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Ch. 16 Regulation of Gene Expression Learning objectives After this lecture you should be able to… • Describe how gene expression is regulated in proand eukaryotes emphasizing the many different levels this can be achieved on. • Account for the mechanisms for interbacterial transfer of genetic material and how a virus reproduces itself by the use of the host reproductive system. Disposition Virus (including bacteriophages) lytic <> lysogenic cell cycle HIV - a retrovirus that infects humans Bacteria The lac operon Eukaryotes, regulation On chromosome level At transcription initiation By alternative splicing By mRNA degradation By protein degradation Virus Genome: ssDNA dsDNA ssRNA dsRNA They have no cell membranes and don t regulate uptake / excretion of substances. They don t carry out metabolic processes. Capsule of protein (rarely lipids) They cannot reproduce independently of a host cell Can we claim that viruses are “life”? Bacteriophages virus that infects bacteria Bakteriofag T2 E. coli Bacteriophage T4 infecting E. coli cells The lytic life cycle The lysogenic life cycle HIV Retrovirus • The genome is ssRNA • Surface glycoproteins recognize the CD4 receptor on host cells • Inside the host cell, reverse transcriptase generates DNA (cDNA) based on the viral RNA • Integrase integrates the cDNA in the host cell DNA = provirus • Upon activation, the provirus genes are transcribed and translated • Host terminator protein binds viral DNA making the RNA polymerase fall of. HIV tat protein binds the terminator protein and inhibits the interaction • New virus particles are generated by viral RNA and proteins and the host cell s membrane. HIV drugs 1) Co-receptor inhibitors 2) Reverse transcriptase inhibitors 3) Protease inhibitors 4) Integrase inhibitors Loss of facial fat Gain of abdominal fat Regulating Gene Expression in Bacteria Adapting to changing environments Example: The lac operon The lac operon RNA polymerase Lactose Lac genes Adapted from Gonick og Wheelis The Cartoon Guide to Genetics The lac operon A more silly variant of Figure 16.11… Adapted from Gonick and Wheelis ”The Cartoon Guide to Genetics” More on the lac operon... Glucose is also involved in the regulation of trancription from the lac operon: Low glucose level => High cAMP level => cAMP binds the cAMP receptor protein (CRP) => CRP binds the promoter of the lac operon => Increased RNA polymerase attraction to the promoter => increased transcription of the lac genes. High glucose level = > Low cAMP level => cAMP doesn t bind CRP => CRP doesn t bind promoter => RNA polymerase doesn t bind promoter => No transcription of the lac genes. Imagine an E. coli in surroundings with plenty of glucose and lactose. How is the level of transcription from the lac operon? Is lactose degraded? Regulating Gene Expression in Multi-Cellular Organisms Same genome – different expression at different stages Same genome – different expression in different places Chromatin (DNA+histones) vsrp.uhnres.utoronto.ca/research1.htm Euchromatin: Lightly packed chromatin. The genes are transcribed. Heterochromatin: Tightly packed chromatin. The genes are normally not transcribed. Remodelling of chromatin structure before transcription Gene expression can be regulated here Barr body – one of the X chromosomes of women are inactivated as heterochromatin During the development of a female embryo, one of the two X chromosomes are inactivated. The female body is a mosaic, where some areas contain cells in which one X chromosome is inactivated, while cells in other areas have the other X chromosome inactivated. XX XX XX XX XX X X X X X X X X X X X X XX X X X X X X © Sinauer Associations, Inc. The timing of the decision on which X chromosome is inactivated determines how large areas with orange / black coat is ... BREAK Epigenetics Epigenetics: The study of changes in the level of expression of genes or sets of genes that are not caused by changes in the DNA sequence (but, e.g., DNA methylation). Epigenetic changes can be inherited by the offspring! Caused by: - Methylation of cytosin - Changes of the chromosomal proteins (histone proteins) Cytosin 5 -Methylcytosin Lamarck Heritability of acquired characteristics. Darwin and natural selection Initiation of trancription Here gene expression can be regultated More on initiation of transcription RNA polymerase II Activator Enhancer Repressor Silencer Repressor Activator TFIID Silencer Enhancer Initiation of transcription Promoter The same transcription factor can bind/regulate several genes! Gene expression can be regulated here The MAOA gene (involved in aggression) - Multiple repeats of 30 bp are present in the promoter of the MAOA gene - Different alleles have different number of repeats - The level of transcription from the MAOA gene varies depending on the allele Fra Sabol et al, Hum Genet (1998); 103(3):273-9. - For men, the alleles with a low level of transcription is associated with an aggressive and antisocial behavior, especially if the men had a bad childhood or drink too much. Structure of eukaryotic proteinencoding genes Start codon Exons: Code for amino acids Stop codon Introns: Code for nothing Promoter – Technically not part of the gene Here RNA polymerase II binds Terminator Signals to the RNA polymerase to stop transcribing © Sinauer Associations, Inc. Animated Tutorial 14.3 RNA Splicing Alternative Splicing Exon1 Intron1 Exon2 Intron2 Exon3 Pre-mRNA Mature mRNA Gene expression can be regulated here MicroRNA – regulation mRNA longevity • Small RNA molecules, ≈ 22 to 23 nucleotides • Bind via basepairing to the 3’ end of the mRNA • Binding inhibits translation and sometimes the mRNA is even degraded • Computers predict that more than 1000 genes encode microRNA • It is also predicted that microRNA molecules regulate the expression of more than 1/3 of all human genes Gene expression can be regulated here Post translational regulation of expression Ubiquitination - or Gene expression can be regulated here Examining gene expression using micro arrays