Question

a. 4 ways in which gene expression can be regulated in eukaryotes

b. A substantial mutation occurs in the regulator region of a the lacI gene. What effects do you imagine this will have on the lac operon in a)the presence of lactose and b) in the absence of lactose?

Answer 1

A. 'Four ways of eukaryotic gene expression:

1. RNA processing: RNA processing steps in eukaryotes include:
• “Capping” of the 5'-end of the primary transcript
• Addition of a polyadenylate “tail” to the 3'-end of transcription
• And excision of “intron” regions to generate spliced “exons” in the mature m-RNA molecule.

2. Gene amplification: In this mechanism, the expression of a gene is increased several-fold. This is commonly observed during the developmental stages of eukaryotic organisms.
Examples:
i. In the fruitfly, Drosophila, there occurs during oogenesis an amplification of a few pre-existing genes such as those for the chorion egg shell proteins S36 and S38.
ii. Methotrexate therapy in cancer: It has been demonstrated in patients receiving methotrexate for treatment of cancer that malignant cells can develop drug resistance by increasing the number of genes for the enzyme dihydrofolate reductase.
3. Gene rearrangement: Gene rearrangement can be studied from the mechanism of the synthesis of light chains of immunoglobulins (Igs).
• Each light chain is encoded by three distinct segments:
i. The variable VL
ii. The joining JL, and
iii. The constant CL segments.
The mammalian haploid genome contains over 500 VL segments, 5 or 6 JL segments and perhaps 10 or 20 CL segments.
• During the differentiation of a lymphoid B-cell, a VL segment is brought from a distant site on the same chromosome to a position in the same chromosome close to the region of the genome containing the JL and CL segments.
• This DNA rearrangement then allows the VL, JL and CL segments to be transcribed as a single m-RNA precursor and later the m-RNA proper to produce a specific antibody light chain. The DNA rearrangement is referred to as the VJ joining of the light chain.

4. Stability of m-RNA:

The stability of m-RNA molecules in the cytoplasm can clearly affect the level of gene expression in a positive or negative direction. It is a potential control site that can be influenced by hormones and other effectors. Some hormones influence the synthesis and degradation of specific m-RNAs. For example, estradiol prolongs the half-life of vitellogenin m-RNA from a few hours to more than 200 hours. This coupled with the fact that estrogens enhance the rate of transcription of this gene by four to six-fold results in a tremendous increase of vitellogenin m-RNA.

B.

if the mutation make the the ability of the Repressor molecule to bind with the operator region, then the expression of the lac operon will be decreased.

but, if the mutation makes the repressor protein less active, then it will loss its activity to bind the operator region of lac operant, and the expression of the operon will not depend on presence of lactose. It will be activated now and then and st times when not required wuthout any intervention from the lactose molecule.