2 Growth of Bacterial Populations Growth of bacteria refers to number rather than size of cellsUnder optimal conditions, a single prokaryote cell divides to produce two daughter cells every ~ 1-3 hoursEach round of division is a generationBacterial population growth is therefore rapid and exponential1 cell 2 cells 4 cells 8 cells 16 cells etc.A colony from a single cell in 12 hours
3 Bacteria Divide by Binary Fission Figure12.11!Prokaryotes reproduce asexually by cell division called binary fissionFirst, the bacterial chromosome begins to replicate, starting at the origin of replicationReplication continues, one origin moves to the opposite side of the cell, and the cell elongatesReplication finishes, the plasma membrane grows inward, and a new cell wall is laid downProduces two genetically identical daughter cells – clonesBinary fission is the evolutionary precursor to mitosis
4 Mechanisms That Produce Variation If binary fission produces clonal offspring, why are bacteria so genetically diverse???Two factors contribute to genetic diversity among and within bacterial speciesMutationRecombination
5 MutationA mutation is a change in the DNA of a gene, ultimately leading to genetic diversityMutations can be spontaneous or caused by mutagensSpontaneous - Errors during DNA replicationMutagens - Chemical or physical factors that damage DNASpontaneous mutations are extremely rare, occurring on average only once in 10 million cell divisions, per geneBecause bacteria divide rapidly & exponentially, mutation is a relevant factor generating genetic diversity
6 Why Mutation Is Important For Bacterial Populations Example That’s 2 X 1010 / 10 million = 2000 E. coli with a mutation in a single gene per day.Multiplied by the number of genes in the E. coli genome, 4300, that’s ~ 9 million mutated bacteria per day in a single human host.2 X 1010 new E. coli are produced per day in the human intestineSay the human population of ~ 6 billion replaces itself about once every 25 years. Because humans have about 30,000 genes per genome that’s about 18 million mutations in 25 years or only ~ 2000 per day, in the entire human population.
7 Genetic Recombination The combining of DNA from two sourcesIn sexually reproducing organisms this is the main way genetic variation is producedIn eukaryotes, it involves the sexual processes of meiosis and fertilizationIn prokaryotes three other processes are used – transformation, transduction, and conjugation (= bacterial “sex”)Results in horizontal gene transfer – the transfer of genetic material within a generation, instead of from one generation to the next – a major force in the long-term evolution of bacteria
8 ConjugationThe direct transfer of genetic material between two bacteria cells that are temporarily joinedDNA transfer is one-way, from “male” to “female”The donor (“male”) uses an appendage called the sex pilus that forms a cytoplasmic mating bridgeDNA gets transferred via this bridge in the form of a plasmidThe plasmid encodes the ability to mate as well as other traits such as antibiotic resistance
9 TransformationThe alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environmentMany bacteria possess cell surface proteins that facilitate transformation in natural populationsE. coli is used in biotechnology applications of genetic recombination (genetic engineering)Cells are cultured in high CaCl2 to become “competent”Cells are then transformed with human genes that code for proteins such as insulin or growth hormone that are needed in large amounts
10 TransductionPhages (viruses that infect bacteria) carry bacterial genes from one host cell to another as a result of mistakes in the phage reproductive cycleIn the process called generalized transduction, this transfer is randomFigure 18.16!
11 Bacterial Populations Evolve Rapidly Natural selection operates on genetic (heritable) variation, such as is generated readily by mutation in bacteriaA mutation that confers a reproductive advantage increases in frequency in subsequent generations, and eventually becomes fixed in the populationBacteria reproduce quickly and therefore have a short generation time relative to most other organismsThe rapid evolution of antibiotic resistance in bacteria is a medically important example of natural selection at work