Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Gene Regulation results in differential Gene Expression, leading to
Document related concepts
no text concepts found
Transcript
GENE REGULATION RESULTS IN DIFFERENTIAL GENE EXPRESSION, LEADING TO CELL SPECIALIZATION Eukaryotic DNA Epigenetics – TED Talk Watch to see how your environment and your choices influence inheritance… http://youtu.be/JTBg6hqeuTg Differential Gene Expression Nucleosome Packing: DNA wraps around histone proteins to form a structure called a nucleosome. Nucleosomes help pack DNA into eukaryotic chromosomes. When acetyl groups attach to the histone proteins the DNA in chromosomes loosens to allow for transcription. The addition of methyl groups to histone proteins can cause DNA to condense thus preventing transcription. In Genomic Imprinting, methylation regulates expression of either the maternal or paternal alleles of certain genes at the start of development. Fig. 18-8-3 Organization of Typical Eukaryotic Genes Enhancer (distal control elements) Poly-A signal sequence Proximal control elements Termination region Exon Intron Exon Intron Exon DNA Upstream Promoter Primary RNA 5 transcript Downstream Transcription Exon Intron Exon Intron Exon RNA processing Cleaved 3 end of primary transcript Poly-A signal Intron RNA Coding segment mRNA 3 5 Cap 5 UTR Start codon Stop codon 3 UTR Poly-A tail 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 © 2011 Pearson Education, Inc. An activator is a protein that binds to an enhancer and stimulates transcription of a gene Activators have two domains, one that binds DNA and a second that activates transcription Bound activators facilitate a sequence of proteinprotein interactions that result in transcription of a given gene © 2011 Pearson Education, Inc. Some transcription factors function as repressors, inhibiting expression of a particular gene by a variety of methods Some activators and repressors act indirectly by influencing chromatin structure to promote or silence transcription © 2011 Pearson Education, Inc. The Roles of Transcription Factors RNA polymerase requires the assistance of transcription factors in order to transcribe DNA into RNA. Regulatory Proteins, repressors and activators, operate similarly to those in prokaryotes, influencing how readily RNA polymerase will attach to a promoter region. In many cases, numerous activators are acting in concert to influence transcription. Fig. 18-9-3 Promoter Activators DNA Enhancer Distal control element Gene TATA box General transcription factors DNA-bending protein Group of mediator proteins RNA polymerase II RNA polymerase II Transcription initiation complex RNA synthesis Coordinately controlled eukaryotic genes A particular combination of control elements can activate transcription only when the appropriate activator proteins are present. All cells of an organism have all chromosomes/genes but certain genes are only active in certain cells. The transcription factors present in the cell determine which genes will be active and which won’t (but they are both still present) Fig. 18-10 Enhancer Control elements Promoter Albumin gene Crystallin gene LIVER CELL NUCLEUS LENS CELL NUCLEUS Available activators Available activators Albumin gene not expressed Albumin gene expressed Crystallin gene not expressed (a) Liver cell Crystallin gene expressed (b) Lens cell Post Transcriptional Regulation Alternate Gene Splicing - different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns Fig. 18-11 Exons DNA Troponin T gene Primary RNA transcript RNA splicing mRNA or Protein Processing and Degradation After translation, various types of protein processing, including cleavage and the addition of chemical groups, are subject to control Proteasomes are giant protein complexes that bind protein molecules and degrade them Ubiquitin Proteasome Protein to be degraded Ubiquitinated protein Proteasome and ubiquitin to be recycled Protein entering a proteasome Protein fragments (peptides) Ubiquintin tags proteins for degradation by proteasomes. Noncoding RNAs role in gene expression RNA Interference, noncoding RNAs play multiple roles in controlling gene expression. MicroRNAs (miRNAs) and Small inserting RNAs (siRNAs) are small single-stranded RNA molecules that can bind to mRNA. These can degrade mRNA or block its translation. The difference between the two is that they form from different RNA precursors. Fig. 18-13 Hairpin miRNA Hydrogen bond Dicer miRNA 5 3 (a) Primary miRNA transcript mRNA degraded miRNAprotein complex Translation blocked (b) Generation and function of miRNAs Cytoplasmic Determinants and Inductive Signals An egg’s cytoplasm contains RNA, proteins, and other substances that are distributed unevenly in the unfertilized egg Cytoplasmic determinants are maternal substances in the egg that influence early development As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression © 2011 Pearson Education, Inc. Figure 18.17a (a) Cytoplasmic determinants in the egg Unfertilized egg Sperm Fertilization Zygote (fertilized egg) Mitotic cell division Two-celled embryo Nucleus Molecules of two different cytoplasmic determinants The other important source of developmental information is the environment around the cell, especially signals from nearby embryonic cells In the process called induction, signal molecules from embryonic cells cause transcriptional changes in nearby target cells Thus, interactions between cells induce differentiation of specialized cell types © 2011 Pearson Education, Inc. Figure 18.17b (b) Induction by nearby cells Early embryo (32 cells) NUCLEUS Signal transduction pathway Signal receptor Signaling molecule (inducer) Figure 18.20 Homeotic genes control pattern formation in late embryo, larva, and adult stages; mutations in homeotic genes cause a misplacement of structures in an animal. Eye Leg Antenna Wild type Mutant Tumor-Suppressor Genes Tumor-suppressor genes help prevent uncontrolled cell growth Mutations that decrease protein products of tumorsuppressor genes may contribute to cancer onset Tumor-suppressor proteins Repair damaged DNA Control cell adhesion Inhibit the cell cycle in the cell-signaling pathway © 2011 Pearson Education, Inc. Suppression of the cell cycle can be important in the case of damage to a cell’s DNA; p53 prevents a cell from passing on mutations due to DNA damage Mutations in the p53 gene prevent suppression of the cell cycle © 2011 Pearson Education, Inc. Figure 18.24b 2 Protein kinases 3 Active form of p53 UV light 1 DNA damage in genome DNA Protein that inhibits the cell cycle (b) Cell cycle–inhibiting pathway MUTATION Defective or missing transcription factor, such as p53, cannot activate transcription.
