Replication in bacteria

1. Reproduction in
bacteria
By:
Mrs. Mali Dhanashri R.
Assistant professor,
GES’s Sir Dr. M. S. Gosavi College of Pharmaceutical
Education and Reseach

2. Reproduction
 Cell growth and reproduction by cell division are
tightly linked in unicellular organisms.
 Bacteria are prokaryotic organisms that reproduce
asexually.
 Bacteria grow to a fixed size and then reproduce
through asexual reproduction
 Under optimal conditions, bacteria can grow and
divide extremely rapidly, and bacterial populations
can double as quickly as every 9.8 minutes.
 In cell division, two identical clone daughter cells
are produced.

3. Types of reproduction
Binary fission
Budding
Fragmentation
Formation of Conidiospores or
sporangiospores

4. Multiplication & divisional
cycle
 Many bacterial cell multiply by binary fusion.
 Means, each individual cell increase in size until it is large
enough to divide into two identical daughter cells.
 At that time of separation each daughter cell must be
capable of growth & reproduction.
 While each daughter cell will automatically contain mRNA,
rRNA, ribosomes, enzymes, cytochromes etc. from mother
cell.

5. Binary fission
 Most prokaryotes reproduce by a process of binary
fission
 In binary fission, the cell grows in volume until it
divides in half to yield two identical daughter cells.
 Each daughter cell can continue to grow at the
same rate as its parent.
 For this process to occur, the cell must grow over
its entire surface until the time of cell division, when
a new hemispherical pole forms at the division
septum in the middle of the cell.

6.  The septum grows inward from the plasma
membrane along the midpoint and forms as
the side wall which pinches inward, dividing
the cell in two.
 In order for the cell to divide in half, the
peptidoglycan structure must be different in
the hemispherical cap than in the straight
portion of the cell wall, and different wall-
cross-linking enzymes must be active at the
septum than elsewhere.

7.  Binary fission begins with the single DNA
molecule replicating and both copies
attaching to the cell membrane.
 Next, the cell membrane begins to grow
between the two DNA molecules. Once the
bacterium just about doubles its original
size, the cell membrane begins to pinch
inward.
 A cell wall then forms between the two DNA
molecules dividing the original cell into two
identical daughter cells

9. Budding
 A group of environmental bacteria reproduces by
budding.
 In this process a small bud forms at one end of the
mother cell
 As growth proceeds, the size of the mother cell
remains about constant, but the bud enlarges.
 When the bud is about the same size as the mother
cell, it separates.
 This type of reproduction is analogous to that
inbudding fungi, such as brewer’s yeast
(Saccharomyces cerevisiae).

10.  One difference between fission and budding
is that, in the latter, the mother cell often has
different properties from the offspring.
 Ex: In some strains, mother cells have a
flagellum and are motile, whereas the
daughter buds lack flagella.


12. Fragmentation
 Becteria may produce extensive filamentous
growth.
 It reproduce by fragmentation of fillaments into
small bacillary or coccoid cells.
 Each filament grows and forms new cells.
 E.g. Norcardia species.

13. Formation of Conidiospores
 Conidia formation takes place in filamentous
bacteria like Streptomyces etc., by the
formation of a transverse septum at the apex
of the filament
 The part of this filament which bears conidia
is called conidiophore.
 After detachment from the mother and
getting contact with suitable substratum, the
conidium germinates and gives rise to new
mycelium.

15. Growth & Genetic exchange
 For many years it was thought that bacteria, unable to
exchange genetic material and could only adapt and evolve
through mutation of genes.
 The bacteria has profound ability to exchange and share
DNA across diverse genera.
 This is of particular significance because it enables
bacterial populations to adapt rapidly to changes in their
environment,.
 This also help in the deployment of antibacterial chemicals
and antibiotics.
 Three major process involved in genetic exchange….
 Transformation
 Transduction
 Conjugation

16. Transformation
 Discovered by Frederick Griffith in 1928.
 The early work on the transfer of virulence in the
pathogen Streptococcus pneumoniae .
 The stage for the research that first showed that DNA
was the genetic material.
 Griffith found that if he boiled virulent bacteria and
injected them into mice, the mice were not affected
and no pneumococci could be recovered from the
animals.
 When he injected a combination of killed virulent
bacteria and a living nonvirulent strain, the mice died;
moreover, he could recover living virulent bacteria
from the dead mice.
 Griffith called this change of nonvirulent bacteria into
virulent pathogens transformation.

18.  Many bacteria can acquire new
genes by taking up DNA
molecules (ex: plasmid) from their
surroundings.
 When bacteria undergo lysis, they
release considerable amounts of
DNA into the environment.
 This DNA may be picked up by a
competent cell- one capable of
taking up the DNA and undergoing
a transformation.
 To be competent, bacteria must be
in the logarithmic stage of growth,
and a competence factor
needed for the transformation
must be present.

19. Transduction
 It is defined as a phenomenon causes
genetic recombination in bacteria wherein
DNA is carried from one specific bacterium
to another by a bacteriophage.
 Bacterial viruses ( bacteriophages)
transfer DNA fragments from one bacterium
(the donor) to another bacterium (the
recipient).
 The viruses involved contain a strand of
DNA enclosed in an outer coat of protein.

21.  After a bacteriophage enters a bacterium, it may
encourage the bacterium to make copies of the
phage.
 At the conclusion of the process, the host
bacterium undergoes lysis and releases new
phages. This cycle is called the lytic cycle.
 Under other circumstances, the virus may attach
to the bacterial chromosome and integrate its
DNA into the bacterial DNA.
 It may remain here for a period of time before
detaching and continuing its replicative process.
This cycle is known as the lysogenic cycle.

22.  Under these conditions, the virus does not
destroy the host bacterium, but remains in a
lysogenic condition with it. The virus is called a
temperate phage, also known as a prophage.
 At a later time, the virus can detach, and the lytic
cycle will ensue.
 It will express not only its genes, but also the
genes acquired from the donor bacterium.
 As well as temperate phage will be active once
again at a low frequency & phasing between
temperate & lytic forms ensures the long-term
survival of the virus.

24. Conjugation
 Conjugation is a natural process
representing the early stages in a true
sexually reproductive process.
 In that transcribed to produce singular
viral elements, which cannot assemble
or lyse the host cell. Such DNA strand
are known as plasmids.
 Plasmids are circular & can either be
integrated into the main chromosome, in
which case they are replicated along
with chromosome & passed to daughter
cells or they are separate from it & can
replicate independently.
 The simplest form of plasmid is F-factor
(fertility factor).
 The cells containing an F-factor are
designed F+