Initiation:
- The process of translation or protein synthesis begins with attachment of mRNA with small subunit of ribosome.
- The ribosome binds to the mRNA at the start codon (AUG).
- AUG is recognized by the initiator tRNA.
Elongation:
- Larger subunit attached with the initiation complex.
- Larger subunit has two site ‘A’ site and ‘P’ site.
- Initiator tRNA accommodated in ‘P’ site of large subunit, the subsequent amino-acyl-tRNA enters into the ‘A’ site.
- The sub subsequent tRNA selected according to the codon of the mRNA.
- Codon of mRNA and anticodon of tRNA are complementary to each other.
- Formation of peptide bond between two amino acids of ‘P’ and ‘A’ site, catalyzed by ribozyme, (23S rRNA in bacteria)
- The moves from codon to codon along the mRNA called translocation.
Termination:
- Elongation continues until a stop codon arrives at ‘P’ site.
- There is no tRNA for stop codon.
- A release factor binds to the stop codon.
- Further shifting of ribosome leads to separation of polypeptide.
- An mRNA also has some additional sequences that are not translated called untranslated regions (UTR).
REGULATION OF GENE EXPRESSION:
- Regulation of gene expression in eukaryotes takes place in different level:
- Transcriptional level (formation of primary transcript)
- Processing level (regulation of splicing)
- Transport of mRNA from nucleus to the cytoplasm.
- Translational level.
- In prokaryotes control of rate of transcriptional initiation is the predominant site for control of gene expression.
- The activity of RNA polymerase at the promoter is regulated by accessory proteins, which affects its ability to recognize the start site.
- The regulatory proteins can acts both positively (activators) or negatively (repressor)
- The regulatory proteins interact with specific region of DNA called operator, which regulate the accessibility of RNA polymerase to promoter.
Lac operon:
- Francois Jacob and Jacque Monod first to describe a transcriptionally regulated system of gene expression.
- A polycistronic structural gene is regulated by common promoter and regulatory genes. Such regulation system is common in bacteria and is called operon.
- Lac operon consists of:-
- One regulator gene ( i-gene)
- Three structural genes (z,y,a)
- Operator. (binding site of repressor protein)
- Promoter.(binding site of the RNA polymerase)
- The i-gene codes for repressor of the lac operon.
- The structural gene consist of three gene (z, y and a)
- ‘z’-gene codes for beta-galactosidase, which hydrolyze lactose into Galactose and glucose.
- ‘y’ –gene codes for permease, which increases the permeability of bacterial cell to lactose.
- ‘a’-gene codes for transacetylase.
- All three genes are required for the metabolism of lactose in bacteria.
- Inducer: lactose is the substrate for β- galactosidase and it regulates the switching on and off of the lac operon. Hence it is called inducer.
- In the absence of glucose, if lactose is added in the growth medium of the bacteria, the lactose is transported into the cell by permease.
- Very low level of expression of lac operon has to be present in the cell all the time; otherwise lactose cannot enter the cell.
Mechanism of regulation of lac operon:
- The repressor protein is synthesized from i-gene (all time constitutively)
- In the absence of the inducer i.e. lactose the active repressor binds to the operator and prevents RNA polymerase from transcribing the structural gene
- In the presence of the inducer such as lactose or allolactose, the repressor is inactivated by interaction with inducer.
- This allows RNA polymerase access to the promoter and transcription proceeds.
- The regulation of lac operon by repressor is referred to as negative regulation.
HUMAN GENOMIC PROJECT:
- Genetic make-up of an organism or an individual lies in the DNA sequences.
- Two individual differs in their DNA sequences at least in some places.
- Finding out the complete DNA sequence of human genome.
- Sequencing human genome was launched in 190.
Goals of HGP:
- Identify all the approximately 20.000 – 25000 genes in human DNA.
- Determine the sequence of all 3 billion chemical base pairs.
- Store this information in data bases.
- Improve tools for data analysis.
- Transfer related technologies to other sectors, such as industries.
- Address the ethical, legal, and social issues (ELSI) that may arise from the project.
CBSE Biology (Chapter Wise) Class XII ( By Mr. Hare Krushna Giri )
Email Id : harekrushnagiri@yahoo.com
