Gene Expression
The two steps of central dogma i.e., transcription and translation are collectively called gene expression. In eukaryotes, the expression of genes can be controlled at different levels. Different methods of controlling gene expression in eukaryotes are described below.
1. Transcriptional Control of Gene Expression
The RNA synthesis depends on RNA polymerase enzymes. Numerous proteins are called transcription factors to help in the action of these enzymes. The RNA polymerase and transcription factor bind to specific sequences of promoter.
The transcription factors bind selectively to enhancer regions of DNA. This enhancer region of the DNA is present thousands of nucleotides away from the promoter. The specific associations between transcription factors and enhancer sites in the genome play role in the control of expression.
2. Post-transcriptional Control of Gene Expression
Gene expression may be blocked or stimulated at any post-transcriptional step. Gene expression can be controlled during RNA processing and export of mRNA. The cell must process initial transcripts before they act as an mRNA, tRNA, or rRNA.
mRNA transcript must receive a 5’ cap and a poly-A tail. The introns must be removed and exons spliced together of RNA. Then this RNA passes from the nucleus to the cytoplasm. In the cytoplasm, mRNA interacts with a number of specific proteins and may associate with ribosomes.
Each step in RNA processing provides an opportunity for control of expression.
3. Translational and Post-translational Control
Translation in eukaryotic cells involves many protein factors like initiation factors. Thus, there are opportunities for the control of gene-level at the level of translation. The last opportunities for controlling gene expression occur after translation.
The control of hormone insulin is an example of post-translational control.
4. Role of hormones in Gene Expression Control
a. Action of hormones in invertebrates:
Polytene chromosomes are present in the salivary glands in certain insect larvae. The presence of these chromosomes provides evidence for the control of gene expression at the transcriptional level.
The chromosome puff appears at specific sites of the polytene. It makes the DNA in that region more accessible to RNA polymerase. The formation of puff is controlled by the hormone ecdysone which causes molting in insects.
b. Action of steroid hormones in vertebrates:
Sex hormones and other steroid hormones alter their gene expression in target cells in vertebrates. When a cell is exposed to steroids, hormone diffuses across the plasma membrane and crosses the cytoplasm.
It enters the nucleus and binds to the soluble receptor protein. The binding of the steroid hormone to the receptor causes the release of inhibitory protein. Therefore, the activated receptor protein can now attach to specific sites on DNA.
These sites are present within enhancer regions that control steroid-responsive genes. Thus, steroid acts as a chemical signal to switch on specific genes in certain cells.