Regulation in Eukaryotic Cells
Gene expression is the ability of a gene to produce a biologically active protein. This process is regulated by the cells of an organism, it is very important to the survival of organisms at all levels. This is much more complex in eukaryotes than in prokaryotes. A major difference is the presence in eukaryotes of a nuclear membrane, which prevents the simultaneous transcription and translation that occurs in prokaryotes. Initiation of protein transcription is started by RNA polymerase. The activity of RNA polymerase is regulated by interaction with regulatory proteins; these proteins can act both positively, as activators, and negatively as repressors. An example of gene regulation in cells is the activity of the trp operon. The trp operon encodes the genes for the synthesis of tryptophan. This type of gene, like the lac operon, is regulated by a repressor that binds to the operator sequences. The activity of the trp repressor is enhanced when it binds tryptophan; in this capacity, tryptophan is known as a corepressor. Since the activity of the trp repressor is enhanced in the presence of tryptophan, the rate of expression of the trp operon is graded in response to the level of tryptophan in the cell. Another example of gene regulation in cells is gene amplification. This is a Technique by which selected DNA from a single cell can be duplicated indefinitely until there is a sufficient amount to analyse by conventional genetic techniques.
The expression of genes is very complicated and is regulated by a number of things. Something that has a major effect on enzyme regulation and stability is temperature. Since enzymes are biochemical catalysts, made up at least partially of protein, they are sensitive in varying degrees to heat. Raising temperatures of the environment generally multiplies the degree of activity by the enzyme. Once an optimum temperature has been reached, however, temperatures that are too high will denature the enzyme it will loose its ability to function. Cofactors are agents that are necessary for an enzyme to carry out a transformation. An apoenzyme is the enzyme without the cofactor and the holoenzyme is the active species made by combining the cofactor with the apoenzyme. The coenzymes are small organic molecules that are non-peptide in nature and are sometimes also known as a prosthetic group. Vitamins are small organic molecules that act as coenzymes although not all coenzymes are vitamins. A vitamin is a molecule that cannot be produced within the body and must be introduced i.e. through diet. Another example of enzyme regulation is that of competitive inhibitors. If a reversible inhibitor can bind to the enzyme active site in place of the substrate, it is described as a “competitive inhibitor.” In pure competitive inhibition, the inhibitor is assumed to bind to the free enzyme but not to the enzyme-substrate complex. When a non-competitive inhibitor binds to the enzyme at the regulatory site, the shape of the active site changes so that it can no longer bind its substrate or catalyze the production of product. The enzyme will remain inhibited until the non-competitive inhibitor leaves the regulatory site.
The survival of organisms greatly depends on the regulatory mechinisims of the density of a population. The number of individuals per unit area (or volume) is the population density. Population dynamics are changes in structure resulting from reproduction, growth, energy gathering, dispersal and death of members of a population. In some populations, nearly all individuals survive for the potential life span and die almost simultaneously (e.g., human). In others, survivorship remains the same throughout the life span. In some others, survivorship of young individuals is low but survivorship is high for the remainder of the life span (e.g., many marine invertebrates with planktonic larvae). Populations have the potential for exponential growth because the number born far exceeds the number which mature and reproduce. Expressed mathematically: G = (b-d) * N where G is the rate of increase in number of individuals, b and d are the average birth rate and death rate per individual, respectively, and N is the number of individuals. Interspecific competition refers to the competition between two or more species for some limiting resource. This limiting resource can be food