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Chapter 11 How Genes Are Controlled PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko Regulation of Eukaryotic Gene Expression Prokaryotes and eukaryotes employ regulatory proteins that – bind to specific segments of DNA and – either promote or block the binding of RNA polymerase, turning the transcription of genes on and off. In eukaryotes, activator proteins seem to be more important than repressors. Thus, the default state for most genes seems to be off. A typical plant or animal cell needs to turn on and transcribe only a small percentage of its genes. © 2012 Pearson Education, Inc. Regulation of Eukaryotic Gene Expression Eukaryotes also need to regulate gene expression during embryonic development Differentiation = cell specialization, in structure and function – is controlled by turning specific sets of genes on or off. Almost all of the cells in an organism contain an identical genome. The differences between cell types are due to selective gene expression. © 2012 Pearson Education, Inc. Figure 11.12 Root of carrot plant Single cell Root cells cultured in growth medium Cell division in culture Plantlet Adult plant Figure 11.7 Chromosome Chromosome DNA unpacking Other changes to the DNA DNA Gene Gene Transcription Exon RNA transcript Intron Addition of a cap and tail Splicing Tail Cap mRNA in nucleus Flow through NUCLEUS nuclear envelope CYTOPLASM mRNA in cytoplasm Breakdown of mRNA Brokendown mRNA Translation Polypeptide Polypeptide Cleavage, modification, activation Active protein Active protein Breakdown of protein Amino acids DNA Packaging and Modification Eukaryotic chromosomes undergo multiple levels of folding and coiling – Nucleosomes are formed when DNA is wrapped around histone proteins. DNA double – helix Each (2-nm diameter) nucleosome bead includes DNA plus eight histones. Metaphase chromosome Nucleosome (10-nm diameter) Tight helical fiber (30-nm diameter) Linker “Beads on a string” Histones Supercoil (300-nm diameter) 700 nm DNA Packaging and chemical modifications can affect gene expression DNA packing can prevent gene expression by preventing RNA polymerase from contacting the DNA. More tightly compacted DNA - less likely DNA will be transcribed Euchromatin - transcriptional active regions of DNA Heterochromation - transcriptional silent (highly compacted) Animation: DNA Packing © 2012 Pearson Education, Inc. DNA Packaging and chemical modifications can affect gene expression Methylation of DNA – Certain enzymes can add a methyl group to DNA bases, without changing the sequence of the bases. – Methylation generally inhibits gene expression – Example: X-Chromosome inactivation © 2012 Pearson Education, Inc. DNA packaging and chemical modifications can affect gene expression X-chromosome inactivation – In female mammals, one of the two X chromosomes is highly compacted and transcriptionally inactive. – An inactivated X chromosome is called a Barr body. © 2012 Pearson Education, Inc. Figure 11.2B Early Embryo Adult Two cell populations X chromosomes Allele for orange fur Cell division and random X chromosome Active X inactivation Inactive X Allele for black fur Inactive X Active X Orange fur Black fur Figure 11.7 Chromosome Chromosome DNA unpacking Other changes to the DNA DNA Gene Gene Transcription Exon RNA transcript Intron Addition of a cap and tail Splicing Tail Cap mRNA in nucleus Flow through NUCLEUS nuclear envelope CYTOPLASM mRNA in cytoplasm Breakdown of mRNA Brokendown mRNA Translation Polypeptide Polypeptide Cleavage, modification, activation Active protein Active protein Breakdown of protein Amino acids Regulation of Transcription Eukaryotic RNA polymerase requires the assistance of proteins called transcription factors. Transcription factors recruit RNA polymerase to gene’s promoter Activator proteins, which bind to DNA sequences called enhancers and help increase rate of gene transcription. Silencers are repressor proteins that – may bind to DNA sequences and – inhibit transcription. Animation: Initiation of Transcription © 2012 Pearson Education, Inc. Figure 11.3 Enhancers Promoter Gene DNA Activator proteins Transcription factors Other proteins RNA polymerase Bending of DNA Transcription Figure 11.7 Chromosome Chromosome DNA unpacking Other changes to the DNA DNA Gene Gene Transcription Exon RNA transcript Intron Addition of a cap and tail Splicing Tail Cap mRNA in nucleus Flow through NUCLEUS nuclear envelope CYTOPLASM mRNA in cytoplasm Breakdown of mRNA Brokendown mRNA Translation Polypeptide Polypeptide Cleavage, modification, activation Active protein Active protein Breakdown of protein Amino acids Regulation of mRNA Processing Alternative RNA splicing Animation: RNA Processing – produces different mRNAs from the same transcript, – results in the production of more than one polypeptide from the same gene Exons 1 DNA 2 4 3 Introns RNA transcript 5 Introns Tail Cap 1 2 5 4 3 RNA splicing or mRNA © 2012 Pearson Education, Inc. 1 2 3 5 1 2 4 5 Figure 11.7 Chromosome Chromosome DNA unpacking Other changes to the DNA DNA Gene Gene Transcription Exon RNA transcript Intron Addition of a cap and tail Splicing Tail Cap mRNA in nucleus Flow through NUCLEUS nuclear envelope CYTOPLASM mRNA in cytoplasm Breakdown of mRNA Brokendown mRNA Translation Polypeptide Polypeptide Cleavage, modification, activation Active protein Active protein Breakdown of protein Amino acids 11.6 Later stages of gene expression are also subject to regulation After mRNA is fully processed and transported to the cytoplasm, gene expression can still be regulated by – breakdown of mRNA, – initiation of translation, Animation: Blocking Translation Animation: mRNA Degradation – protein activation, and Animation: Protein Degradation – protein breakdown. Animation: Protein Processing © 2012 Pearson Education, Inc. Figure 11.7 Chromosome Chromosome DNA unpacking Other changes to the DNA DNA Gene Gene Transcription Exon RNA transcript Intron Addition of a cap and tail Splicing Tail Cap mRNA in nucleus Flow through NUCLEUS nuclear envelope CYTOPLASM mRNA in cytoplasm Breakdown of mRNA Brokendown mRNA Translation Polypeptide Polypeptide Cleavage, modification, activation Active protein Active protein Breakdown of protein Amino acids Figure 11.6 Folding of the polypeptide and the formation of S—S linkages Cleavage S S Initial polypeptide (inactive) Folded polypeptide (inactive) S S Active form of insulin 11.7 Review: Multiple mechanisms regulate gene expression in eukaryotes These controls points include: 1. chromosome changes and DNA unpacking, 2. control of transcription, 3. control of RNA processing including the – addition of a cap and tail and – splicing, 4. flow through the nuclear envelope, 5. breakdown of mRNA, © 2012 Pearson Education, Inc. 11.7 Review: Multiple mechanisms regulate gene expression in eukaryotes 6. control of translation, and 7. control after translation including – cleavage/modification/activation of proteins and – breakdown of protein. © 2012 Pearson Education, Inc.