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As stated before, prokaryotic cells do not have a nucleus, but there is still a plasma membrane, DNA, cytoplasm and other proteins that might be needed within the cell for survival. The central dogma of cellular biology is applicable to prokaryotes – DNA is transcribed into RNA, which is translated into proteins that are needed within the cell. However, since there is no nucleus, transcription and translation occur simultaneously. Because the environment that prokaryotes are subjected to often change, they need to be able to adapt quickly and as a result, gene regulation is less complex than eukaryotic cell gene regulation. Prokaryotic cell regulation occurs at the transcriptional level, which allows for these adaptations.
Prokaryotic cells’ DNA is much like eukaryotes’ DNA in that they are both double stranded. However, there are a variety of differences between prokaryote DNA and eukaryote DNA. A specific type of prokaryote that helps to demonstrate gene regulation in prokaryotic cells is the bacterial cell. Bacteria cell’s DNA is organized into operons, which are clustered genes that allows for feedback mechanistic regulation.
Feedback is a process that occurs within both eukaryotes and prokaryotes to helps to either maintain homeostasis (in eukaryotes) or ensure that proteins that are needed in the cell are created. There is both positive feedback and negative feedback.
Positive Feedback – Occurs within bacteria with the Lac Operon. The Lac Operon is divided into different regulatory portions. The lacl gene is responsible for a protein that binds to the operator. The operator is a portion of DNA that is found upstream from the lacZ, lacY and lacA genes and if the protein made from the lacl gene is bound to it, RNA polymerase is not able to continue transcription past the operator. The lacZ, lacY and lacA genes are responsible for proteins that digest lactose – a nutrient that is not always found around the bacteria.
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When lactose is found in the environment surrounding the bacterial cell, these genes need to be expressed, so this is when the positive feedback is initiated. Lactose is broken down into allolactose, which is a substrate for the protein that binds to the operator, and when bound to that protein, alters the shape of the protein so it no longer fits onto the operator. As a result, the operator stays open and RNA polymerase is able to transcribe the lacZ, lacY and lacA genes, which are needed to digest the lactose that is present.
The extent that the lactose digesting proteins are expressed depends on the amount of lactose found in the environment. More lactose means that the protein that binds to the operon will be bound to allolactose and therefore inactive, which means that lactose digesting proteins will be expressed more, to digest the high levels of lactose. As the levels of lactose decreases, due to digestion by β-Galactosidase, Permease and Transacetylase (the lacZ, lacY and lacA gene proteins), there is less lactose present to bind to the protein that binds to the operator. Because the operator is not bound to lactose as often, it is in the shape that fits onto the operator, again blocking expression of lactose expressing genes more regularly. The process ensures that the lactose digesting protein are only expressed when they are needed.
Negative Feedback – Occurs within bacteria with the Trp Operon. Tryptophan (Trp) is an amino acid that can be found in turkey, among other foods. It is important that there is tryptophan present in a cell in order for protein synthesis to take place. The Trp operon is divided into different regulatory portions. The TrpR gene is responsible for a protein that binds to the operator. The operator is a portion of DNA that is found upstream from the trpE, trpD, trpC, trpB and trpA genes. These genes code for enzymes, which are responsible for tryptophan synthesis. However, tryptophan only needs to be made when the environment surrounding the cell is lacking it. When a cell has an abundance of tryptophan, likely from it’s environment, it is bound to the protein that the trpR gene makes. When this protein is bound to tryptophan, it is in a formation that allows it to bind to the operator, upstream from the trpE, D, C, B and A genes – blocking RNA polymerase from expressing these genes.
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When the environment that the cell is in lacks tryptophan, the cell needs to create it’s own to ensure protein synthesis occurs properly. This is where the negative feedback comes into the picture. Because there is little tryptophan in the cell, it will not be bound to the trpR protein often. This means that the operator will be vacant and RNA polymerase will be able to transcribe the genes that code for the enzymes that are involved in tryptophan synthesis.
These examples of the Lac and Trp operons help to demonstrate how prokaryotic cells regulate at the transcriptional level. The genes that need to be expressed at certain times are expressed because of various mechanisms of control.
The following powerpoint is available for download to act as an aide for presenting this information.
Reference
SBI4U: Biology, Grade 12, University Preparation, Unit 3: Molecular Genetics, Activity 4: Protein synthesis control mechanism. Obtained from: https://download.elearningontario.ca/repository/1225700000/SBI4UCU03A04/content.html
These examples of the Lac and Trp operons help to demonstrate how prokaryotic cells regulate at the transcriptional level. The genes that need to be expressed at certain times are expressed because of various mechanisms of control.
The following powerpoint is available for download to act as an aide for presenting this information.
Reference
SBI4U: Biology, Grade 12, University Preparation, Unit 3: Molecular Genetics, Activity 4: Protein synthesis control mechanism. Obtained from: https://download.elearningontario.ca/repository/1225700000/SBI4UCU03A04/content.html