Question

1. Describe the difference between a repressible operon and an inducible operon. Give an example of either type of operon and describe the mechanism by which the operon responds to an environmental cue.
2. Describe the differences between general transcription factors and specific transcription factors and their impact on gene expression.
3. Compare and contrast microRNA (miRNA) and synthetic RNA (siRNA). Be sure to include their occurrence in nature, and their mechanism(s) of action.

Answer 1

Q1) Operon refers to a group of structural genes having a common promoter, operator and a regulator gene. As such all genes are transcribed simultaneously when the RNA polymerase binds the promoter site.

there are two types of operons- inducible operons and repressible operons.

Inducible operons are those operons which are induced to express in the presence of inducer (a substance which generally acts as the substrate for the enzymes to be synthesized by this operon). In absence of inducer, the operon is switched off as the repressor protein binds the operator region and block the binding of RNA polymerase to the promoter site. in the presence of inducer, repressor cannot bind the operator, hence RNA polymerase beinds the promoter site and favours transcription. Lac-operon is an example of inducible operon.

Repressible operons are those which are repressed in the presence of Corepressor (a substance which generally prevents the expression of genes for the enzymes to be synthesized by this operon). In absence of co-repressor, the operon is switched on as the repressor protein couldnot bind the operator region and promotes the binding of RNA polymerase to the promoter site. in the presence of co-repressor, repressor binds the operator site and blocks its therby prevents RNA polymerase to bind the promoter site and stops transcription. Tryptophan operon is an example of repressible operon.

Q2) Transcription factors are the protein complexes which controls the gene expression by binding on the cistron at the specific sequences in the promoter region or to RNA polymearse. the function of these transcription factors is to regulate the expression of genes by switching the transcription on and off. there are two types of transcription factors- general transcription factors and specific transcription factors.

General transcription factors also known as basaal transcription factors which are required for the transcription of all the genes and are necessary for the transcription to occur. some of these transcription factors directly bind to the promoter region of DNA or form complexes with the RNA polymerase directly as pre-initiation complexes which helps in the recruitment of RNA polymerase to the promoter site. the examples of general transcription factors are TFIIA, TFIIB, TFIID etc.

Specific transcription fcators are not ubiquitious and are required to upregulate or downregulate the expression of specific genes known as regulated genes. these trancription factors bind the upstream positions of the cistron and work as either enhancer or repressor of transcription e.g. Leucine zipper transcription factor, MYb transcription factor, heat shock factor etc.

Q3) MicroRNA or miRNA is a small RNA molecule which is generally noncoding with 22 bases and these RNA molecules play an imoprtant role in the gene silencing or posttrancriptional regulation of gene expression. The miRNA forms complementary base pairs with the mRNA molecule ans render it inactive and prevent its translation and promotes its degradration by RNases.It regulates the expression of multiple mRNAs

synthetic RNA (Si RNA) actually stands for Small interfering RNA is a double stranded non coding small RNA molecue of 20-25 bp which brings out the gene silencing of specific genes by binding at the mRNA sequence complenemntray to i. before getting involved in gene silencing, these siRNA molecules are unwound to produce single stranded structure