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.
Plot the following on log linear paper
Time after inoc (h)
0
Cell Number
1 x10 2
Log
2.0
1
2
3
2.4 x10 2
4.8 x10 2
8.5 x10 2
~ 2.3
~2.6
~2.9
4
5
6
1.6 x 10 3
3.2 x 10 3
6.4 x 10 3
~3.2
~3.5
~3.8
7
1.28 x 10 4
~4.1
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
Number of generations
= minutes/generation
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
14 generations
See page 877 Totora
= 21 minutes/generation
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