Main Body
Chapter 4- How gene expression is regulated ?
Prokaryotic Gene Expression
Dr.V.Malathi
Learning Objectives
By the end of this chapter, readers will be able to:
- Define Operons
- Explain the roles of Lac I, Lac Z, and Lac Y of the lactose Operon
- Predict the effects on lactose metabolism when the concentration of lactose is changed.
- Explain about the Trp Operon
- Define attenuation
- Explain molecular mechanisms that control Eukaryotic gene regulation
- Define Epigenetics
- Discuss the role of epigenetics in gene expression and regulation
Prokaryotic Gene regulation
The process of turning on a gene to produce RNA and protein is called gene expression. For a cell to function properly appropriate proteins must be synthesized at the appropriate time. Whether in a simple unicellular organism or a complex multi-cellular organism, each cell controls when and how its genes are expressed.
Prokaryotic organisms are single-celled organisms that lack a defined nucleus; therefore, their DNA floats freely within the cell cytoplasm. To synthesize a protein, the processes of transcription (DNA to RNA) and translation (RNA to protein) occur almost simultaneously. The expression of a gene is a highly regulated process and ensures that a cell’s resources are not wasted making proteins that the cell does not need at that time.
Prokaryotic transcription and translation occur simultaneously in the cytoplasm, and regulation occurs at the transcriptional level.
Regulation of Gene expression in Prokaryotes
Operons
In prokaryotes , genes with related functions for example the genes that encode the enzymes of a single biochemical pathway— are found next to each other on the DNA or clustered together and regulated together . Such gene clusters shares same promoter and a regulatory sequence that controls the transcription of the entire unit. The organization of genes in this manner is called an This arrangement enables the prokaryote to rapidly adapt to changes in the environment.
Prokaryotic mRNAs are , meaning they contain the information to make more than one protein. The promoter has simultaneous control over the regulation of the transcription of these structural genes.
The operon includes two main components namely
- Structural genes – genes that encode proteins used in metabolism or biosynthesis or that play a structural role in the cell and
- Regulatory elements- includes the promoter and the region surrounding the promoter and the genes that code for the regulatory proteins
In prokaryotic cells, there are three types of regulatory molecules that can affect the expression of operons namely ;
- Repressors– are proteins that suppress transcription of a gene in response to an external stimulus,
- Activators – are proteins that increase the transcription of a gene in response to an external stimulus and
- Inducers – are small molecules that either activate or repress transcription depending on the needs of the cell and the availability of substrate.
Genes that encode for enzymes with related functions are clustered together and regulated together. This organization of genes is called an Operon
m RNAs containing the information to make more than one proteins . Prokaryotes have Polycistronic mRNA