A gene is the fundamental unit of heredity. Every individual has two copies of each gene, one inherited from each parent. Although most people contain the same genes, there is a small fraction that is slightly different amongst people. A gene with a small difference in its sequence of DNA bases forms different alleles, contributing to different phenotypes.
However, only 1% of the DNA is composed of genes that encode proteins; the rest, 99% is non-coding DNA. This non-coding DNA performs different functions within a cell but is mainly involved in gene regulation. They act as regulatory elements, providing a site for transcription factors or other regulatory proteins to bind and activate or repress the gene activity.
An average human cell genome contains approximately 30,000 to 120,000 genes, but only a fraction of them are expressed in any specific cell at one time. This is why despite containing the same set of DNA and genes, each cell type in a human body looks different and performs different functions. It is accomplished through gene regulation, i.e., expression of specific genes at proper time and location.
Gene regulation can happen at the level of DNA or RNA. Cells have internal chemical mechanisms that control when a gene is expressed to make RNA and protein, how much of the protein is made, and when to stop making that protein because it is no longer needed. The cells thus turn on the genes only when required, helping them save space and energy. If the cells were to express all genes at once, not only would they require a significant amount of energy but also enormous size because DNA must be unwound from its tightly coiled structure to be transcribed and translated into protein.
Prokaryotic and eukaryotic cells regulate gene expression at different levels. In prokaryotic organisms, the primary control of gene expression is mainly at the transcriptional level, which means that the RNA is continuously transcribed until a protein is required, and it stops when the protein is no longer needed.
In contrast, in eukaryotic cells, the processes of transcription and translation are physically separated by the nuclear membrane, adding to the complexity. Transcription occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. Here, the regulation of gene expression can occur at different stages of the process. It can be regulated when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors (epigenetic level), when the RNA is transcribed (transcriptional level), when the RNA is processed and exported to the cytoplasm after it is transcribed (post-transcriptional level), when the RNA is translated into protein (translational level), or after the protein has been made (post-translational level).
This text is partially adapted from Openstax, Biology 2e, Section 16.1: Regulation of Gene Expression.
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