PowerPoint Presentation On Gene Expression

PPT On Gene Expression

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Gene Expression Presentation Transcript:

1. Gene is basic unit of heredity and is a segment for genomic information. It consist of both ‘coding’ and ‘non-coding’ sequences. These coding and non-coding sequences are copied in a process called Transcription producing RNA which directs the synthesis of proteins. Phenotype of an organism is an resultant of genes interacting with each other. Every cell has same set of genes so it is necessary to have a mechanism which allows desired gene to function at particular time. Expression of gene is not a single step process rather a vast system comprising a complex network. Activity of undesired genes has to be restricted which requires time and space.

2. Operon Negative control of lactose operon Positive control of lactose operon Mutation experiments by Jacob and Monod

3. F. Jacob and J. Monod in 1961 proposed a model to explain induction or repression of enzyme synthesis. This model is known as ‘Operon Model’ which assumes presence of coordinated control of protein synthesis and this unit is called ‘Operon’. Operon is a group of linked genes linked on chromosome under control of promoter and these linked genes gives rise to a single m-RNA. Operon consists of:- An operator gene which control activity Structural gene which participates in protein synthesis.

4. Histidine operon(his operon)- it has RNA element. Tryptophan operon(trp operon)- a repressible system which controls biosynthesis of tryptophan from precursor chorismic acid. Lactose operon(lac operon)- regulated by factors including presence of glucose and lactose.

5. An operon required for transport and metabolism of lactose in E.coli. E.coli cells when transferred to medium containing lactose instead of glucose, they stop growing for a certain period of time. They again starts growing as during lag phase cells turn on lac operon. Galactoside permease is the enzyme used for transport of lactose within cells. Lactose is made up of glucose and galactose linked by β-galactosidic bond. Three genes encoding above mentioned enzymes are: Lac Z- encodes for β galactosidase Lac Y- encodes for galactoside permease Lac A- encodes for galactoside transacetylase

 6. It is lac repressor coded by a gene called gene 1(lac 1). Repressor occupies operator and blocks access of RNA polymerase which in turn cannot transcribe lac Z, Y and A genes. Lac operon is repressed till glucose is available. In presence of lactose operon is turned on so that lactose can be metabolized. Repressor is an allosteric protein which changes its confirmation when it binds to small molecules. Inducer binds to repressor which is an alternate form of lactose called allolactose. Allolactose is glucose linked to galactose by β- 1,6 bond.

7. Allolactose acts as inducer by binding to repressor causing conformational shift. When repressor is , RNA polymerase is free to bind to lac promoter. It then transcribe three structural genes i.e. lac Z, lac A, lac Y.

8. E.coli cells metabolize glucose easily as compared to lactose Selection of glucose among other energy sources is influenced due to some breakdown products. It is thus called as Catabolite Repression. A nucleotide called cyclic AMP (cAMP) responds to glucose concentration. Its concentration responds inversely proportional to glucose concentration.

9. Positive controller is a complex comprising of two parts: cAMP Binding protein CAP (catabolite activator protein) Complex binds to promoter and turns it on by binding RNA polymerase Complex when binds, it causes promoter to bend and makes easier the DNA strands forming open promoter complex.

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