Genes and Heredity

Operons and Gene Transfer

Operons

Collections of genes in bacteria that code for products with a coordinated function are operons. Genetic regulation of protein synthesis relies on operons—some are inducible and some are repressible.

Eukaryotic cells utilize proteins called transcription factors to regulate gene expression. In prokaryotic bacteria, gene regulation utilizes operons. An operon is a collection of genes that is transcribed together and contains at least two regions of control called a promoter (a short DNA segment where RNA polymerase binds to initiate transcription) and an operator (a regulatory sequence to which a repressor can bind to halt transcription). Operons typically control inducible enzymes that are created based upon environmental stimuli.

An example of a bacterial operon is the lacZ operon. The lacZ operon is found in E. coli and other enteric or intestinal bacteria. The three genes of the lacZ operon encode enzymes for processing lactose into usable material for the bacteria. When the protein lactose is absent from the environment, a repressor binds to the operator, thereby keeping the lacZ operon turned off. A repressor is a protein that binds to the operator to prevent transcription of the operon genes. When lactose is present, the lactose acts as an inducer and binds to the repressor, effectively inactivating it so that RNA polymerase can bind to the promoter and transcription can occur. A repressor protein is considered a negative regulator of a gene, meaning a protein that prevents transcription or translation. An inducer is a protein that binds to and inactivates the repressor in order to permit transcription of the operon genes. An inducer is a positive regulator of a gene, meaning a protein that promotes transcription or translation.

The lacZ Operon

The lacZ operon is a collection of genes in some bacteria that are differentially expressed based on environmental conditions. The presence or absence of lactose in the environment of the bacterium dictates whether or not transcription occurs.

Gene Transfer

Organisms can acquire new genes not directly inherited from a parent through gene transfer. In bacteria, this is completed by one of the three types of horizontal gene transfer: transformation, transduction, and conjugation.

Bacteria use asexual reproduction to generate offspring. The transmission of genes from parent to offspring is called vertical gene transfer. Bacteria are also capable of horizontal gene transfer. Horizontal gene transfer, or lateral gene transfer, is the direct transfer of DNA between individual organisms without reproduction. Three main types of horizontal gene transfer take place in bacteria: transformation, transduction, and conjugation.

Transformation is incorporation of foreign DNA by a bacterial cell; this DNA is subsequently inserted into the genome. Bacterial cells must be at the right lifecycle stage to be able to absorb DNA. "Naked" DNA fragments, those that are no longer inside a cell, are taken up by bacterial cells. Inside the bacteria, the naked DNA is incorporated into the chromosome, resulting in a change in genotype, the genetic makeup of the organism. This may, or may not, also lead to a change in phenotype, the physical characteristics of the organism.

Transduction occurs when a bacteriophage (a virus that infects bacteria) injects DNA into a bacterial cell. Bacteriophage genomes contain the genes necessary for their life cycles as well as DNA acquired from bacteria in previous infection events. Once the viral DNA is inside the bacterium, a viral protein incorporates it into the host’s genome. Transduction takes advantage of the bacterium's normal reproductive cycle by remaining dormant in the host genome. Each time the bacterium replicates, the viral DNA is also replicated.

Conjugation is the direct transfer of genetic material between two bacteria. The bacteria are joined via external structures called pili. The genetic material is transferred through a single pilus in the form of a small piece of circular DNA called the plasmid. A plasmid is a small, circular piece of bacterial DNA that replicates on its own and can be transferred between cells. Once the donor pilus has reached the surface of the other bacterium, the plasmid is replicated and the displaced single strand is transferred through the pilus to the donor cell’s cytoplasm where complementary synthesis recreates the original double stranded plasmid. In order for conjugation to occur, bacterial cells must be within close proximity to one another. In addition, one bacterium has to be carrying a self-transmissible plasmid, or F plasmid. An F plasmid encodes the enzymes and proteins necessary to carry out conjugation. This includes coding for the formation of a pilus through which genetic exchange takes place. Bacteria containing the F plasmid are called F+ cells. Those that do not are called F– cells. If a bacterial population containing only F– cells never comes in contact with F+ bacteria, then they will remain F– and conjugation will not take place.

Horizontal Gene Transfer

There are three types of horizontal gene transfer in bacteria: transformation, transduction, and conjugation. They represent the transfer of bacterial DNA between individuals without reproduction.