Bacteria replication, recombination, and transformation By Dustin Heuterman and Monica Reeder Bacteria replication, recombination, and transformation

The Bacterial Genome and Its Replication The main component of the genome in most bacteria is one double-stranded circular DNA molecule that is associated with a small protein. A nucleoid is a dense region of DNA that is tightly coiled with the help of certain proteins and is not bounded by membrane like the nucleus of a eukaryotic cell. Bacterial cells divide by binary fission, which is preceded by replication of the bacterial chromosome. Bacteria can prosper in a favorable environment causing them to divide rapidly, whether an artificial or organic environment. E. Coli growing under optimal conditions can divide every 20 minutes.

Mutation and Genetic Recombination as Sources of Genetic Variation Binary Fission is an asexual process so that means most of the bacteria in a colony are genetically identical to the parent cell unless a mutation occurs in a bacteria cell. Bacteria that are well equipped for a local environment divide more prolifically than bacteria that are ill-equipped. Even though most bacterial cells reproduce asexually, genetic recombination does occur in bacteria (sexual reproduction). Such as an experiment where two mutated E. Coli strands couldn’t synthesize either tryptophan or arginine, but when both cells were incubated together, cells emerged that could synthesize both amino acids.

Mechanisms of Gene Transfer and Genetic Recombination in Bacteria Meiosis combines DNA in eukaryotes, but in prokaryotes, three other processes are used: Transformation – The alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment. Transduction – Phages (the viruses that infect bacteria) carry bacterial genes from one host cell to another as a result of aberrations in the phage reproductive cycle. Conjugation and Plasmids – The direct transfer of genetic material between two bacterial cells that are temporarily joined. F factor – The ability to form sex pili and donate DNA during conjuration results from the presence of a special piece of DNA. Plasmid – A small, circular, self-replicating DNA molecule separate from the bacterial chromosome. Episome – A genetic element that can replicate either as part of the bacterial chromosome or independently of it.

The F Plasmid and Conjugation F Plasmid – The F factor and its plasmid form. F Plasmid = F+ Cells. The F Plasmid Cells function as DNA donors during conjugation. Cells lacking the F Factor are F- cells. When F- cells and F+ cells conjugate the F- becomes F+. An Hfr cell with the F factor built into its chromosome. The F factor DNA moves into the F- partner dragging along adjacent chromosomal DNA. If the new DNA aligns with the F- chromosome, segments of DNA can be exchanged.

R Plasmids and Antibiotic Resistance Mutations in genes can give rise to antibiotic resistance. An R Plasmid are plasmids that carry genes conferring antibiotic resistance. Resistant strains of pathogens are becoming more common making treatment of certain bacterial infections more difficult. R Plasmids have genes that encode sex pili and enable plasmid transfer from one bacterial to another by conjugation.

Transposition of Genetic Elements Transposable Elements are the DNA of a single cell that can also undergo recombination. They never exist independently but are always part of chromosomal or plasmid DNA. The movement of these elements is called Transposition. They can move from a plasmid to the chromosome or from one plasmid to another, sometimes called “Jumping Genes.” During bacterial transformation, generalized transduction, and conjuation, recombination occurs between homologous regions of DNA.

Insertion Sequences Insertion Sequence contains a single gene which codes for transposase, an enzyme that catalyzes movement of the insertion sequence from one site to another within the genome. During transposition, molecules of the enzyme bind to the inverted repeats and to a target site elsewhere in the genome and catalyze the necessary DNA cutting and resealing. They can cause mutations if it transposes into the coding sequence of a gene or into a DNA region that regulates gene expression.

Transposons Transposons are transposable elements longer and more complex than insertion sequences and move about in the bacterial genome. Extra genes are sandwiched between two insertion sequences so it is as though two insertion sequences happened to land relatively close together as a single transposable element. Transposons may help bacteria adapt to new environments. Transposons can add a gene for antibiotic resistance to a plasmid already carrying genes for resistance to other antibiotics. Transposons are not unique to bacteria and are important components of eukaryotic genomes.