1. List and briefly explain different mechanisms of post-transcriptional
control of gene expression (think about examples)? **
a. Localization of mRNA to specific regions: mRNA is positioned close to the sites
where the proteins coded by that particular mRNA are required. This is done with the
help of the cytoskeleton (microtubules, microfilaments and actin filaments) which
provides the pathway for the transport of the mRNA. E.g.: Axis formation during
development (bicoid mRNA is localized in the anterior part of the egg) & beta-actin
mRNA localization in the leading edge of fibroblasts.
b. RNA editing: They are alterations in the sequence of the mRNA.
In protozoan mitochondria: Addition or deletion of U residues.
In Mammals: Single-nucleotide site-specific editing (A to I) and C to U.
E.g.: Synthesis os the two forms of apolipoprotein-B proteins (apoB)
CAA (glutamine) is changed UAA (stop codon) causing an early termination and
formation of Apo-B48 in intestines. This doesn’t occur in the liver and Apo-B100 is
formed from the same mRNA.
c. Post-transcriptional gene silencing (PTGS) by small RNA molectules:
A specific protein complex (DICER) will cut long dsRNA into siRNA or iRNA as
well as short non-coding dsRNA into miRNA. siRNAs and miRNAs are then
incorporated into nuclease complex to form the RNA-induced silencing complex
(RISC). Finally, RISC, recognizes complementary RNA and stops its translation or
degrades already formed mRNA. (RISC degrades the sense strand of the siRNA, the
anti-sense strand binds to the mRNA and RISC degrades it. miRNA inhibit
transcription rather than degrade mRNA)
E.g.: Defence against viral infections (RNA viruses). Also, used to control expression
of genes required only at a certain stage of development.
d. Translational control switch:
In order to save energy and resources, cells don’t translate proteins that aren’t
required. Some mRNAs have coupled functions (mainly antagonists) and such
switches allow the cell to change the protein expressed according to the
E.g.: Ferritin (storage) and transferritin (transport into the cell) are proteins involved
in the regulation of the iron content of the cell. Both the mRNAs have Iron
Responsive Element (IRE) that binds IRE-BP in either the 5’UTR (ferritin) or the
3’UTR (transferritin). In low Fe, translation of Ferritin is switched off and that of
transferritin is switched on and vice versa.
e. mRNA longevity: Stabilizing elements are present in the 3’UTR of mRNA as well as
in the coding sequence. Instability elements are also found on mRNA but aren’t
E.g.: Potentially harmful proteins or ones that are only required in small quantities
would be expected to degrade faster and are therefore likely to contain instability
elements. Also, use of cyclohexamide (Cx- protein synthesis inhibitor) prevents
formation of protein that binds to instability elements causing degradation of mRNA.
Thus, the protein coded by the mRNA is translated and can be assayed using
Note: this question requires only listing and brief explanation, the answer above also applies
to Q2. 2. Explain the role of mRNA stability (or editing, or iRNA, or translational
control switch or mRNA localization – five different questions possible) in
control of gene expression.
Solved above. But Dragana’s answer is shorter and incomplete
3. Explain the connection between pre m-RNA splicing and transport of
mRNA from the nucleus.
Only mature mRNA can be transported out of the nucleus (i.e. 5’ CAP, splicing and 3’
Poly-A tail). The transport ma