Topic 6 Growth

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Topic 6 Growth Reproduction of Bacteria

• Biology 1001
• October 5, 2005

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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

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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

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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

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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

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Why Mutation Is Important For Bacterial
PopulationsExample
Thats 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, thats 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 thats about
18 million mutations in 25 years or only 2000
per day, in the entire human population.
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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

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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

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Transformation

• The alteration of a bacterial cells 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

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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!?

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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