Regulation of Gene Expression in Eukaryotes

Transcription

1 Ch. 19 Regulation of Gene Expression in Eukaryotes BIOL 222 Differential Gene Expression in Eukaryotes Signal Cells in a multicellular eukaryotic organism genetically identical differential gene expression Creates differences between cell types Cap RNA NUCLEUS Chromatin Chromatin modification Gene available for transcription Gene Transcription Exon Primary transcript Intron RNA processing Tail mrna in nucleus Transport to cytoplasm CYTOPLASM mrna in cytoplasm Degradation of mrna Translation Polypeptide Protein processing Degradation of protein Active protein Transport to cellular destination Cellular function Regulation of Chromatin Structure highly packed heterochromatin Genes are usually not expressed Chemical modifications to histones and of chromatin influence both chromatin structure and gene expression 1

2 histone acetylation Histone Modifications acetyl groups are attached to positively charged lysines in histone tails loosens chromatin structure histone methylation promoting transcription can condense chromatin Can cause long term gene inactivation Leading to altered cell differentiation double helix (a) Histone tails protrude outward from a nucleosome Histone tails Amino acids available for chemical modification phosphorylation next to a methylated amino acid can loosen chromatin Unacetylated histones Acetylated histones (b) Acetylation of histone tails promotes loose chromatin structure that permits transcription Epigenetic Inheritance epigenetic inheritance traits transmitted by mechanisms not directly involving the nucleotide sequence Due to methylation, acetylation, or phosphorylation Organization of a Typical Eukaryotic Gene control elements segments of noncoding help regulate transcription by binding certain proteins critical to the precise regulation of gene expression in different cell types Enhancer (distal control elements) Proximal control elements Poly-A signal sequence Termination region Upstream Promoter Exon Intron Exon Intron Transcription Exon Downstream Primary RNA transcript 5ʹ Exon Intron Exon Intron Exon RNA processing Cleaved 3ʹ end of primary transcript Intron RNA Poly-A signal Coding segment mrna 5ʹ Cap 5ʹ UTR Start codon Stop codon 3ʹ UTR Poly-A tail 3ʹ 2

3 Enhancers and Specific Transcription Factors Proximal control elements located close to the promoter Distal control elements Activators Distal control Enhancer element Promoter TATA box Gene enhancers activator PLAY -bending protein General transcription factors Group of mediator proteins binds to an enhancer RNA polymerase II stimulates transcription of a RNA polymerase II gene Transcription initiation complex RNA synthesis RNA Processing Post-transcriptional control alternative RNA splicing different mrna molecules from the same primary transcript depending on which exons are used Primary RNA transcript mrna Exons Troponin T gene RNA splicing or mrna Degradation Life span of mrna molecules key to determining protein synthesis determined in part by sequences in the leader and trailer regions Eukaryotic mrna more long lived than prokaryotic mrna Methylated cap, Poly-A tail 3

4 Protein Processing and Degradation protein processing The cleavage and/or addition of chemical groups subject to control Proteasomes giant protein complexes bind protein molecules and degrade them Ubiquitin Proteasome Proteasome and ubiquitin to be recycled Protein to be degraded Ubiquitinated protein Protein entering a proteasome Protein fragments (peptides) Initiation of Translation Other forms of regulation include regulation of the initiation of translation of selected mrnas can be blocked by regulatory proteins in the cytosol that bind to sequences or structures of the mrna Alternatively, translation of all mrnas in a cell may be regulated simultaneously Ie. translation initiation factors are simultaneously activated in an egg following fertilization Noncoding RNAs Only a small fraction of codes for proteins, rrna, and trna A significant amount of the genome may be transcribed into noncoding RNAs Noncoding RNAs regulate gene expression at two points chromatin configuration mrna translation 4

5 MicroRNAs and Small Interfering RNAs RNA interference (RNAi) Process of inhibition of gene expression by RNA molecules Hairpin mirna Hydrogen bond caused by small interfering Dicer RNAs (sirnas) and mirna s 5ʹ 3ʹ (a) Primary mirna transcript mirna mirnaprotein complex MicroRNAs (mirnas) small single-stranded RNA molecules mrna degraded Translation blocked (b) Generation and function of mirnas bind to mrna can degrade mrna or block its translation MicroRNAs and Small Interfering RNAs sirnas and mirnas similar but form from different RNA precursors sirnas play a role in heterochromatin formation by facilitating methylation can block large regions of the chromosome may also block transcription of specific genes Differential Gene Expression and Cell Differentiation embryonic development 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 5

6 Differential Gene Expression and Cell Differentiation Cell differentiation process by which cells become specialized in structure and function morphogenesis physical processes that give an organism its shape 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 An egg s cytoplasm contains Cytoplasmic Determinants RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg Cytoplasmic determinants maternal substances in the egg that influence early development During mitosis cells contain different cytoplasmic determinants leads to different gene expression Zygote Sperm Fertilization Mitotic cell division Two-celled embryo Unfertilized egg cell Two different cytoplasmic determinants (a) Cytoplasmic determinants in the egg Nucleus Inductive Signals induction A process where signal molecules from embryonic cells cause transcriptional changes in nearby target cells Thus, interactions between cells induce differentiation of specialized cell types Early embryo (32 cells) Signal transduction pathway Signal receptor Signal molecule (inducer) (b) Induction by nearby cells NUCLEUS 6

7 Sequential Regulation of Gene Expression Determination Nucleus Embryonic precursor cell Master regulatory gene myod OFF Other muscle-specific genes OFF commits a cell to its final fate mrna OFF precedes differentiation Myoblast (determined) MyoD protein (transcription factor) Cell differentiation is marked by the production of tissue-specific proteins Part of a muscle fiber (fully differentiated cell) mrna mrna mrna mrna MyoD Another transcription factor Myosin, other muscle proteins, and cell cycle blocking proteins Types of Genes Associated with Cancer Cancer can be caused by mutations to genes that regulate cell growth and division Tumor viruses can cause cancer in animals including humans Oncogenes cancer-causing genes Proto-oncogenes corresponding normal cellular genes responsible for normal cell growth and division Conversion of a proto-oncogene to an oncogene can lead to abnormal stimulation of the cell cycle Oncogenes and Proto-Oncogenes Proto-oncogenes can be converted to oncogenes by Movement of 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: causes an increase in gene expression Proto-oncogene Translocation or transposition: Gene amplification: within a control element Point mutation: within the gene New promoter Oncogene Oncogene Normal growthstimulating protein in excess Normal growth-stimulating protein in excess Normal growthstimulating protein in excess Hyperactive or degradationresistant protein 7

8 Tumor-Suppressor Genes Tumor-suppressor genes help prevent uncontrolled cell growth Mutations that decrease protein products of tumor-suppressor genes may contribute to cancer onset Tumor-suppressor proteins Repair damaged Control cell adhesion Inhibit the cell cycle in the cell-signaling pathway The Multistep Model of Cancer Development Multiple mutations needed for cancer incidence increases with age A cancerous cell is characterized by at least one active oncogene and the mutation of several tumor-suppressor genes Colon EFFECTS OF MUTATIONS 1 Loss of tumorsuppressor gene Colon wall APC (or other) 2 Activation of ras oncogene 4 Loss of tumor-suppressor gene p53 Normal colon epithelial cells Small benign growth (polyp) 3 Loss of 5 tumor-suppressor gene DCC Larger benign growth (adenoma) Additional mutations Malignant tumor (carcinoma) You should now be able to: 1. Explain the concept of an operon and the function of the operator, repressor, and corepressor 2. Explain the adaptive advantage of grouping bacterial genes into an operon 3. Explain how repressible and inducible operons differ and how those differences reflect differences in the pathways they control 4. Explain how methylation and histone acetylation affect chromatin structure and the regulation of transcription 5. Define control elements and explain how they influence transcription 6. Explain the role of promoters, enhancers, activators, and repressors in transcription control 8

9 7. Explain how eukaryotic genes can be coordinately expressed 8. Describe the roles played by small RNAs on gene expression 9. Explain why determination precedes differentiation 10. Describe two sources of information that instruct a cell to express genes at the appropriate time 11. Explain how mutations in tumor-suppressor genes can contribute to cancer 9