Transcript Topic 6. Growth & Reproduction of Bacteria

Topic 6
Growth & Reproduction of Bacteria
Biology 1001
October 5, 2005
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
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Each round of division is a generation
Bacterial population growth is therefore rapid and exponential
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1 cell  2 cells  4 cells  8 cells  16 cells etc.
A colony from a single cell in 12 hours
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
Figure
12.11!
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
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
Why Mutation Is Important For Bacterial Populations
Example
2 X 1010 new E. coli are produced
per day in the human intestine
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.
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.
Genetic Recombination

The combining of DNA from two sources

In sexually reproducing organisms this is the main way genetic
variation is produced
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In eukaryotes, it involves the sexual processes of meiosis and
fertilization
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In prokaryotes three other processes are used – transformation,
transduction, and conjugation (= bacterial “sex”)
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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
Conjugation

The direct transfer of genetic material between two bacteria
cells that are temporarily joined

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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
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
Transduction

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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!
Bacterial Populations Evolve
Rapidly

Natural selection operates on genetic (heritable) variation,
such as is generated readily by mutation in bacteria
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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
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The rapid evolution of antibiotic resistance in bacteria is a
medically important example of natural selection at work