Each gene encodes instructions for proteins but genes can be turned on or off and non-genetic or epigenetic factors influence what genes can or cannot do.
Jacob, Lwoff and Monod, French scientists, studied genes related to the metabolism of lactose in a simple and common bacterium, Escherichia coli. Jacob, Lwoff and Monod were able to develop a model that was unique and true, won them a 1965 Nobel prize in Physiology/Medicine and to this day serves as a basic research model that has been expanded and developed for a large array of different genes. The Jacob, Lwoff, Monod lactose operon is discussed following.
Lactose Operons: Operator and Promoter Gene Regions Inducers and Repressor Proteins
Here are some important things to keep in mind as you study genes and gene regulation:
- DNA physically is arranged as a double-helix and resembles a twisted ladder.
- DNA chemically is composed of a deoxyribose-phosphate backbone and four different kinds of nitrogen bases: adenine, thymine, guanine and cytosine (see photo below).
- Genes are DNA codes that are specific and defined sequences of these four bases. Some genes are bigger than others, therefore the size of a gene (total number of base pairs) indicates the approximate size of the final protein.
- DNA specifies the messenger-RNA (m-RNA) the ribosomal-RNA (r-RNA) and the transfer-RNA (t-RNA or s-RNA). The m-RNA instructs for the amino acid sequence of the protein, the r-RNA complexes with structural proteins to form a site for the m-RNA and t-RNA activities to be accomplished for making the protein. Each of the 20 different t-RNA carries one specific kind of amino acid and the anti-codon matches against the codon of the m-RNA to sequence the protein correctly.
- Genes can be regulated or controlled in such a way as to be turned on (active), or off (inactive). One way this is accomplished is by histone proteins, small basic and positively charged molecules that can interact with the negatively-charged regions of DNA and close off the DNA from being "read". The complex formed by the genes and protein is termed chromatin or chromosome.
- Operons are regulated gene sites which may be repressed (off) or induced (on). Genes have a regulatory coded DNA area (operator) that can be recognized by proteins. Proteins that are "repressors" can attach to operator sites and turn the gene off. When this occurs the repressor blocks an enzyme called RNA polymerase from attaching to the DNA. Thus the RNA polymerase can not transcribe the gene and make a copy of the m-RNA needed as the instruction sheet for making the protein.
- Repressor proteins can be removed from the operator site by means of inducers which are molecules that inactivate a repressor protein and remove it from the gene operator site. In the E.coli model the repressor protein is removed from the operator site when it interacts with lactose.
- When the repressor protein is removed the RNA polymerase finds the promoter region and starts to move along and transcribe the m-RNA. In the lactose operon the RNA polymerase begins to move down from the promoter and causes three genes to be transcribed as distinct m-RNA: B-galactosidase (Z site), permease (Y site) and another enzyme thiogalactoside transacetylase ("A" site).
How the Jacob and Monod Operon Model Works – Five Things to Know
Here are 5 things to remember about the lactose operon;
- When glucose is available the lactose operon is turned off and not working. The repressor protein is attached to the operator (on-off switch) gene site and blocks the RNA polymerase from attachment at the promoter region of the operon.
- If glucose is used up and lactose is available some lactose enters the cell and interacts with the repressor protein. The lactose-protein interaction results in a change of the repressor protein and the protein is released from binding to the DNA at the operator site. Lactose is classified as an inducer.
- When the repressor protein is released from the DNA, then the RNA polymerase begins to transcribe the genes including B-galactosidase (converts lactose to glucose and galactose) and lactose permease (helps carry lactose into the cell rapidly).
- The lactose enters the cell is broken down by the enzyme B-galactosidase into glucose and galactose and new energy is available to the cell for growth and survival.
- When the lactose is all used up and no more is "visible" inside the cell the repressor protein assumes its normal shape, attaches to the operator site and shuts the lactose operon down (off). No lactose-processing enzymes are needed when lactose is absent and the cell has a form of what we might say is "molecular intelligence". These mechanisms of feedback inhibition or feedback control help cells to conserve energy and molecular resources.
There are molecules such as sugars, amino acids and even alcohol (Miles, M.F.) that can control the expression of genes. Genes may be turned "off" by repressor proteins and turned "on" by inducers that inactivate repressors and turn the gene on. Scientists continue to discover much more about the complexities and intricacies of gene control and regulation as they delve deeper into many different genes, proteins and other biochemical molecules of importance.
Source
Miles, M.F. "Alcohol’s effects on gene expression." Alcohol Health & Research World 19(3): 237–243