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Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Differential Gene Expression Almost all the cells in an organism are genetically.

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Presentation on theme: "Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Differential Gene Expression Almost all the cells in an organism are genetically."— Presentation transcript:

1 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Differential Gene Expression Almost all the cells in an organism are genetically identical Differences between cell types result from differential gene expression, the expression of different genes by cells with the same genome Errors in gene expression can lead to diseases including cancer Gene expression is regulated at many stages

2 Fig DNA Signal Gene NUCLEUS Chromatin modification Chromatin Gene available for transcription Exon Intron Tail RNA Cap RNA processing Primary transcript mRNA in nucleus Transport to cytoplasm mRNA in cytoplasm Translatio n CYTOPLASM Degradation of mRNA Protein processing Polypeptide Active protein Cellular function Transport to cellular destination Degradation of protein Transcription

3 Fig. 18-6a DNA Signal Gene NUCLEUS Chromatin modification Chromatin Gene available for transcription Exon Intron Tail RNA Cap RNA processing Primary transcript mRNA in nucleus Transport to cytoplasm CYTOPLASM Transcription

4 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Overview: Conducting the Genetic Orchestra 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

5 Fig. 18-6b mRNA in cytoplasm Translation CYTOPLASM Degradation of mRNA Protein processing Polypeptide Active protein Cellular function Transport to cellular destination Degradation of protein

6 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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

7 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Histone Modifications In histone acetylation, acetyl groups are attached to positively charged lysines in histone tails This process 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 Animation: DNA Packing Animation: DNA Packing

8 Fig Histone tails DNA double helix (a) Histone tails protrude outward from a nucleosome Acetylated histones Amino acids available for chemical modification (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription Unacetylated histones

9 X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated by DNA methylation during embryonic development The inactive X condenses into a Barr body If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10 Fig X chromosomes Early embryo: Allele for orange fur Allele for black fur Cell division and X chromosome inactivation Two cell populations in adult cat: Active X Inactive X Black furOrange fur

11 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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

12 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Proximal control elements on the DNA are located close to the promoter Distal control elements, groups of which are called enhancers, may be far away from a gene or even located in an intron Transcription Factors

13 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings An activator is a protein that binds to an enhancer and stimulates transcription of a gene Bound activators cause mediator proteins to interact with proteins at the promoter Animation: Initiation of Transcription Animation: Initiation of Transcription

14 Fig Enhancer TATA box Promoter Activators DNA Gene Distal control element Group of mediator proteins DNA-bending protein General transcription factors RNA polymerase II RNA polymerase II Transcription initiation complex RNA synthesis

15 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Some transcription factors function as repressors, inhibiting expression of a particular gene Some activators and repressors act indirectly by influencing chromatin structure to promote or silence transcription

16 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A particular combination of control elements can activate transcription only when the appropriate activator proteins are present Combinatorial Control of Gene Activation

17 Fig Control elements Enhancer Available activators Albumin gene (b) Lens cell Crystallin gene expressed Available activators LENS CELL NUCLEUS LIVER CELL NUCLEUS Crystallin gene Promoter (a) Liver cell Crystallin gene not expressed Albumin gene expressed Albumin gene not expressed

18 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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

19 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings RNA Processing 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 Animation: RNA Processing Animation: RNA Processing

20 Fig or RNA splicing mRNA Primary RNA transcript Troponin T gene Exons DNA

21 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 The mRNA life span is determined in part by sequences in the leader and trailer regions Animation: mRNA Degradation Animation: mRNA Degradation

22 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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 Animation: Blocking Translation Animation: Blocking Translation

23 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Protein Processing and Degradation After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control Proteins destined for degradation are Ubiquinated and sent for degradation either to a lysosome or Proteasomes which are giant protein complexes that bind protein molecules and degrade them Animation: Protein Degradation Animation: Protein Degradation Animation: Protein Processing Animation: Protein Processing

24 Fig Proteasome and ubiquitin to be recycled Proteasome Protein fragments (peptides) Protein entering a proteasome Ubiquitinated protein Protein to be degraded Ubiquitin

25 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 18.4: 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

26 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings A Genetic Program for Embryonic Development The transformation from zygote to adult results from cell division, cell differentiation, and morphogenesis

27 Fig (a) Fertilized eggs of a frog (b) Newly hatched tadpole

28 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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


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