Binary fission Procaryotic cell division:

– Binary fission is the process by which most procaryotes replicate.

– Binary fission generally involves the separation of a single cell into two more or less identical daughter cells, each containing, among other things, at least one copy of the parental DNA.

– Stepwise process: • The first steps of binary fission include cell elongation

and DNA replication. • The cell envelope then pinches inward, eventually

meeting. • A cross wall is formed and ultimately two distinct cells

are present, each essentially identical to the original parent cell.

Generation time [doubling time] Procaryotic cell division: A bacterial generation time is also know as its doubling

time.Doubling time is the time it takes a bacterium to do one

binary fission starting from having just divided and ending at the point of having just completed the next division.

Generation times vary with organism and environment and can range from 20 minutes for a fast growing bacterium under ideal conditions, to hours and days for less than ideal conditions or for slowly growing bacteria.

•     Definition: The generation time is the time it takes for the number of cells in a culture to double. 

•     It is usually determined only during exponential phase growth. 

 

Generation Time • •     Two ways one can determine:• –  [1] mathematically• –  [2] graphically

• Graphical Determination of Generation Time.• •     Plot the number of cells/ml against time is plotted on

semi-log paper.  • –   X-axis (time) on linear scale• –   Y-axis (number cells/ml) on logarithmic scale• •     Generation time is determined by the time it takes

the number of cells to double. 

Time after inoc (h) Cell Number Log

0 1 x10 2 2.0

1 2.4 x10 2 ~ 2.3

2 4.8 x10 2 ~2.6

3 8.5 x10 2 ~2.9

4 1.6 x 10 3 ~3.2

5 3.2 x 10 3 ~3.5

6 6.4 x 10 3 ~3.8

7 1.28 x 10 4 ~4.1

Plot the following on log linear paper

Calculating Generation time

• 2 number of generations - describes how the cells increase exponentially

• Initial number of cells x 2 number of generations = final cell concentration

• e.g. if 5 cells divided 9 times

5 x 2 9 =2560 cells

• To calc. the number of generations, cell numbers must be converted to

logs

No. of generations= log cell # at end –log cell number at start

0.301

(Note: 0.301 is log of 2)

• To calc generation time for a population

60min/hr x hours = minutes/generation

Number of generations

To calc generation time for a population

60min/hr x hours = minutes/generation

Number of generations

e.g. 100 bacteria growing for 5 hours produce 1720320 cells

Log 1720320-log 100 = 14 generations

0.301

60min/hr x 5 hours = 21 minutes/generation

14 generations

See page 877 Totora

Generation times for some common bacteria under optimal

conditions of growth Bacterium Medium Generation Time (minutes)

Escherichia coli Glucose-salts 17

Bacillus  megaterium Sucrose-salts 25

Streptococcus lactis Milk 26

Streptococcus lactis Lactose broth 48

Staphylococcus aureus Heart infusion broth 27-30

Lactobacillus acidophilus Milk 66-87

Rhizobium japonicum Mannitol-salts-yeast extract 344-461

Mycobacterium tuberculosis Synthetic 792-932

Treponema pallidum Rabbit testes 1980