Gene Expression. Lesson 6

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1 Gene Expression Lesson 6

2 Regulation of gene expression Gene regulation turning on or off specific genes depending on the requirements of an organism Housekeeping genes are always switched on (vital life functions e.g. metabolism, growth, replication) Not all proteins are required by the cell at all time à waste of cell s resources

3 Prokaryotic gene control mechanism Regulated in response to the concentration specific molecules Two common examples of gene control in prokaryotes: 1. Lactose 2. Tryptophan Operon cluster of genes under the control of a promotor and operator Promotor binds RNA polymerase and starts transcription Operator region in the operon that regulatory factors (i.e. repressor protein) bind to

4 The Lac Operon Lac operon Cluster of 3 genes (lactose metabolism) under the control of a promotor and operator lacz, lacy, laca Genes code for proteins required for bacteria to break down and use lactose as a nutrient Lactose à glucose + galactose

5 When lactose is absent When lactose is not present à lac repressor is active Repressor protein binds to the operator to repress transcription laci repressor protein (produced by the laci gene) Keeps RNA polymerase from binding to the promotor

6 When lactose is present When lactose is present à lac repressor is inactive Inducer molecule that triggers the expression of an operon s genes Lactose functions as an inducer à bind to repressor protein and make it inactive RNA polymerase is able to bind to the promotor à transcription begins

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8 Inducible operon Inducible operon always off!! Turned on by an inducer molecule (lactose) As concentration of lactose in the cell changes, so does the rate of transcription of the lactose-metabolizing enzymes

9 What would happen if There was a mutation in the laci protein gene?

10 What would happen if there was a mutation in the laci protein gene? laci protein will be dysfunctional Repressor protein will not be able to bind to either the operator or lactose Transcription of the three genes will occur, irrespective of the level of lactose in the cell

11 The Trp operon Trp operon cluster of 5 genes under the control of a promotor and an operator Regulates the production of the amino acid tryptophan

12 When tryptophan is absent When tryptophan is not present à repressor is inactive Repressor will not bind to the operator RNA polymerase is able to bind to the promotor Transcription occurs for the tryptophan-making enzymes

13 When tryptophan is present When tryptophan is present à Repressor is active Trp will bind to the repressor à repressor will bind to the operator à stop transcription Corepressor a single molecule that binds to a regulatory protein to reduce the expression of an operon s genes

14 Repressible operon Repressible operon always on!!! Turned off by the end product (tryptophan)

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16 Are these molecules corepressors or inducers?

17 Are these molecules corepressors or inducers? A = corepressor When levels of A are low, transcription of the genes is high Presence of A does not block transcription When levels of A are high, gene transcription is low B = inducer Presence of A has blocked transcription When B is low, transcription is also low Indicating B must be available for the transcription of the genes to take place

18 Eukaryotic gene control mechanism Requires a larger number of steps à more complex Do not use the operon system 4 categories: 1. Transcriptional DNA à mrna 2. Post-transcriptional as mrna is being processed 3. Translational mrna à polypeptide chain 4. Post-translational after polypeptide chain is synthesized

19 Transcriptional regulation Regulates which genes are transcribed and the transcription rate Examples: Activator/repressor proteins bind to the promotor and enhance or decrease rate of transcription (i.e. general transcription factors (proteins) bind to TATA box region)

20 Histone modification Loosening of histones to access promotor region Methylation methyl group (-CH3) is added to the cysteine bases in the promoter region to inhibit transcription (silencing)

21 Post-Transcriptional regulation Controls availability of mrna s by affecting changes in premrna processing and degradation Examples: Alternative splicing (pre-mrna à different mrnas) Capping and tailing Binding masking proteins à keep mrnas inactive Regulatory molecules (i.e. proteins, hormones) can directly or indirectly affect the rate of mrna breakdown

22 Translational regulation Controls how often and how fast mrna is translated into protein (during protein synthesis) Example: Changing length of Poly(A) tail of mrna by adding/deleting adenine at the 3 end (may shorten or lengthen lifetime of mrna)

23 Post-Translational regulation Controls when and how long proteins will be functional (after polypeptide chain is synthesized) Cell can regulate expression by limiting functional proteins Example: Chemical modifications (adding/removing sections of the polypeptide chain or certain chemical groups) Degradation (small protein tags called ubiquitin that are recognized by degradation mechanisms)

24 Type of Control Description Examples Transcriptional Post- Transcriptional Translational Regulates which genes and the rate at which they are transcribed Controls the availability of mrna molecules to ribosomes Controls how often and how fast mrna is translated into protein Activators/repressor proteins, access to promoters, methylation Alternative splicing, masking proteins, degradation Variation of poly(a) tail Post- Translational Controls when proteins when and how long proteins will be functional processing (i.e. chemical modifications), tagged for degradation

25 Homework Vocabulary Cards Handout Questions Answers (Lesson 4-6) given tomorrow