gene is not expressed and not actively transcribed
In different tissues and times this may change
o Gene is individually fine tuned
o Gene’s tuning is dynamic and aware of its surrounding
Regulation of Gene Expression in Prokaryotes
o Simple, single celled organisms with generation times in minutes
o Rapid and reversible alterations so they can adapt quickly to changes in their
o Genes are organized into a functional unit called an operan i.e. coordinated synthesis
of proteins with related functions.
o Each operon can contain several to many genes and each is transcribed as a
unit from a promoter into a single messenger RNA
The lac operan is an example of transcriptional regulation
demonstrating how genes are turned on and off
Operator a short segment to which a regulatory protein binds
Regulation of Gene Expression in Eukaryotes
Multicellular cells
Produce a large number of cells
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Produce a large number of different types of cells
More complicated since nuclear DNA is bound to histones, thus
need chromatin remodeling to loosen histone DNA interaction
(acetylases add acetyle groups to histones) or slide nucleosomes
away from gene’s promoter region
Chromatin Remodeling
Remodeling is a crucial initial event because it opens the way for
transcription to occur
Promoter DNA not accessible to proteins (RNA polymerase, TF’s)
that bind to DNA to initiate transcription.
DNA is wrapped around a core of two molecules each made of
histones H2A, H2B, H3 and H4. Forming the nucleosome. Higher
level of compaction when histone H1 is linked to adjacent
o Gene is inactive
Using ATP energy, chromatin remodeling complex binds and
slides nucleosomes along DNA
Steps in Gene Expression in Eukaryotes
- Which genes are transcribed
- Types & availability of mRNAs to ribosomes
- Rate at which proteins are made
- Availability of finished protein
DNA that does not encode mRNA
Promoters, enhancers
Intergenic sequence
Repeats telomeres and centromeres
DNA that does encode for mRNA, & does get translated into a protein in eukaryote
Genes are scattered around the genome
Genes consist of protein-coding sequences and adjacent regulatory sequences
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