Both heterochromatin and euchromatin can be found packaged in particular regions of each of the distinct chromosomes. As a consequence of the DNA in heterochromatin being extremely compacted and the transcriptional machinery being unable to access it, heterochromatin is transcriptionally inactive. In human females, one copy of each of the two X chromosomes is packaged into a heterochromatin to form a Barr body. This renders that copy of each X chromosome completely inactive. The fact that the Cys residues in the DNA that makes up heterochromatin are heavily methylated provides some evidence that methylation may play a role in the preservation of heterochromatin. Genes that were dormant and found in heterochromatin can become active again when treated with drugs that interfere with the methylation process.
The control of the lac Operon system
Two distinct proteins work together to control the level of activity of the promoter that is responsible for regulating the expression of the lac operon. One of the proteins acts as a negative control by preventing RNA polymerase from carrying out transcription, while the other protein helps RNA polymerase bind more tightly to the promoter (positive control).
Eukaryotic Mechanisms for the Control of Gene Expression
In comparison to a prokaryotic cell, the amount of genetic information contained in a human cell is approximately one thousand times greater. The fact that different cell types have to express different subsets of genes at different stages of an organism’s development adds an additional layer of complexity to the situation. It is a very complicated process to regulate gene expression in such a way that only a particular subset of genes is expressed in a specific tissue at a specific point in development. The increased complexity of regulation makes it more likely that there will be malfunctions, which can lead to disease. In this article, we will discuss three of the ways in which eukaryotic organisms regulate gene expression: the modification of gene content or position, regulation of transcription, and alternative RNA processing.
1. Modification of the Sequence or Position of a Gene
The quantity of a gene’s copies as well as its position on a chromosome can have a significant impact on the amount of that gene’s expression. The content of genes or their locations can be changed through processes such as gene amplification, gene reduction, or gene rearrangement.
The process of amplifying genes
It is possible to boost the expression of a specific gene by increasing the number of copies of that gene. Since almost all eukaryotic cells require large amounts of histone proteins and rRNA, the genes that code for histone proteins and rRNA are always present in an amplified state. Amplification of genes can cause complications when trying to treat cancer with chemotherapeutic drugs. Dihydrofolate reductase is the enzyme that is responsible for regenerating the folates that are used in nucleotide synthesis. Methotrexate has the ability to inhibit this enzyme. The gene that codes for dihydrofolate reductase is frequently amplified by several hundred fold in tumour cells, which results in more enzyme production than the drug is able to handle. As a result, tumour cells frequently become resistant to the drug.
Trans acting elements regulate gene expression. At least 100 proteins are known, many for gene regulation. Others regulate gene expression similarly to how CAP-cAMP activates prokaryotic genes. Trans-acting factors have DNA-binding, transcription-activating, and ligand-binding domains.