2. DNA exchange among bacteria
DNA exchange among bacteria
• DNA can be exchanged among bacteria in three
ways:
1. Conjugation
The transfer of genetic material between
bacterial cells by direct cell-to-cell contact or by a
bridge-like connection between two cells.
2. Transduction
A phage carries DNA from one bacterium to
another.
3. Transformation
Cells take up free DNA directly from their
environment.
4. History
History
• In 1928, Fred Griffith found that one form of the
pathogenic pneumococci (now called
Streptococcus pneumoniae) could be
mysteriously “transformed” into another form.
• Griffith made a conclusion that the dead
pathogenic bacteria gave off a “transforming
principle” that changed the live nonpathogenic
rough-colony-forming bacteria into the
pathogenic smooth-colony form.
5. Competence
Competence
• The ability of some bacteria to take up naked
DNA from their environment.
• It is genetically programmed. Generally, more
than a dozen genes are involved, encoding both
regulatory and structural components.
• The general steps that occur in natural
transformation differ in Gram-negative
and positive bacteria.
7. Types of Transformation
Types of Transformation
There are two types of transformation:
1.Natural transformation
2.Artificial transformation
8. Natural Transformation
Natural Transformation
• In this case DNA take-up occurs without outside
help.
• Naturally competent bacterium – They can
take up DNA from the environment without
requiring special treatment.
• About 40 species have been found to be
naturally competent or transformable.
• Examples: Bacillus subtilis, Streptococcus
pneumoniae, Haemophilus influenzae, Neisseria
gonorrhoeae, Helicobacter pylori, Acinetobacter
baylyi, and some species of marine
cyanobacteria.
9. Steps involved in natural
Steps involved in natural
transformation
transformation
Following proteins are involved in this process:
•ComEA: binds directly extracellular double-stranded DNA.
•The comF genes encode proteins that translocate the
DNA into the cell.
• ComFA provides the energy for translocation of DNA
through the membrane.
•ComEA, ComEC, and ComFA form a sort of ATP-
DNA into the cell.
•The genes in the comG operon encode proteins that
might form a “pseudopilus” which helps move DNA
through the ComEC channel.
They might bind to extracellular DNA, perhaps acting
through the ComEA DNA-binding protein, and then retract,
drawing the DNA into the cell.
10. Conti…
Conti…
• The comE, comF and comG operons are all under the
transcriptional control of ComK.
• Some of genes involved in the transformation process
are not designated as com, because such genes were
first discovered on the basis of their involvement in other
processes.
1. The nucA gene product makes double-strand breaks
in extracellular DNA.
2. Other examples are single-stranded-DNA binding
protein (SSB), and RecA functions in the
recombination of transforming DNA with chromosome
DNA.
11.
12. Conti….
Conti….
• The lengths of single-stranded DNA
incorporated into the recipient chromosome are
about 8.5 to 12 kb on cotransformation of
genetic markers
• incorporation takes only few minutes to be
completed.
15. Artificially induced competence
Artificially induced competence
• Bacteria can be sometimes be made competent by
certain chemical treatments or DNA can be forced into
bacteria by a strong electric field in a process called
electroporation.
1. Chemical Treatment (with calcium ions).
• Chemically induced transformation is usually
inefficient, and only a small percentage of the cells are
ever transformed.
• The cells must be plated under conditions, selective
for the transformed cells.
• Therefore, the DNA used for the transformation should
contain a selectable gene such as encoding resistance
to an antibiotic.
16. Artificially induced competence
Artificially induced competence
2. Electroporation
• The bacteria are mixed with DNA and briefly
exposed to a strong electric field.
• The bacteria first be washed extensively in
buffer with very low ionic strength such as
distilled water.
• The brief electric field across the cellular
membranes might create artificial pore of H2O
lined by phospholipid head groups. DNA can
pass through these temporary hydrophilic
pores.
• Electroporation requires specialized equipment.
17.
18. Applications of Transformation
Applications of Transformation
Bacterial transformation is used:
•To make multiple copies of DNA, called DNA
cloning.
•To make large amounts of specific human
proteins, for example, human insulin, which can
be used to treat people with Type I diabetes.
•To genetically modify a bacterium or other cell.
