Gene structure and function, lac operon, trytophan operon

Gene:-

Definition:- 

“A functional segment of DNA which manufacture protein, regulate gene expression and renowned as a hereditary unit is known as a gene.”

History:-

- Mendel first discovered the concept of the “inheritance of traits”, despite, he fails to describe it.

- The term “gene” was coined and studied by Wilhelm Johannsson. But he was unable to describe the chemical structure of it. In 1953, James Watson and Francis Crick defined the chemical structure of the DNA viz gene.

Structure of gene:-

- Genes are actually DNA segment thus are made up of the nucleotide chain. The chemical structure of a gene comprises nucleotides.

- Genes are made up of A, T, G and C nucleotides. With the nucleotides of the opposite strand, it binds with hydrogen bonds and with the adjacent nucleotide, it binds with phosphodiester bonds.

- The nucleotides are the combination of nitrogenous bases (A, T, G and C), phosphate and pentose sugar.

- In general, the gene structure consists of two types of components: core components and regulatory components.

- The core components or sequences actually take parts in protein formation. While the regulatory components maintain gene expression.

- Exons are core elements. Sequences on the other side like promoters, enhancers and silencers are regulatory elements of a gene.

- The third type of element called maintenance elements possesses information for DNA repair, modification and replication. The functional or physical structure of a gene comprises introns, exons, promoters, enhancers and UTRs.

- Introns are non-coding sequences removed from the final transcript.

- Exons are coding part of a gene which are joined after splicing and constructs the final transcript.

- Regulatory components are located on the extreme ends of a gene.

- Promotes are non-coding sequences but facilitates binding sites for enzymes and transcriptional factors to work. The promoter consists of TATA box and CCAAT sequences for enzyme binding.

- The entire promoter region is located on the 5’ end and made up of core promoter and proximal promoter sequences.

Functions of gene:-

- The main function of a gene is to form or manufacture a protein. 

- Some genes can’t form protein, although they transcribe into mRNA. For instance, the microRNAs are the type of tiny ribonucleic acid formed from some genes but it doesn’t undergo protein formation. It helps in gene regulation instead.

Gene Regulation:-

·  Gene expression results in the formation of a polypeptide.

·   In eukaryotes, the regulation includes the following levels:-

i. Transcriptional level (formation of primary transcript)

ii. Processing level (regulation of splicing)

iii. Transport of mRNA from nucleus to the cytoplasm

iv. Translational level.

·   The metabolic, physiological and environmental conditions regulate expression of genes.

E.g.-

-   In E. coli the enzyme, beta-galactosidase hydrolyses lactose into galactose and glucose. In the absence of lactose, the synthesis of beta-galactosidase stops.

-   The development and differentiation of embryo into adult are a result of the expression of several set of genes.

Operon Concept:-

·  “Each metabolic reaction is controlled by a set of genes”

·  All the genes regulating a metabolic reaction constitute an Operon. E.g. lac operon, trp operon, ara operon, his operon, val operon etc.

·  When a substrate is added to growth medium of bacteria, a set of genes is switched on to metabolize it. This is called induction.

·  When a metabolite (product) is added, the genes to produce it are turned off. This is called repression.

1. Lac operon in E. coli:-

·  The operon controlling lactose metabolism. It consists of -

a) A regulatory or inhibitor (i) gene]:- It codes for the repressor.

b) 3 structural genes:-

i. z gene:- It codes for β-galactosidase which hydrolyze lactose to galactose and glucose.

ii. y gene:- It codes for permease which increase permeability of the cell to lactose.

iii. a gene:- It codes for transacetylase.

·  The genes present in the operon function together in the same or related metabolic pathway. There is an operator region for each operon.

·  In the absence of inducer:- If there is no lactose (inducer), Lac operon remains switched off. The regulator gene synthesizes mRNA to produce the repressor protein; this protein binds to the operator genes and blocks RNA polymerase movement. So the structural genes are not expressed.

·  In the presence of inducer:- If lactose is provided in the growth medium, the lactose is transported into the E. coli cells by the action of permease. Lactose (inducer) binds with repressor protein. So repressor protein cannot bind to operator gene. The operator gene becomes free and induces the RNA polymerase to bind with promoter gene. Then transcription of structural genes starts. It is also known as indusible operon. Regulation of lac operon by repressor is called negative regulation.

2. Trp operon in E. coli:- 

·    The Trp operon, found in E. coli bacteria, is a group of genes that encode biosynthetic enzymes for the amino acid tryptophan.

·   The Trp operon is regulated by the tryptophan co-repressor. So it is also known as repressible operon. It is positive regulation.

i. The Trp operon is expressed (turned "on") when tryptophan levels are low.

ii. The trp operon is repressed (turned "off") when tryptophan levels are high.