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Given that all the genetic material of a cell is present inside the cell at all times, how does the cell know when and how much to transcribe and translate a given gene? And how does this gene expression affect phenotypic differences? While we have focused so far on the sections of DNA that code for proteins, there are also sections of DNA that are designed to interact with proteins and control transcription. These sections of DNA are called regulatory sequences. There are numerous types and functions of regulatory sequences, some of which we will discuss in this section. Regulation of gene expression is important because the combination of genes that are expressed and the levels at which they are expressed dictates what happens in a cell and an organism. It also helps maintain order and conserve energy and resources within a cell.
Promoters and Transcription Factors
Promoters are specific DNA sequences where RNA polymerase binds to start transcription. These play a critical role in the initiation of transcription and ultimately gene expression. They are located immediately before the gene they control. This allows for very targeted control of gene expression. Transcription factors are proteins that bind to promoters, or other DNA sequences, and activate or repress transcription. They do this by controlling if RNA polymerase binds and therefore whether transcription occurs. Transcription factors that promote gene expression are called activators and those that inhibit gene expression are called repressors. Repressors may also block activators and thereby block transcription.
Transcription factors have broad applications and play an integral role in life. Some transcription factors help organisms respond to environmental signals like heat shock. Others regulate genes involved in metabolism. Changes in transcription factors can also have impacts on phenotypic traits such as height and skin color.
Groups of Genes
Cells often have related genes grouped together. This allows for efficiency of production as expression can be regulated together. In prokaryotes, groups of genes are called operons. During transcription, since the operon is controlled by a single promoter, all of the genes in a given operon are transcribed together in a single mRNA molecule. In eukaryotes, the transcription of groups of genes are coordinated by the genes having the same transcription factors.
One example of an operon is the lac operon. This operon is found in bacteria and controls the conversion of lactose to glucose and galactose. When lactose is present, it binds to the lacl repressor protein, forcing it to release from the DNA, and allowing the lac genes to be transcribed. Translation of these genes produces an enzyme that facilitates the breaking down of lactose. When lactose is not present, the lacl repressor protein prevents transcription of the gene.
Epigenetic Changes
Epigenetic regulation is from changes to the DNA that affect how the DNA acts in the cell but does not alter the DNA sequence. Two main examples of this are DNA methylation and histone modification. DNA methylation is the addition of methyl groups to nucleotides in DNA. This can prevent gene expression from stopping transcription factors from binding to the promoter region. Histone modification can also change which genes are expressed and which are not. Histones are the proteins around which DNA is wrapped. Modifications to these can either make the DNA more tightly wound or more relaxed. When the DNA is tightly wound, it is less accessible to the molecules that lead to gene expression. On the contrary, when the DNA is more relaxed, more gene expression occurs. Through processes like these, two cells (or organisms), that have identical genetic material, could present very different phenotypes.
