Control of gene expression

Definition, Principle, gene expression in eukaryotes, levels of gene control, genes and regulatory elements, Operon system

1. Control of gene expression Presented by : Gurleen Kaur Roll no. = 60235 M.Sc. Biotechnology

2. Gene expression is the process by which information from gene is used in the synthesis of a functional gene products .

3. General Principle of gene regulation One of the major themes of molecular genetics is the central dogma Which stated that genetic information flows from DNA to RNA to proteins and provided a molecular basis for the connection between genotype and phenotype. Although the central dogma brought coherence to early research in molecular genetics, it failed to address a critical issue: How is the flow of information along the molecular pathway regulated? Consider Escherichia coli a bacterium that resides in your intestine your eating habits determine the nutrients available to this bacteria it can’t seek out nourishment when nutrients are scare; nor can it move away when confronted with unpleasant changes.

4. E. coli makes, up for its inability to alter the external environmental by a being internally flexible. For example, if Glucose is present, E. coli uses it to generate ATP; If there is no Glucose it utilizes Lactose, Arabinose, Maltose, Xylose or many other no. of sugars. If amino acids are available E. coli uses to synthesize proteins. If particular amino acid is absent, E. coli produces the enzymes needed to synthesize that amino acid.

5. Thus, E. coli responds to the environmental changes by rapidly altering its biochemistry. Producing all enzymes necessary for every environmental conditions is highly energetically expensive. E. coli maintain this biochemical flexibility by gene regulation. CONSEPTS Gene regulation is key to both unicellular flexibility and multicellular specialization and it is critical to success of all living organisms.

6. In bacteria Gene regulation maintains internal flexibility , turning genes ON and OFF in response to environmental changes. In multicellular eukaryotes Gene regulation brings about cellular differentiation . LEVELS OF GENE CONTROL A gene may be regulated at no. of points along the pathway of information flow from genotype to phenotype. First Regulation ; First regulation arises through the alteration of structure of gene. Modification to DNA or its packaging may influence which sequence are available for transcription.

7. DNA methylation and changes in chromatin are two processes that play a pivotal role in gene regulation. Second point ; At level of transcription gene can be regulated. For sake of cellular economy it makes sence to limit protein production early in the transfer of information from DNA to protein. Transcription is an important point of gene regulation in both bacterial and eukaryotic cell . Third point; mRNA processing is third point of gene regulation. Eukaryotic messenger RNA is extensively modified before its translation. A 5’ cap is added. 3’ end is cleaved and polyadenylation .

8. Introns are removed. These modifications determine the stability of mRNA; Whether mRNA can be translated. The rate of translation The amino acid sequence of the protein produced. Fourth point; Regulation of RNA stability for the control of gene expression. The amount of protein produced depends not only the amount of the m RNA but also on the rate at which m RNA is degraded, so RNA plays an important role in gene expression.

9. Fifth point; At level of translation. Gene regulation at this level requires a large number of enzymes , protein factors and RNA molecules. All of these factors, Availability of amino acids , Sequences in m RNA . Influences the rate at which proteins are produced and therefore provides points at which gene expression may be controlled.

10. Concepts; Gene expression may be controlled at any of a number of points along the molecular pathway from DNA to protein , including gene structure, transcription, m RNA processing, RNA stability, translation and post translation modifications.

11. Gene expression may be controlled at multiple levels

12. Genes and regulatory elements; Genes and DNA sequences that are transcribed into RNA. Regulatory elements are DNA sequences that are not transcribed but effect the expression of genes. Regulatory gene’s product either RNA or proteins, interact with other sequences and effect their transcription or translation. Genes include DNA sequences that encode proteins, as well as sequences that encodes r RNA , t RNA, sn RNA AND Other types of RNA. Structural genes encodes proteins that are used in cellular metabolism .Much of gene regulation take place through the action of proteins ,produced by regulatory genes that recognize and bind to regulatory elements.

13. DNA binding proteins Much of gene regulation is accomplished by proteins that binds to DNA sequences and influences their expression. Regulatory proteins have function parts- domains, consisting of 60-90 amino acids- that are responsible for binding with DNA. Within a domain only a few amino acids actually make contact with DNA . ( asparagine, arginine, glutamine, glycine, lycine) often form hydrogen bonds with the bases or interect with the sugar phosphate backbone of DNA.

14. DNA binding proteins can be grouped into several distinct types on the basis of characteristic structure called a motif, found within the domain. Common DNA – binding motifs Location : Bacterial regulatory proteins; related motifs in eukaryotic proteins. Characteristics: Two alpha helices Binding site: Major groove of DNA

15. Location; Eukaryotic regulatory and other proteins. Characteristic: Loop of amino acids with zinc at base. Binding site in DNA : Major groove

16. Location: Eukaryotic transcription factors Characteristics : helix of lucine residues and basic arm; two lucine rasidue interdigitate. Binding site in DNA two adjacent major grooves

17. Location: Eukaryotic proteins Characteristics: Two alpha helices separated by a loop of amino acids. Binding site in DNA: Major groove

18. Gene regulation in bacterial cells The mechanism of gene regulation were first investigated in bacterial cells. When the study of these mechanism in eukaryotic cell begin , it seemed clear that bacterial and eukaryotic gene regulation were quite different . * One significant difference in bacterial and eukaryotic gene control lies in the organization of functionally related genes . * In bacterial cells the genes that have related function are clustered and under control of single promoter . These genes are often transcribed in single mRNA. * Eukaryotic genes in contrast are dispersed , and typically each is transcribed into a separate mRNA.

19. OPERON SYSTEM An operon is part of genetic material which act as single regulated unit having one or more structural genes , an operator, a promoter, a regulator gene, a repressor , an inducer or corepressor. Operons are of two types ; *Inducible operon system - Lac operon *Represible operon system

20. Inducible operon system – Lac operon It is unit of genetic material which is switched ON in response to the presence of chemicals. It comprised of following parts: Structural genes - synthesizes m RNA controls metabolic activity of cytoplasm An operon has one or more structural genes (Z,Y,A). They transcribe polycistronic m RNA molecules that helps in the synthesis of three molecules: Beta galactosidase galactosidase permease thiogalactosidase acetylase The three enzymes are however,produced in different molar concentrations. Initial entry of Lactose into bacterium would only occur due to this activity .

21. Operator gene It comprises of only 27 base pairs . It directly control the synthesis of the m RNA over the structural genes. It is switched OFF by the presence of a repressor. An inducer can take away the repressor and switch ON the gene. Promoter gene Act as initiation signal for RNA polymerase. It is functional only when operator gene allows passage of RNA polymerase to structural gene.

22. Regulator gene It produces inhibitor or repressor for blocking of operator gene. It controls the function of operator gene. It function through the formation of an m RNA of repressor. Repressor is proteinaceous substance synthesized by the regulator gene i gene. Repressor as two allosteric sites one for attaching to operator and second for binding to inducer. After coming in contact with inducer the repressor undergoes conformational changes in such a way that it is unable to combine with operator.

23. Inducer *It is chemical which after coming contact with the repressor, changes into the non DNA binding state so as to free the operator gene. CAP (catabolic activator pathway) *It exerts negative control in lac operon . *In its absence RNA polymerase is unable to recognise promoter gene. CAP site is present near the lac promoter. CAP activates Lac genes only when glucose is absent.

24. The trp operon of E. coli The trp operon controls the biosynthesis of the amino acid tryptophan in E. coli and it is repressible operon . In repressible operon transcription is normally turned on and must be repressed

25. When trp is low: The trp repressor is normally inactive, it cannot bind to the operator and transcription takes place. When trp is high : The trp binds to the repressor and makes it active. The trp repressor then binds to the operator and shuts transcription OFF.

26. When glucose level is high, cAMP levels are low, c AMP is likely to bind to CAP. RNA polymerase can’t bind to DNA as efficiently so transcription is at a low rate. When glucose level is low c AMP levels are high. CAP readily binds c AMP, and the CAP-c AMP complex bind DNA, increasing the efficiency of polymerase binding. The result is high rate of transcription and translation of the structural genes and the production of glucose from lactose.

27. THANK YOU