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Gene Expression

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Regulation of Gene Expression
Prokaryotic Gene Regulation
Eukaryotic Transcription Gene Regulation
Eukaryotic Epigenetic Gene Regulation
Eukaryotic Post-transcriptional Gene Regulation
Eukaryotic Translational and Post-translational Gene
Development on the Cellular Level
Cancer and Gene Regulation

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Regulation of Gene Expression
•  The Process and Purpose of Gene Expression Regulation
•  Prokaryotic versus Eukaryotic Gene Expression

The Process and Purpose of Gene Expression Regulation
•  Every cell within an organism shares the same genome (with exceptions, i.e. mature red blood cells), but has variation between its proteomes.
•  Gene expression involves the process of transcribing DNA into RNA and then translating RNA into proteins.
•  Gene expression is a highly complex and tightly-regulated process.

Prokaryotic versus Eukaryotic Gene Expression
•  Prokayotic gene expression is primarily controlled at the level of transcription.
•  Eukaryotic gene expression is controlled at the levels of epigenetics, transcription, post-transcription, translation, and post-translation.
•  Prokaryotic gene expression (both transcription and translation) occurs within the cytoplasm of a cell due to the lack of a defined nucleus; thus, the DNA is freely located within the cytoplasm.
•  Eukaryotic gene expression occurs in both the nucleus (transcription) and cytoplasm (translation).


Prokaryotic Gene Regulation
•  The trp Operon: A Repressor Operon
•  Catabolite Activator Protein (CAP): An Activator Regulator
•  The lac Operon: An Inducer Operon

The trp Operon: A Repressor Operon
•  The operator sequence is encoded between the promoter region and the first trp-coding gene.
•  The trp operon is repressed when tryptophan levels are high by binding the repressor protein to the operator sequence via a corepressor which blocks RNA polymerase from transcribing the trp-related genes.
•  The trp operon is activated when tryptophan levels are low by dissociation of the repressor protein to the operator sequence which allows RNA polymerase to transcribe the trp genes in the operon.

Catabolite Activator Protein (CAP): An Activator Regulator
•  Catabolite activator protein (CAP) must bind to cAMP to activate transcription of the lac operon by RNA polymerase.
•  CAP is a transcriptional activator with a ligand-binding domain at the N-terminus and a DNA-binding domain at the C-terminus.
•  cAMP molecules bind to CAP and function as allosteric effectors by increasing CAP’s affinity to DNA.

The lac Operon: An Inducer Operon
•  The lac operon contains an operator, promoter, and structural genes that are transcribed together and are under the control of the catabolite activator protein (CAP) or repressor.
•  The lac operon is not activated and transcription remains off when the level of glucose is low or non-existent, but lactose is absent.
•  The lac operon encodes for the genes needed to utilize lactose as an energy source.


Eukaryotic Transcription Gene Regulation
•  The Promoter and the Transcription Machinery
•  Transcriptional Enhancers and Repressors

The Promoter and the Transcription Machinery
•  The purpose of the promoter is to bind transcription factors that control the initiation of transcription.
•  The promoter region can be short or quite long; the longer the promoter is, the more available space for proteins to bind.
•  To initiate transcription, a transcription factor (TFIID) binds to the TATA box, which causes other transcription factors to subsequently bind to the TATA box.
•  Once the transcription initiation complex is assembled, RNA polymerase can bind to its upstream sequence and is then phosphorylated.
•  Phosphorylation of RNA polymerase releases part of the protein from the DNA to activate the transcription initiation complex and places RNA polymerase in the correct orientation to begin transcription.
•  Transcription factors respond to environmental stimuli that cause the proteins to find their binding sites and initiate transcription of the gene that is needed.

Transcriptional Enhancers and Repressors
•  Enhancers can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or thousands of nucleotides away.
•  When a DNA-bending protein binds to the enhancer, the shape of the DNA changes, which allows interactions between the activators and transcription factors to occur.
•  Repressors respond to external stimuli to prevent the binding of activating transcription factors.
•  Corepressors can repress transcriptional initiation by recruiting histone deacetylase.
•  Histone deactylation increases the positive charge on histones, which strengthens the interaction between the histones and DNA, making the DNA less accessible to transcription.


Eukaryotic Epigenetic Gene Regulation
•  Epigenetic Control: Regulating Access to Genes within the Chromosome

Epigenetic Control: Regulating Access to Genes within the Chromosome
•  DNA is packaged by wrapping around histone proteins into structures called nucleosomes, which resemble beads on a string.
•  When DNA is to be transcribed, the nucleosomes can slide away from that region of DNA, opening it up to the transcription machinery of the cell.
•  Chemical modifications to either the histone proteins or the DNA itself signals whether or not a particular region of the genome should be “open” or “closed” to the transcription machinery.
•  Modifications such as acetylation or methylation of the histones can alter how tightly DNA is wrapped around them, while methylation of DNA changes how the DNA interacts with proteins, including the histone proteins that control access to the region.
•  This type of genetic regulation is called epigenetic regulation (“above genetics”) as it does not change the nucleotide sequence of the DNA.


Eukaryotic Post-transcriptional Gene Regulation
•  RNA Splicing

RNA Splicing
•  Introns are intervening sequences within a pre-mRNA molecule that do not code for proteins and are removed during RNA processing by a spliceosome.
•  Exons are expressing sequences within a pre-mRNA molecule that are spliced together once introns are removed to form mature mRNA molecules that are translated into proteins.
•  Alternative splicing allows for the production of various protein isoforms from one single gene coding.
•  A spliceosome is a complex comprised of both RNA molecules and proteins which determine which introns to leave out and which exons to keep and bind together.


Eukaryotic Translational and Post-translational Gene
•  The Initiation Complex and Translation Rate
•  Chemical Modifications, Protein Activity, and Longevity

The Initiation Complex and Translation Rate
•  The components involved in ribosome assembly are brought together by the help of proteins called initiation factors which bind to the small ribosomal subunit.
•  Initiator tRNA is used to locate the start codon AUG (the amino acid methionine) which establishes the reading frame for the mRNA strand.
•  GTP carried by eIF2 is the energy source used for loading the initiator tRNA carried by the small ribosomal subunit on the correct start codon in the mRNA.
•  GTP carried by eIF5 is the energy source for assembling the large and small ribosomal subunits together.

Chemical Modifications, Protein Activity, and Longevity
•  Proteins can be chemically modified by adding methyl, phosphate, acetyl, and ubiquitin groups.
•  Protein longevity can be affected by altering stages of gene regulation, including but not limited to altering: accessibility to chromosomal DNA for transcription, rate of translation, nuclear shuttling, RNA stability, and post-translational modifications.
•  Ubiquitin is added to a protein to mark it for degradation by the proteasome.


Development on the Cellular Level
•  Adding cells through cellular division
•  Differentiating cells to serve different functions
•  Mechanics of differentation: induction and gene expression
•  Establishing the body axes and patterns
•  Cell migration
•  Genes provide positional information
•  Programmed cell death

Adding cells through cellular division
•  Symmetric cell division of stem cells ensures that a constant pool of stem cells is available by giving rise to two identical daughter cells both endowed with stem cell properties.
•  Asymmetric division of stem cells results in the production of only one stem cell and a progenitor cell with limited self-renewal potential.
•  Progenitor cells that are produced via asymmetric cell division will go through additional rounds of cell division until they are terminally differentiated into a mature, specialized cell.
•  Asymmetric division can be controlled by both intrinsic and extrinsic factors.
•  Intrinsic factors involve differing amounts of cell-fate determinants being distributed into each daughter cell, while extrinsic factors involve interactions with neighboring cells and the micro and macro environment of the precursor cell.

Differentiating cells to serve different functions
•  The three major cell types in the mammalian body include germ cells (which develop into gametes), somatic cells (diploid cells that develop into a majority of the human body) and stem cells (cells that can divide indefinitely).
•  In human development, the inner cell mass exhibits the ability to differentiate and form all tissues of the body; however, they cannot form an organism.
•  The various types of stem and progenitor cells included in the body that will differentiate to develop more specialized cells includes: hematopoietic stem cells, mesenchymal stem cells, epithelial stem cells and muscle satellite cells.
•  To develop a multicellular oragnisms, cells must differentiate to specialize for different functions.

Mechanics of differentation: induction and gene expression
•  Different types of stem cells exhibit varying abilities to differentiate into specialized cells (from the most unlimited stem cell to the most restricted): totipotent, pluripotent, multipotent to oligopotent.
•  Totipotent cells have the potential to differentiate into any of the cells needed to enable an organism to grow and develop; pluripotent cells have the potential to differentiate into any type of human tissue but cannot support the full development of an organism.
•  A multipotent stem cell has the potential to differentiate into different types of cells within a given cell lineage or small number of lineages, while an oligopotent stem cell is limited to becoming one of a few different cell types.
•  The process of cellular differentiation is under strict regulation by transcription factors which can either activate or repress expression of genes that will affect the proteome of the cell and thus, provide the necessary components it needs to become a specialized cell.
•  All cells contain the same complement of DNA, or genome, but once differentiation occurs, it is the changes in the proteome that will distinguish one cell type from another.

Establishing the body axes and patterns
•  The three axes of the animal body are established in development via the expression of specific sets of genes that regulate which cells will develop into specific structures.
•  During development, the dorsal cells are genetically programmed to develop into the notochord and define the axis.
•  The neural tube can develop in two ways: primary or secondary neurulation, which are used by organisms in varying degrees to establish the neural tube that will develop into the central nervous system (brain and spinal cord).
•  Specific patterns along the neural tube that are established via secretion and production of specific signaling molecules (such as Wnt, Shh, BMP and retinoic acid) play a key role in patterning the dorsal and ventral axes.

Cell migration
•  The disruption or dysfunction of cell migration processes can lead to formation of various diseases such as metastasis, tumor formation and vascular disease.
•  In prokaryotic organisms, and some eukaryotic cells such as sperm cells, cell migration occurs via the use of a cilia or flagella to propel forward.
•  In eukaryotic organisms, cell migration is a much more complex process and can include, but is not excluded to, changes in the cytoskeleton, motor proteins, blebbing, and cytoplasmic displacement; it involves both external and internal signals that mediate these processes.

Genes provide positional information
•  Organogenesis results in the formation of the various organs in the body; however it will only occur if specific sets of genes are expressed to determine ultimate cell type.
•  The ability of specific cells to migrate to the the edge of the ectoderm is highly regulated by specific gene expression and allows for differentiation into epidermal cells; in contrast, the cells which remain in the center will develop into the neural plate.
•  The expression of specific sets of genes will also regulate the reorganization of the mesoderm into distinct groups of cells, called somites, which develop into the ribs, lungs, spine muscle and notochord.

Programmed cell death
•  Programmed cell death can provide an advantage to an organism during development, for instance by maintaining homeostasis and protection against potentially disruptive issues which may arise during the life of a cell.
•  Apoptosis is a process of programmed cell death that is regulated by numerous biochemical events and appears to be genetically mediated.
•  Autophagy is a process of programmed cell death that is characterized as a catabolic process via formation of an autophagolysosome which degrades damaged cellular contents.
•  Necrosis occurs when cellular death is caused by external factors and is characterized as an alternate form of programmed cell death, called necroptosis.


Cancer and Gene Regulation
•  Cancer: Disease of Altered Gene Expression
•  Cancer and Epigenetic Alterations
•  Cancer and Transcriptional Control
•  Cancer and Post-transcriptional Control
•  Cancer, Translational/Post-translational Control, and Targeted Therapies

Cancer: Disease of Altered Gene Expression
•  Cancer results from a gene that is not normally expressed in a cell, but is switched on and expressed at high levels due to mutations or alterations in gene regulation.
•  Alterations in histone acetylation, activation of transcription factors, increased RNA stability, increased translational control, and protein modification are all observed in cancer cells.
•  Tumor suppressor genes, active in normal cells, work to prevent uncontrolled cell growth.
•  Proto-oncogenes, which are positive cell-cycle regulators, can become oncogenes and cause cancer when mutated.

Cancer and Epigenetic Alterations
•  The DNA in the promoter region of silenced genes in cancer cells is methylated on cytosine DNA residues in CpG islands.
•  Histone proteins that surround the promoter region of silenced genes lack the acetylation modification that is present when the genes are expressed in normal cells.
•  When the combination of DNA methylation and histone deacetylation occur within cancer cells, the gene present in that chromosomal region is silenced.
•  Epigenetic changes that are altered in cancer can be reversed and may, therefore, be helpful in new drug and therapy design.

Cancer and Transcriptional Control
•  The mutations that activate transcription factors can increase the binding of a transcription factor to its binding site in a promoter leading to increased transcriptional activation of that gene and resulting in altered cell growth.
•  A mutation in the DNA of a promoter or enhancer region may increase the binding ability of a transcription factor, which may then lead to the increased transcription and anomalous gene expression that is seen in cancer cells.
•  Studying how to control the transcriptional activation of gene expression in cancer cells along with identifying how a transcription factor binds or a pathway activates where a gene can be turned off has led researchers to new drugs and novel ways of treating cancer.

Cancer and Post-transcriptional Control
•  Specific cancers have altered expression of miRNAs; changes in the miRNA population of particular cancers varies depending on the type of cancer.
•  Having too many miRNAs can dramatically decrease the RNA population leading to a decrease in protein expression.
•  Studies have found that some miRNAs are specifically expressed only in cancer cells.

Cancer, Translational/Post-translational Control, and Targeted Therapies
•  Protein modifications from the increased translation of a protein to changes in protein phosphorylation to alternative splice variants of a protein are found in cancer cells.
•  The expression of the wrong protein dramatically alters cell function and contributes to the progression of cancer.
•  Gene regulation and gene function provide scientists with the opportunity to design medicines and therapies that specifically target diseased cells or exploit the overexpression of specific proteins as cancer treatment.


Appendix

Key terms
•  acetylation the reaction of a substance with acetic acid or one of its derivatives; the introduction of one or more acetyl groups into a substance
•  activator any chemical or agent which regulates one or more genes by increasing the rate of transcription
•  anencephaly a lethal birth defect in which most of the brain and parts of the skull are missing; absence of the encephalon
•  apoptosis a process of programmed cell death 
•  autologous derived from part of the same individual (i.e. from the recipient rather than the donor)
•  autophagy a type of programmed cell death accomplished through self-digestion
•  blastocyst the mammalian blastula formed during development where the inner cell mass can be found which forms the embryo
•  bleb an irregular bulge in the plasma membrane of a cell
•  cancer a disease in which the cells of a tissue undergo uncontrolled (and often rapid) proliferation
•  cancer a disease in which the cells of a tissue undergo uncontrolled (and often rapid) proliferation
•  chemotaxis the movement of a cell or an organism in response to a chemical stimulant
•  differentiate to produce distinct cells, organs or to achieve specific functions by a process of development
•  enhancer a short region of DNA that can increase transcription of genes
•  epigenetic the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence
•  epigenetics the study of heritable changes caused by the activation and deactivation of genes without any change in DNA sequence
•  epigenetics the study of heritable changes caused by the activation and deactivation of genes without any change in DNA sequence
•  exon a region of a transcribed gene present in the final functional RNA molecule
•  exosome a vesicle responsible for the selective removal of plasma membrane proteins
•  extracellular matrix All the connective tissues and fibres that are not part of a cell, but rather provide support.
•  gastrulation the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells
•  genome the cell’s complete genetic information packaged as a double-stranded DNA molecule
•  histone any of various simple water-soluble proteins that are rich in the basic amino acids lysine and arginine and are complexed with DNA in the nucleosomes of eukaryotic chromatin
•  inner cell mass a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals
•  intron a portion of a split gene that is included in pre-RNA transcripts but is removed during RNA processing and rapidly degraded
•  laminar of fluid motion, smooth and regular, flowing as though in different layers
•  metastasis the transference of a bodily function or disease to another part of the body; specifically the development of a secondary area of disease remote from the original site, as with some cancers
•  methylation the addition of a methyl group to cytosine and adenine residues in DNA that leads to the epigenetic modification of DNA and the reduction of gene expression and protein production
•  microRNA a single-stranded, non-coding form of RNA, having only about 20-30 nucleotides, that has a number of functions including the regulation of gene expression
•  morula a spherical mass of blastomeres that forms following the splitting of a zygote; it becomes the blastula
•  neural tube hollow longitudinal dorsal tube formed in the folding and subsequent fusion of the opposite ectodermal folds in the embryo that gives rise to the brain and spinal cord
•  neurulation the process by which the beginnings of the vertebrate nervous system is formed in embryos
•  notochord a flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine
•  nucleosome any of the subunits that repeat in chromatin; a coil of DNA surrounding a histone core
•  nucleosome any of the subunits that repeat in chromatin; a coil of DNA surrounding a histone core
•  oncogene any gene that contributes to the conversion of a normal cell into a cancerous cell when mutated or expressed at high levels
•  operator a segment of DNA to which a transcription factor protein binds
•  operon a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together
•  operon a unit of genetic material that functions in a coordinated manner by means of an operator, a promoter, and structural genes that are transcribed together
•  organogenesis the formation and development of the organs of an organism from embryonic cells
•  phosphorylation the addition of a phosphate group to a compound; often catalyzed by enzymes
•  pluripotent able to develop into more than one mature cell or tissue type, but not all
•  pluripotent able to develop into more than one mature cell or tissue type, but not all
•  post-translational modification the chemical modification of a protein after its translation; one of the later steps in protein biosynthesis, and thus gene expression, for many proteins
•  progenitor cell a biological cell that, like a stem cell, has a tendency to differentiate into a specific type of cell, but is already more specific than a stem cell and is pushed to differentiate into its “target” cell.
•  promoter the section of DNA that controls the initiation of RNA transcription
•  promoter the section of DNA that controls the initiation of RNA transcription
•  proteasome a complex protein, found in bacterial, archaeal and eukaryotic cells, that breaks down other proteins via proteolysis
•  proteome the complete set of proteins encoded by a particular genome
•  proteome the complete set of proteins encoded by a particular genome
•  proteome the complete set of proteins encoded by a particular genome
•  proto-oncogene a gene that promotes the specialization and division of normal cells that becomes an oncogene following mutation
•  reading frame either of three possible triplets of codons in which a DNA sequence could be transcribed
•  repressor any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
•  repressor any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
•  repressor any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
•  RNA polymerase a DNA-dependent RNA polymerase, an enzyme, that produces RNA
•  somatic part of, or relating to the body of an organism
•  somite one of the paired masses of mesoderm distributed along the sides of the neural tube that will eventually become dermis, skeletal muscle, or vertebrae
•  spliceosome a dynamic complex of RNA and protein subunits that removes introns from precursor mRNA
•  targeted therapy a type of medication that blocks the growth of cancer cells by interfering with specific targeted molecules rather than by interfering with rapidly dividing cells
•  TATA box a DNA sequence (cis-regulatory element) found in the promoter region of genes in archaea and eukaryotes
•  totipotency the ability of a cell to produce differentiated cells upon division
•  transcription the synthesis of RNA under the direction of DNA
•  transcription factor a protein that binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to mRNA
•  transcription factor a protein that binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to mRNA
•  ubiquitin a small polypeptide present in the cells of all eukaryotes; it plays a part in modifying and degrading proteins

Neural Tube Formation
The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.

Somites
In this five-week old human embryo, somites are segments along the length of the body.

Cell Migration
Phase images of BSC 1 cells migrating in a scratch assay in the absence of serum over a period of 15 hours.

Alternative Splicing
Pre-mRNA can be alternatively spliced to create different proteins.

Mechanism of Splicing
Alternative splicing can result in protein isoforms.

Ubiquitin Tags
Proteins with ubiquitin tags are marked for degradation within the proteasome.

MicroRNA
Overexpression of miRNAs could be detrimental to normal cellular activity because miRNAs bind to the 3′ UTR of RNA molecules to degrade them.Specific types of miRNAs are only found in cancer cells.

Translation Initiation Complex
Gene expression can be controlled by factors that bind the translation initiation complex.

Nucleosomes can change position to allow transcription of genes
Nucleosomes can slide along DNA.When nucleosomes are spaced closely together (top), transcription factors cannot bind and gene expression is turned off.When the nucleosomes are spaced far apart (bottom), the DNA is exposed.Transcription factors can bind, allowing gene expression to occur.Modifications to the histones and DNA affect nucleosome spacing.

Modifications to histones and DNA can alter gene expression
Histone proteins and DNA nucleotides can be modified chemically.Modifications affect nucleosome spacing and gene expression.

DNA Packaging
DNA is folded around histone proteins to create (a) nucleosome complexes.These nucleosomes control the access of proteins to the underlying DNA.When viewed through an electron microscope (b), the nucleosomes look like beads on a string.

Alternative Splicing
There are five basic modes of alternative splicing.

Epigenetic Alterations in Cancer Cells
In cancer cells, silencing genes through epigenetic mechanisms is a common occurrence.Mechanisms can include modifications to histone proteins and DNA associated with these silencing genes.

Using Gene Expression in Targeted Therapy
Scientists are using knowledge of the regulation of gene expression in individual cancers to develop new ways to treat target diseased cells and prevent the disease from occurring.Target therapies exploit the overexpression of a specific protein or gene mutation to develop new medications against the specific cancer.

Promoters
A generalized promoter of a gene transcribed by RNA polymerase II is shown.Transcription factors recognize the promoter.RNA polymerase II then binds and forms the transcription initiation complex.

Cell Differentiation
Mechanics of cellular differentiation can be controlled by growth factors which can induce cell division.In asymetric cell division the cell will be induced to differentiate into a specialized cell and the growth factors will work in tandem.

Programmed Cell Death
This histological section of a foot of a 15-day-old mouse embryo, visualized using light microscopy, reveals areas of tissue between the toes, which apoptosis will eliminate before the mouse reaches its full gestational age at 27 days.

Apoptosis
This video describes the process of apoptosis, or programmed cell death.

The lac Operon
Transcription of the lac operon is carefully regulated so that its expression only occurs when glucose is limited and lactose is present to serve as an alternative fuel source.

The trp operon
The five genes that are needed to synthesize tryptophan in E. coli are located next to each other in the trp operon.When tryptophan is plentiful, two tryptophan molecules bind the repressor protein at the operator sequence.This physically blocks the RNA polymerase from transcribing the tryptophan genes.When tryptophan is absent, the repressor protein does not bind to the operator and the genes are transcribed.

Vertebrate Axis Formation
Animal bodies have three axes for symmetry:lateral-medial (left-right), dorsal-ventral (back-belly), and anterior-posterior (head-feet).

Transcription Factors
Transcription factors, especially some that are proto-oncogenes or tumor suppressors, help regulate the cell cycle; however, when regulation gives rise to cancer cells, then transcriptional control of gene expression is affected.

Symmetric and Asymmetric Division
This diagram illustrates stem cell division and differentiation, through the processes of (1) symmetric stem cell division, (2) asymmetric stem cell division, (3) progenitor division, and (4) terminal differentiation.Stem cells are indicated by (A), progenitor cells by (B), and differentiated cells by (C).

Proto-oncogenes Can Become Oncogenes
When mutated, proto-oncogenes can become oncogenes and cause cancer due to uncontrolled cell growth.

Hematopoiesis: the differentiation of multipotent cells
The process of hematopoiesis involves the differentiation of multipotent cells into blood and immune cells.The multipotent hematopoietic stem cells give rise to many different cell types, including the cells of the immune system and red blood cells.

Transcription Factors Regulate Gene Expression
While each body cell contains the organism’s entire genome, different cells regulate gene expression with the use of various transcription factors.Transcription factors are proteins that affect the binding of RNA polymerase to a particular gene on the DNA molecule.

Enhancers
An enhancer is a DNA sequence that promotes transcription.Each enhancer is made up of short DNA sequences called distal control elements.Activators bound to the distal control elements interact with mediator proteins and transcription factors.

Stem Cells
Pluripotent, embryonic stem cells originate as inner cell mass (ICM) cells within a blastocyst.These stem cells can become any tissue in the body, excluding a placenta.Only cells from an earlier stage of the embryo, known as the morula, are totipotent, able to become all tissues in the body and the extraembryonic placenta.

Catabolite Activator Protein (CAP) Regulation
When glucose levels fall, E. coli may use other sugars for fuel, but must transcribe new genes to do so.As glucose supplies become limited, cAMP levels increase.This cAMP binds to the CAP protein, a positive regulator that binds to an operator region upstream of the genes required to use other sugar sources.

Neural Tube
Transverse section of half of a chick embryo of forty-five hours’ incubation.The dorsal (back) surface of the embryo is toward the top of this page, while the ventral (front) surface is toward the bottom.(Neural tube is in green.)

Neural Tube Formation
The central region of the ectoderm forms the neural tube, which gives rise to the brain and the spinal cord.

Gene Expression
The genetic content of each somatic cell in an organism is the same, but not all genes are expressed in every cell.The control of which genes are expressed dictates whether a cell is (a) an eye cell or (b) a liver cell.It is the differential gene expression patterns that arise in different cells that give rise to (c) a complete organism.

Prokaryotic vs Eukaryotic Gene Expression
Prokaryotic transcription and translation occur simultaneously in the cytoplasm, and regulation occurs at the transcriptional level.Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm.Further regulation may occur through post-translational modifications of proteins.

Attribution
•  Connexions. “Introduction.” CC BY 3.0 http://cnx.org/content/m44533/latest/?collection=col11448/latest
•  Wiktionary. “proteome.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/proteome
•  Wiktionary. “genome.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/genome
•  Wiktionary. “somatic.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/somatic
•  Connexions. “Regulation of Gene Expression.” CC BY 3.0 http://cnx.org/content/m44534/latest/?collection=col11448/latest
•  Wiktionary. “epigenetics.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/epigenetics
•  Wiktionary. “nucleosome.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/nucleosome
•  Connexions. “Prokaryotic Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44535/latest/?collection=col11448/latest
•  Wiktionary. “operon.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/operon
•  Wiktionary. “repressor.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/repressor
•  Connexions. “Prokaryotic Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44535/latest/?collection=col11448/latest
•  Wikipedia. “Catabolite activator protein.” CC BY-SA 3.0 http://en.wikipedia.org/wiki/Catabolite_activator_protein
•  Wikipedia. “RNA polymerase.” CC BY-SA 3.0 http://en.wikipedia.org/wiki/RNA%20polymerase
•  Wiktionary. “promoter.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/promoter
•  Wiktionary. “operon.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/operon
•  Connexions. “Prokaryotic Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44535/latest/?collection=col11448/latest
•  Wikibooks. “An Introduction to Molecular Biology/Gene Expression.” CC BY-SA 3.0 http://en.wikibooks.org/wiki/An_Introduction_to_Molecular_Biology/Gene_Expression

•  Wiktionary. “repressor.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/repressor
•  Wikipedia. “operator.” CC BY-SA 3.0 http://en.wikipedia.org/wiki/operator
•  Connexions. “Eukaryotic Epigenetic Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44536/latest/?collection=col11448/latest
•  Wiktionary. “nucleosome.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/nucleosome
•  Wiktionary. “epigenetics.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/epigenetics
•  Wiktionary. “histone.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/histone
•  Connexions. “Eukaryotic Transcription Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44538/latest/?collection=col11448/latest
•  Wiktionary. “transcription factor.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/transcription+factor
•  Wiktionary. “TATA box.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/TATA+box
•  Wiktionary. “promoter.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/promoter
•  Connexions. “Eukaryotic Transcription Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44538/latest/?collection=col11448/latest
•  Connexions. “Eukaryotic Transcription Gene Regulation.” CC BY 3.0 http://cnx.org/content/m44538/latest/?collection=col11448/latest
•  Wikibooks. “An Introduction to Molecular Biology/Gene Expression.” CC BY-SA 3.0 http://en.wikibooks.org/wiki/An_Introduction_to_Molecular_Biology/Gene_Expression#Enhancer
•  Wiktionary. “repressor.” CC BY-SA 3.0 http://en.wiktionary.org/wiki/repressor
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