Gene Expression Lecture 5 PDF

Lecture 5 The Regulation of Gene Expression Learning Objectives Understand how bacteria regulate gene expression due to different environmental conditions Understand how eukaryotes regulate gene expression and how this gives rise to different cell types.  Regulation of a met...

Lecture 5 The Regulation of Gene Expression Learning Objectives Understand how bacteria regulate gene expression due to different environmental conditions Understand how eukaryotes regulate gene expression and how this gives rise to different cell types.  Regulation of a metabolic pathway long short term term control control  The Operon Model Discovered in 1961 by Jacob and Monod functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and translated together in the cytoplasm, The result of this is that the genes contained in the operon are either expressed together or not at all. Several genes must be co-transcribed to define an operon. trp operon – repressible operon  lac Operon – inducible operon  Gene regulation in eukaryotes 1. Chromatin Structure Histone modifications 2. DNA methylation 3. Specific Transcription Factors 4. Alternative RNA splicing 5. mRNA degradation 6. Protein degradation  1. Histone modifications Chemical modifications to the histones Act as switches that alter the confirmation of chromatin Histone acetylation – Acetyl groups are attached (-COCH3) Histone methylation – Methyl groups attached (-CH3) – Attach to specific aa in the histone protein  2. DNA methylation Covalent attachment of methyl groups (-CH3) Usually Cytosine bases at CpG dinucleotides  3. Gene Transcription Control Elements Sections of DNA that are non- coding to which transcription factors bind. Proximal control elements (near to promoter) Distal control elements (can be thousands of nts up or down stream) Distal control elements are grouped together and called enhancer (or silencer) Gene expression can be increased or decreased by the binding of Specific transcription factors – activators or repressors – to the control elements 1. These activators and repressors regulate changes in chromatin structure and histone acetylation and deacetylation influencing the ability of general transcriptions factors to bind to promoters. 2. Activators and repressors directly regulate assembly of transcription- initiation complexes and the rate at which they initiate transcription. Albumin - produced by the liver. Functions to regulate osmotic pressure in blood cells Crystallin – found in the lens and cornea. A structural protein. The highly cooperative assembly of initiation complexes in vivo generally requires several activators. A cell must produce the specific set of activators required for transcription of a particular gene in order to express that gene.  4. Alternative Splicing  5. RNA interference (RNAi) Two types : 1.miRNA – micro RNA 2. siRNA – small interfering Lead to mRNA degradation Or inhibition of translation Small single stranded RNA molecules that bind to mRNA Click link to watch animation on RNA interference - https://youtu.be/cK-OGB1_ELE  6. Protein degradation Protein to be degraded is tagged with ubiquitin Degradation of proteins tagged with ubiquitin occurs at the proteosome. Differential Gene expression  Cytoplasmic determinants Eggs cytoplasm contains: RNA Proteins Not an equal distribution of substances Profound impact on embryonic development Maternal substances in the egg that influence early development = Cytoplasmic Determinants  Induction – cell communication Major source of developmental information is the environment Most influential are the signals Early embryo (32 cells) from other embryonic cells. Signal transduction One group of cells will pathway influence the development of Signal another group of cells. receptor Signal Contact with cell surface molecule (inducer) molecules Binding of growth factors (b) Induction by nearby cells Signals that cause changes in a target cell is called INDUCTION  Determination – tissue specific gene expression Subtle changes set a cell on a path of specialisation The earliest changes are observed at the molecular level These molecular changes then drive the “Determination” process that leads to observable differentiation of a cell. Embryonic cell irreversibly committed to its final fate. Determination is followed by differentiation  Head Thorax Abdomen Pattern 0.5 mm Formation Dorsal Begins in early embryo Right BODY Anterior Posterior AXES Left Ventral Cytoplasmic determinants and (a) Adult inductive signals contribute to the Follicle cell development of spatial organisation 1 Egg cell Nucleus developing within ovarian follicle Egg cell Nurse cell Cytoplasmic determinants provide positional information 2 Unfertilized egg Egg shell for axes in unfertilised egg Depleted nurse cells Molecular cues tell a cell its location Fertilization Laying of egg relative to the body axes and neighbouring cells. 3 Fertilized egg Embryonic development 4 Segmented Cytoplasmic determinants are embryo 0.1 mm synthesised by maternal effect Body segments Hatching (egg polarity) genes 5 Larval stage (b) Development from egg to larva  Tail Bicoid - a maternal effect gene Head codes for a morphogen protein involved in specifying the anterior T T A7 A8 end of the embryo 1 T 2A1 A6 3 A2 A3 A4 A5 Wild-type larva An embryo who has two mutant Tail Tail alleles of the bicoid gene lacks the front half of the body and has two tails. A8 A8 A7 A7 A6 Mutant larva Experiments have shown bicoid (bicoid) mRNA at the far anterior end of a mature egg. Fertilisation = mRNA translated Fertilization, into protein and Bicoid protein 100 µm translation of bicoid Anterior end diffuses from anterior to posterior Bicoid mRNA in mature unfertilized egg mRNA Bicoid protein in early embryo causing a concentration gradient. Nurse cells Egg Increased understanding of the bicoid mRNA mothers critical role in the initial Developing egg Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo stages of embryonic development  Edward Lewis 1940s Mapped mutations to specific genes – homeotic genes Homeotic genes: regulate the development of anatomical structures. control pattern formation in late embryo, larva and adult mutations cause body segments to develop abnormally Wild type Mutant Antennapedia Legs instead of Double thorax antennae Humans also have homeotic or hox gene…….. fusion and malformation Horizontal gaze palsy Facial weakness Under developed external ear  Summary Prokaryotic genes clustered in Operons Eukaryotes have several mechanisms for gene regulation Multicellular organisms have differential gene expression – Maternal effect genes and environmental cues determine cell differentiation AND pattern formation Defects in gene expression can give rise to malformation and disease (e.g. cancer)