Chapter 18

Chapter 18

CHAPTER 18 REGULATION OF GENE EXPRESSION Overview Prokaryotes and eukaryotes alter gene expression in response to their changing

environment In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types RNA molecules play many roles in regulating gene expression in eukaryotes

Bacteria often respond to environmental change by regulating transcription. E. coli cell lives in erratic environment of human colon. Depending on the eating habits of the host. If the environments are lacking in amino acid tryptophan it will respond by activating a

pathway that make tryptophan from another compound. If host eats a tryptophan rich food the bacteria cell stops producing tryptophan. Metabolic control occur on two levels. 1. Cell can adjust the activity of enzymes

already present. that relies on the sensitivity of many enzymes to chemical that increase or decrease their activity. The activity of the first enzyme in the tryptophan synthesis pathway is inhibited by

pathways end product ( feed back inhibition) Second, cells can adjust the production level of certain enzymes by regulating the expression of gens encoding the enzymes In previous example, if environment provide all the tryptophan the cell needs, the cell stop

making the enzymes that catalyze the synthesis of tryptophan In this case the control of enzyme production occurs at the level of TRANSCRIPTION, the synthesis of mRNA coding for these enzymes Generally, many genes of the bacterial

genome are switched on or off by changes in the metabolic status of the cell. Gene expression in bacteria is controlled by the operon model Operons: The Basic Concept A cluster of functionally related genes can be

under coordinated control by a single on-off switch Such as: E. Coli synthesizes the Tryptophan from a precursor is a multistep pathway. Each step is catalyzed by a specific enzyme

The 5 genes those code for these enzymes are clustered together on the chromosome. A single promoter serves all 5 genes, which together constitute a TRANSCRIPTION unit. Thus, transcription give rise to one long mRNA molecule that codes for 5 polypeptides making up the enzymes in the tryptophan pathway

One single on-off switch can control the whole cluster of related genes. The regulatory switch is a segment of DNA called an operator usually positioned within the promoter An operon is the entire stretch of DNA that

includes the operator, the promoter, and the genes that they control The operon can be switched off by a protein repressor The repressor prevents gene transcription by binding to the operator and blocking RNA

polymerase Repressor is specific. The repressor is the product of a separate regulatory gene, which located some distance from the operon Regulatory genes are expressed continuously

at a low rate to prevent a permanent switch off. A corepressor is a molecule that cooperates with a repressor protein to switch an operon off For example, E. coli can synthesize the amino acid tryptophan

By default the trp operon is on and the genes for tryptophan synthesis are transcribed When tryptophan is present, it binds to the trp repressor protein, which turns the operon off The repressor is active only in the presence of its corepressor tryptophan; thus the trp

operon is turned off (repressed) if tryptophan levels are high Repressible and Inducible Operons: Two Types of Negative Gene Regulation A repressible operon is one that is usually on; binding of a repressor to the operator shuts

off transcription The trp operon is a repressible operon An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription Lac operon (lactose) is an inducible operon

The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose By itself, the lac repressor is active and switches the lac operon off A molecule called an inducer inactivates the repressor to turn the lac operon on

Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal Repressible enzymes usually function in anabolic pathways; their synthesis is repressed by high levels of the end product Regulation of the trp and lac operons involves

negative control of genes because operons are switched off by the active form of the repressor Positive Gene Regulation When glucose and lactose both present in its environment, E. coli preferentially uses glucose.

Enzymes for glycolysis are continually present Only when lactose is present and glucose is in short supply only than E. coli synthesize appreciable quantity of the enzymes for lactose breakdown. How does E. coli cell sense the glucose

concentration? The mechanism depends on the interaction of an allosteric regulatory protein with a small organic molecule, in this case it is cyclic AMP (cAMP), which accumulates when glucose is in short supply. The regulatory protein, called catabolic

activator protein (CAP), is an activator, a protein that bind with DNA and stimulate transcription of a gene. When glucose levels increase, CAP detaches from the lac operon, and transcription returns to a normal rate

CAP helps regulate other operons that encode enzymes used in catabolic pathways Eukaryotic gene expression is regulated at many stages All organisms must regulate which genes are expressed at any given time

In multicellular organisms regulation of gene expression is essential for cell specialization in multicellular organisms. Almost all the cells in an organism are genetically identical Eukaryotic gene expression is regulated at

many stages 1. Differential gene expression 2.Histone Modification a. histone acetylation b. DNA methylation

3. Epigenetic inhetitance Differential Gene Expression Differences between cell types result from differential gene expression, the expression of different genes by cells with the same genome A typical human sell might express about 20%

of its protein coding genes at any given time. Function of any cell depends on the appropriate set of genes being expressed The transcription factors of a cell must locate the right gene at the right time. When gene expression proceeds abnormally, serious imbalances and diseases, including

cancer, can arise Gene Expression is regulated at many stages. Regulation of Chromatin Structure The structural organization of chromatin not only packs a cells DNA into a compact form that fit inside the nucleus, but also help in gene expression

in many ways. Genes within heterochromatin, what is highly condensed, are usually not expressed Certain chemical modifications to the histone proteins and to the DNA of chromatin influence both chromatin structure and gene expression

Regulation of Chromatin Structure Genes within highly packed heterochromatin are usually not expressed Chemical modifications to histones and DNA of chromatin influence both chromatin structure and gene expression

Histone Modifications Histones play a direct role in the regulation of gene transcription. The N-terminus of each histone molecule protrudes outside from the nucleosome. In histone acetylation, acetyl groups are attached to positively charged lysines in

histone tails This loosens chromatin structure, thereby promoting the initiation of transcription The addition of methyl groups (methylation) can condense chromatin; the addition of phosphate groups (phosphorylation) next to a

methylated amino acid can loosen chromatin DNA Methylation DNA methylation, the addition of methyl groups to certain bases in DNA, is associated with reduced transcription in some species DNA methylation can cause long-term

inactivation of genes in cellular differentiation In genomic imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development Epigenetic Inheritance

Although the chromatin modifications just discussed do not alter DNA sequence, they may be passed to future generations of cells The inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance

Regulation of Transcription Initiation Chromatin-modifying enzymes provide initial control of gene expression by making a region of DNA either more or less able to bind the transcription machinery Associated with most eukaryotic genes are

multiple control elements, segments of noncoding DNA that serve as binding sites for transcription factors that help regulate transcription Control elements and the transcription factors they bind are critical to the precise regulation of gene expression in different cell types

The Roles of Transcription Factors To initiate transcription, eukaryotic RNA polymerase requires the assistance of proteins called transcription factors General transcription factors are essential for the transcription of all protein-coding genes

In eukaryotes, high levels of transcription of particular genes depend on control elements interacting with specific transcription factors Enhancers and Specific Transcription Factors Some control elements, named proximal control

elements are located close to the promoter The more distant control elements Distal control elements, groupings of which are called enhancers, may be far away from a gene or even located in an intron A given gene can have multiple enhancers, each active at a different time or in a different cell

types or location. The rate of gene expression can be strongly increased or decreased by the binding of specific transcription factor, either ACTIVATORS OR REPOESSORS. An activator is a protein that binds to an

enhancer and stimulates transcription of a gene Activators have two domains, one that binds DNA and a second that activates transcription Bound activators facilitate a sequence of protein-protein interactions that result in

transcription of a given gene Some transcription factors function as repressors, inhibiting expression of a particular gene by a variety of methods Some activators and repressors act indirectly by influencing chromatin structure to promote

or silence transcription Combinatorial Control of Gene Activation A particular combination of control elements can activate transcription only when the appropriate activator proteins are present

Coordinately Controlled Genes in Eukaryotes Unlike the genes of a prokaryotic operon, each of the co-expressed eukaryotic genes has a promoter and control elements These genes can be scattered over different chromosomes, but each has the same combination of control elements Copies of the activators recognize specific

control elements and promote simultaneous transcription of the genes Nuclear Architecture and Gene Expression Loops of chromatin extend from individual chromosomes into specific sites in the nucleus Loops from different chromosomes may

congregate at particular sites, some of which are rich in transcription factors and RNA polymerases These may be areas specialized for a common function Mechanisms of Post-Transcriptional

Regulation Transcription alone does not account for gene expression Regulatory mechanisms can operate at various stages after transcription Such mechanisms allow a cell to fine-tune gene expression rapidly in response to

environmental changes How Cell Can Regulate Gene Expression?

1. RNA Processing 2. mRNA Degradation 3. Initiation of Translation 4. Protein Processing & Degradation RNA Processing

One example of regulation at the RNAprocessing level is alternative RNA splicing. In alternative RNA splicing, different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns

mRNA Degradation The life span of mRNA molecules in the cytoplasm is a key to determining protein synthesis Eukaryotic mRNA is more long lived than prokaryotic mRNA Nucleotide sequences that influence the

lifespan of mRNA in eukaryotes reside in the untranslated region (UTR) at the 3 end of the molecule Initiation of Translation The initiation of translation of selected mRNAs can be blocked by regulatory proteins

that bind to sequences or structures of the mRNA Alternatively, translation of all mRNAs in a cell may be regulated simultaneously For example, translation initiation factors are simultaneously activated in an egg following fertilization

Protein Processing and Degradation After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control Proteasomes are giant protein complexes that bind protein molecules and degrade them

Noncoding RNAs play multiple roles in controlling gene expression Only a small fraction of DNA codes for proteins, and a very small fraction of the nonprotein-coding DNA consists of genes for RNA such as rRNA and tRNA A significant amount of the genome may be

transcribed into noncoding RNAs (ncRNAs) Noncoding RNAs regulate gene expression at two points: mRNA translation and chromatin configuration Effects on mRNAs by MicroRNAs and Small Interfering RNAs

MicroRNAs (miRNAs) are small singlestranded RNA molecules that can bind to mRNA These can degrade mRNA or block its translation The phenomenon of inhibition of gene expression by RNA molecules is called RNA

interference (RNAi) RNAi is caused by small interfering RNAs (siRNAs) siRNAs and miRNAs are similar but form from different RNA precursors Chromatin Remodeling and Effects on

Transcription by ncRNAs In addition to affecting mRNAs, small RNAs can cause remodeling of chromatin structure In some yeasts siRNAs play a role in heterochromatin formation and can block large regions of the chromosome Newly discovered Small ncRNAs is called piwiassociated RNAs (piRNAs) induce heterochromatin,

blocking the expression of parasitic DNA elements in the genome, known as transposons In some yeasts siRNAs play a role in heterochromatin formation and can block large regions of the chromosome Small ncRNAs called piwi-associated RNAs

(piRNAs) induce heterochromatin, blocking the expression of parasitic DNA elements in the genome, known as transposons RNA-based mechanisms may also block transcription of single genes The Evolutionary Significance of Small

ncRNAs Small ncRNAs can regulate gene expression at multiple steps An increase in the number of miRNAs in a species may have allowed morphological complexity to increase over evolutionary time siRNAs may have evolved first, followed by miRNAs and later piRNAs

A program of differential gene expression leads to the different cell types in a multicellular organism During embryonic development, a fertilized egg gives rise to many different cell types Cell types are organized successively into

tissues, organs, organ systems, and the whole organism Gene expression orchestrates the developmental programs of animals A Genetic Program for Embryonic Development

The transformation from zygote to adult results from: Cell Division Cell differentiation and Morphogenesis (The physical processes that give an organism its shape ) All three processes have their basis in cellular

behavior. It take just four days for : cell division Cell Differentiation And

Morphogenesis to transform each of the fertilizes frog egg into a tedpole. Cell differentiation is the process by which cells become specialized in structure and

function The physical processes that give an organism its shape constitute morphogenesis Differential gene expression results from genes being regulated differently in each cell type Materials in the egg can set up gene

regulation that is carried out as cells divide Cytoplasmic Determinants and Inductive Signals An eggs cytoplasm contains: RNA, proteins, and other substances that are

distributed unevenly in the unfertilized egg Cytoplasmic determinants are maternal substances in the egg that influence early development As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which

lead to different gene expression The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells

Thus, interactions between cells induce differentiation of specialized cell types Sequential Regulation of Gene Expression During Cellular Differentiation Determination commits a cell to its final fate This term was coined before biologist to refer

that stage in which earliest changes set a cell on a path of specliztion. Determination precedes differentiation Cell differentiation is marked by the production of tissue-specific proteins Muscle cells develop from embryonic cells, under

particular condition commit them to become muscle cells. The committed cells appear unchanged under microscope, determination has occoured. Myoblasts produce muscle-specific proteins and form skeletal muscle cells MyoD is one of several master regulatory genes

that produce proteins that commit the cell to becoming skeletal muscle The MyoD protein is a transcription factor that binds to enhancers of various target genes Pattern Formation: Setting Up the Body Plan Cytoplasmic determinants and inductive

signals both contribute to the development of a spatial organization in which the tissues and organs of an organism are all in their characteristic places. This process is called PATTERN FORMATION Positional information, the molecular cues

that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster Combining anatomical, genetic, and biochemical approaches, researchers have

discovered developmental principles common to many other species, including humans The Life Cycle of Drosophila In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization

After fertilization, the embryo develops into a segmented larva with three larval stages Genetic Analysis of Early Development Lewis discovered the homeotic genes, which control pattern formation in late embryo, larva, and adult stages

Cancer results from genetic changes that affect cell cycle control The gene regulation systems that go wrong during cancer are the very same systems involved in embryonic development Cancer can be caused by mutations to genes that regulate cell growth and division

Tumor viruses can cause cancer in animals including humans Oncogenes are cancer-causing genes Proto-oncogenes are the corresponding normal cellular genes that are responsible for normal cell growth and division

Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle Proto-oncogenes can be converted to oncogenes by Movement of DNA within the genome: if it ends up near an active promoter, transcription may increase

Amplification of a proto-oncogene: increases the number of copies of the gene Point mutations in the proto-oncogene or its control elements: cause an increase in gene expression Tumor-Suppressor Genes Tumor-suppressor genes help prevent uncontrolled

cell growth Mutations that decrease protein products of tumorsuppressor genes may contribute to cancer onset Tumor-suppressor proteins Repair damaged DNA Control cell adhesion Inhibit the cell cycle in the cell-signaling pathway

Interference with Normal Cell-Signaling Pathways Mutations in the ras proto-oncogene and p53 tumor-suppressor gene are common in human cancers Mutations in the ras gene can lead to production of a hyperactive Ras protein and

increased cell division Suppression of the cell cycle can be important in the case of damage to a cells DNA; p53 prevents a cell from passing on mutations due to DNA damage Mutations in the p53 gene prevent

suppression of the cell cycle The Multistep Model of Cancer Development Multiple mutations are generally needed for full-fledged cancer; thus the incidence increases with age At the DNA level, a cancerous cell is usually

characterized by at least one active oncogene and the mutation of several tumor-suppressor genes Inherited Predisposition and Other Factors Contributing to Cancer Individuals can inherit oncogenes or mutant

alleles of tumor-suppressor genes Inherited mutations in the tumor-suppressor gene adenomatous polyposis coli are common in individuals with colorectal cancer Mutations in the BRCA1 or BRCA2 gene are found in at least half of inherited breast cancers, and tests using DNA sequencing can detect these mutations

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