NEET Biology Notes Molecular Basis of Inheritance Lac Operon
Lac operon is an inducible operop, in which presence of key metabolic substance, i.e. lactose induces transcription of structural genes (or active repressor + inducer is equal to inactive repressor).
The lac operon of E. coli. (a) Arrangement of genes in lac operon. (b) The repressor produced by regulator gene is in an active form and binds to the operator DNA preventing transcription initiation, (c) In the presence of inducer, the repressor molecules are inactivated by the inducer molecules so, that they can no longer bind to the operator DNA. As a result, the structural gens are transcribed.
The tryptophan operon (tip operon) in bacteria is a repressible operon, where repressor is inactive and it becomes active as DNA binding protein complexed with corepressor (i.e. tryptophan). In the absence of tryptophan, the operator site is open to binding by RNA polymerase, which transcribes the structural genes of trp operon.
Regulation of the repressible tryptophan (trp) operon of E. coli
(a) Arrangement of the genes in operon.
(b) The regulator gene (located at a different site in the chromosome) produced an inactive repressor, which is unable to bind to the operator, hence transcription of the structural genes continues
(c) But, when the corepressor, i.e. tryptophan, is present at a threshold concentration, it interacts with and activates the repressor, which now binds the operator. This prevents transcription of the operon.
Human Genome Project (HGP)
HGP was 13 year project that was launched in the year 1990 and completed in 2003. The project was coordinated by US, Department of Energy and the National Institute of Health.
Goals of HGP
- Identify all the genes in human DNA.
- Determine the sequences of the 3 billion base pairs present in human DNA.
- Store the information in databases.
- Improve the tools for data analysis.
- Transfer the technologies to other sectors.
- Address the ethical, legal and social issues that may arise from this project.
Features of HGP
The salient features considered in human genome are listed here
- The human genome contains 3164.7 million nucleotides (base pairs).
- The size of genes varies.
- A total number of genes is estimated at 30000 and 99.9% of the nucleotides are the same in all humans.
- Only less than 2% of the genome codes for proteins.
- Repetitive sequences make up a large portion of the human genome.
- Scientists have identified about 1.4 million locations, where DNA differs in single base in human beings. These are called Single Nucleotide Polymorphisms (SNPs).
Advantages of HGP
Knowledge of the effects of variations of DNA among individuals can provide the ways to diagnose, treat and even prevent a number of diseases/disorders that affect human beings. It provides clues to the understanding of human biology.