Topic 6 Growth & Reproduction of Bacteria

1. Topic 6Growth & Reproduction of Bacteria Biology 1001 October 5, 2005

2. Growth of Bacterial Populations • Growth of bacteria refers to number rather than size of cells • Under optimal conditions, a single prokaryote cell divides to produce two daughter cells every ~ 1-3 hours • Each round of division is a generation • Bacterial population growth is therefore rapid and exponential • 1 cell  2 cells  4 cells  8 cells  16 cells etc. • A colony from a single cell in 12 hours

3. Figure 12.11! Bacteria Divide by Binary Fission • Prokaryotes reproduce asexually by cell division called binary fission • First, the bacterial chromosome begins to replicate, starting at the origin of replication • Replication continues, one origin moves to the opposite side of the cell, and the cell elongates • Replication finishes, the plasma membrane grows inward, and a new cell wall is laid down • Produces two genetically identical daughter cells – clones • Binary 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 species • Mutation • Recombination

5. Mutation • A mutation is a change in the DNA of a gene, ultimately leading to genetic diversity • Mutations can be spontaneous or caused by mutagens • Spontaneous - Errors during DNA replication • Mutagens - Chemical or physical factors that damage DNA • Spontaneous mutations are extremely rare, occurring on average only once in 10 million cell divisions, per gene • Because bacteria divide rapidly & exponentially, mutation is a relevant factor generating genetic diversity

6. Why Mutation Is Important For Bacterial PopulationsExample 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 intestine Say 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 sources • In sexually reproducing organisms this is the main way genetic variation is produced • In eukaryotes, it involves the sexual processes of meiosis and fertilization • In 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. Conjugation • The direct transfer of genetic material between two bacteria cells that are temporarily joined • DNA transfer is one-way, from “male” to “female” • The donor (“male”) uses an appendage called the sex pilus that forms a cytoplasmic mating bridge • DNA gets transferred via this bridge in the form of a plasmid • The plasmid encodes the ability to mate as well as other traits such as antibiotic resistance

9. Transformation • The alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment • Many bacteria possess cell surface proteins that facilitate transformation in natural populations • E. 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. Transduction • Phages (viruses that infect bacteria) carry bacterial genes from one host cell to another as a result of mistakes in the phage reproductive cycle • In the process called generalized transduction, this transfer is random Figure 18.16!

11. Bacterial Populations Evolve Rapidly • Natural selection operates on genetic (heritable) variation, such as is generated readily by mutation in bacteria • A mutation that confers a reproductive advantage increases in frequency in subsequent generations, and eventually becomes fixed in the population • Bacteria reproduce quickly and therefore have a short generation time relative to most other organisms • The rapid evolution of antibiotic resistance in bacteria is a medically important example of natural selection at work