Regulation of Gene Expression Flashcards
Gene Types
These can be constitutive or contingency. Constitutive are always expressed as they are essential for the basics of life e.g. DNA synthesis, replication and repair, RNA and proteins synthesis. Contingency encode for products that confer an advantage under special conditions e.g. heat stress, pH stress, starvation, carbon source availability etc. Every operation in a living cell must be coordinately regulated to ensure that certain processes in certain areas which is required for efficiency and conservation of energy.
Contingency Genes
These are typically activated by triggers as they aren’t necessarily maintained and replicated consistently in order to conserve energy. For example if there is a substrate of a pathway present then catabolic enzymes can act to break substances down. If the substrate either isn’t present or is present but not in its ideal form then enzymes will be synthesised (genes turned on) however if it is in ideal form or not present then enzymes won’t be synthesised. With biosynthetic enzymes the presence of the end product is what regulates where if the end product of the reaction is available then enzymes aren’t synthesised whereas if they aren’t present then enzymes are synthesised (genes turned on) to promote the reaction to produce the end product.
Prokaryote vs Eukaryote Gene Regulation
In prokaryotes the purpose of this is to provide maximum growth to outperform other single-celled organisms whereas in eukaryotes it is to regulate development and differentiation to ensure the correct functions throughout the organism and prevention of cancers. The challenges faced by prokaryotes are continual environmental change whereas for eukaryotes the environment is more constant. The range of gene expression in prokaryotes is that there is rarely ever a total switch off with ‘off’ really meaning operation at a low basal level to ensure quick responses without wasting too much energy whereas for eukaryotes genes are completely turned off and this is common. The production of mRNA in prokaryotes is coupled transcription and translation occurs in the cytoplasm and uses polycistronic (codes for multiple proteins on one strand) mRNA whereas in eukaryotes transcription and RNA splicing occurs in the nucleus and translation occurs in the cytoplasm.
Gene Regulation in Prokaryotes
The effect of these methods means the number of protein molecules produced per unit time by active genes varies from gene to gene and varies in response to the environment. This is done to satisfy the needs of the cell without wasting energy e.g. molecules needed occasionally are only synthesised when required, if there are 2 pathways for energy production the cell will ‘choose’ the one that yields the most energy and enzymes that use up substrates of other enzymes are inhibited if the product of that substrate and enzyme are required. The mechanisms for this are typically on/off regulation where a system is turned on when needed and off when not needed and other multiple ways where both transcription and translation can be controlled.
4 Ways a Cell Control Protein Production
- Transcriptional control: controlling when and how often a given gene is transcribed.
- RNA processing control: controlling how the RNA transcript is spliced or otherwise processed (polyadenylation or capping) which is typically used more by eukaryotes.
- Translation control: selecting which mRNAs in the cytoplasm are translated by ribosomes.
- Post-translational control: an example is protein activity control where proteins are selectively activated or inactivated after they have been made.
Prokaryotes mostly use transcriptional control as the first point of regulation however they also use post-translational control when required.
Coordinate Regulation (Principle of Gene Regulation)
This is a metabolic pathway which converts compounds into other compounds due to the production of polycistronic mRNA which can encode for multiple enzymes to undergo this process. This only occurs in prokaryotes as eukaryotes typically don’t have polycistronic mRNA.
Degradative vs Biosynthetic Pathways (Principle of Gene Regulation)
in catabolic processes larger complex molecules being present leads to enzyme production to break them down to smaller end products. In anabolic processes the presence of smaller precursor products leads to the production of enzymes to produce a larger more complex end product.
Negative vs Positive Regulation (Principle of Gene Regulation)
In this form of control a repressor protein is present in the cell and prevents transcription. In some cases an antagonist inducer is needed to remove the repressor and allow the initiation of transcription. In other cases a corepressor (ligand) binds to the repressor on the DNA to block transcription. In all of these cases when the repressor is bound to DNA it prevents transcription of the gene. In this form of control an activator protein binds to the DNA and allows transcription as long as a repressor isn’t present. A ligand (e.g. sugars, amino acids etc.) may bind to the activator. The specificity of the ligand may determine whether the activator can or can’t bind to the DNA. In all these cases when the activator binds to the DNA it results in the activation of transcription of a gene.
Operator
These sit on the DNA code around the promotor sequences (either between 2 sequences, upstream or downstream from the promoter) of transcription. This is what either the repressor or activator binds to in order to repress or activate transcription.
Genetic Organisation of Bacterial Chromosomes
In a gene the DNA sequence that codes for a polypeptide, tRNA or rRNA is represented as arrows on a genetic map and found on both strands of DNA where they don’t generally overlap. In bacteria genes are arranged singly or in operons.
Operon
A group of genes adjacent to each other on the chromosome that are transcribed from a single promotor into a single mRNA molecule. These are all on the same strand moving in the same direction (either 5’-3’ or 3’-5’). These sequences aren’t found in eukaryotes. These are used for a specific biosynthetic pathway e.g. lipopolysaccharide production where the genes code for an enzyme that produces lipopolysaccharide, enzymes that move it into the membrane etc.
Tryptophan Operon
This is a biosynthesis operon responsible for the production of a specific amino acid. This operon consists of 5 genes that are used to encode this process. The operator region (15bp long) is within the promoter sequence. Each gene produces a specific enzyme necessary for the production of the amino acid.
Promoter
A DNA sequence the RNA polymerase binds to in order to open the DNA double helix and to begin synthesising the mRNA.
Repressor & Activator
A protein that binds to an operator sequence to prevent the transcription of adjacent genes in the operon. A protein that binds to an operator sequence to cause the transcription of adjacent genes in the operon.
Polycistronic mRNA
RNA that has more than one coding region and is translated into a number of different proteins. This is formed when an operon is transcribed.
