2Some rats are genetically programmed to prefer alcohol to water UnderstandingGene expressionCan help alcoholicsget effectiveTreatment orPrevent it altogether.
3Review Level of protein produced varies DNA RNA ProteinLevel of protein produced variesInfluenced by environmental conditions and the developmental stage of cell
4Gene expression is precisely regulated. ExamplesWhen an extracellular signal binds to its receptor, it sets in motion a signal transduction pathway that may end in some genes being activated and some being repressed.During cell cycle, cyclins are synthesized only at specific points.What does this mean?Gene expression is precisely regulated.
5So basically, there are 2 types of genes In some cases:Gene expression is modified to counteract changes in environment to help maintain homeostasisIn other cases, gene expression changes so that the cell can perform a specific function.For example, all of our cells carry the genes to encode keratin (protein in hair) and hemoglobin.But keratin is only made in specialized epithelial cells and hemoglobin is only made by developing RBC’sIn contrast, all cells express the genes that encode for enzymes for metabolismSo basically, there are 2 types of genes
6Two types of genes Our focus Constitutive genes Inducible genes Actively expressed all the timeInducible genesExpressed only when their proteins are needed by the cellOur focus
7Genes are subject to positive and negative regulation Positive regulation – there is a binding of an activator that stimulates transcriptionNegative regulation – there is a binding of a repressor, that prevents transcriptionActivators and repressorsAreTranscription factors!Let’s look at mechanisms of:Viruses, bacteria and eukaryotes
8What are viruses?Non living particle that can only reproduce within a host cellNot cellularCan have ds DNA, ssDNA, dsRNA or ssRNATakes over the host cells protein synthesis machinery within minutes of entering the host2 types of reproductive cyclesLyticLysogenic
9Lytic Cycle Lytic – “break”, meaning host cell is destroyed afterwards 6 stepsBacteriophage (virus infecting a bacteria cell) infects a host cell – viral DNA entersIt uses the bacterium’s RNA polymerase to transcribe early genesOne early protein shuts down host (bacterial) gene transcriptionAnother protein stimulates viral genome replicationAnother protein stimulates late gene transcriptionNew viral capsid proteins and a protein lyses the host cell
11HIV Review Human immunodeficiency virus Typically infects only cells of the immune system that express a surface receptor called CD4.Proteins on the membrane are involved in the infection of new cells, which HIV enters by direct fusion of the viral envelope with the host plasma membraneIt is a retrovirus
12Retrovirus Genome is single stranded RNA Carries an enzyme, reverse transcriptase that makes a DNA strand that is complementary to the RNA, while at the same time degrading the RNA and making a second DNA strand that is complementary to the firstThe resulting dsDNA becomes integrated into the host’s chromosome, where it resides and the virus can become dormant.Eventually cellular triggers result and stimulates transcription of the viral DNA, resulting in mRNAs that are translated into viral proteins, and in new copies of the viral genome
13Negative regulation of HIV Normally a host cell has a negative regulatory system that can repress the expression of invading viral genes.However HIV can counteract this with a virus-encoded protein called Tat (Transactivator of transcription)Tat binds to the viral mRNA along with proteins that allow RNA polymerase to transcribe the viral genome.
16OperonCluster of genes with a single promotor that code for proteins in the DNA of bacteriaCodes for 3 lactose-metabolizing enzymes in E. ColiCalled the lac operonExample of negative regulation4 major components
17Component #1 Promotor region Region of DNA to which the RNA polymerase attaches to begin transcription
18Component #2 Structural genes Contain DNA sequences that code for several enzymes
19Component #3OperatorLocated between the promotor and the structural genesRegion of DNA that is able to control RNA polymerase’s access to structural genesIt’s like a switch that can turn the operon on or off
20Component #4 Repressor protein A substance that can prevent gene expression by binding to the operator and prevents RNA polymerase from transcribing the structural genesTranscription would resume when the repressor is removed by a molecule called an inducer
21Background Information When you consume milk, the disaccharide lactose is soon present in your intestinal tract and available to the E. Coli living there.Before E. Coli can absorb lactose, it must first make beta-galactosidase, the enzyme that breaks down lactose into glucose and galactose.
22Lactose 3 enzymes are needed to metabolize lactose: B-galactoside permease – moves sugar into cellB-galactosidase – hydrolyzes lactose to glucose and galactoseB-galactoside transacetylase – transfers acetyl groups from acetyl CoA to certain B-galactosides. Role is unclear.
23Therefore E. Coli should only make the enzyme when lactose is present. It is in E. Coli’s best interest to focus its energy on using available nutrientsTherefore E. Coli should only make the enzyme when lactose is present.How does E. Coli do this?By an operon!
24So if lactose is absent: RNA polymerasestructural genesthat aren'ttranscribedRepressor binds to operator and it prevents RNA polymerase from binding to promotor, so transcription is blocked.No mRNA is produced, so no enzyme is produced.
25And if lactose is present: Lactose binds to repressor. The repressor becomes inactive!Binds to promotorOperator is free so transcription takes place and the enzymes are made!!
26Now let’s look at A repressible Operon, trp operon Inducible SystemLac operon is called an inducible systemAllolactose (alternative form of lactose) is the inducer and it leads to the synthesis of enzymes in the lactose-metabolizing pathway by binding to the repressor protein and preventing its binding to the operator.Now let’s look atA repressibleOperon, trp operon
27Trp Operon Trp – tryptophan, an amino acid A co-repressor is involved. It is a molecule that binds to the repressor, causing it to change shape and bind to the operator, thereby inhibiting transcription.When tryptophan is adequately present in the cell, it is energy efficient to stop making the enzymes for tryptophan synthesisTherefore tryptophan functions as a co-repressor and binds to the repressor of the trp operon!This causes the repressor to bind to the trp operator to prevent transcription.
30Repressible System Trp operon The product of a metabolic pathway (the co-repressor) binds to the repressor protein, which is then able to bind to the operator and block transcription.
31Eukaryotic CellsCan also regulate the transcription of large stretches of DNA (containing many genes) by reversible, non-sequence-specific alterations to either the DNA or the chromosomal proteinsThese alterations can be passed on to daughter cells after mitosis or meiosisAre called Epigenetic changes (not mutations)
32DNA Methylation1-5% of cytosines in the DNA are chemically modified by the addition of a methyl group to form 5-methyl-cytosine.Catalyzed by the enzyme DNA methyltransferaseUsually occurs when C’s that are adjacent to G’sAreas rich in the methylation are called CpG islands, and are abundant in promotors
33DNA Methylation continued… This change in DNA is heritableWhen DNA is replicated, an enzyme called maintenance methylase catalyzes the formation of 5-methylcytosine in the new DNA strandBut it is reversible by demethylase, which catalyzes the removal of the methyl group from cytosine.Methylated DNA binds specific proteins that are involved in the repression of transcription; thus heavily methylated genes tend to be inactive