Bacteria use three main mechanisms to adapt to changing environments: mutation, gene transfer, and regulation of gene expression. Mutation occurs spontaneously through base substitutions, insertions, deletions, and movement of transposable elements during DNA replication. Gene transfer allows for transfer of DNA between bacteria through three main mechanisms: transformation involving uptake of naked DNA, transduction using bacteriophages, and conjugation mediated by plasmids. Regulation of gene expression allows bacteria to control which genes are expressed in response to environmental conditions.

1. Bacteria use three main mechanisms to adapt to
changing environments
– Mutation
– Gene transfer
– Regulation of gene expression

2. Mutation
Spontaneous mutations occur infrequently
and randomly in the natural environment
Rate of spontaneous mutation
Probability that a mutation will be observed in a given
gene each time the cell divides
Rate is generally between 1 in 10,000 and 1 in a trillion

5. Mutations are stable heritable changes in
the base sequence of DNA
Mutations can be caused by:
Base substitutions
Removal or addition of nucleotides
Transposable elements

6. Base substitutions
Most common type of mutation
Results from mistakes during DNA
replication
Point mutations: when one base pair is
changed
Missense mutation
Nonsense mutation: Mutation that
changes an amino acid codon to a stop
codon

8. Removal and addition of nucleotides
Shifts the translational reading frame
Called frameshift mutation because it affects all amino acids downstream
from addition or deletion (frequently result in premature stops)

9. Transposable elements
Special segments (transposons) of DNA that move
spontaneously from one gene to a different gene
Transposons may disrupt the integrity of the gene and
render its protein product nonfunctional
Read about Barbara McClintock (A Glimpse of History) on pg. 191

10. Mutations are essential for understanding genetics
Mutations can be intentionally produced (induced
mutations) to demonstrate function of particular gene or
set of genes
Mutations can be induced via:
Chemical mutagens
Transposition
Radiation

11. Chemical mutagens
Nitrous acid
Changes cytosine to uracil
Alkylating agents
Alter hydrogen bonding of bases
Nitrosoguanine is common alkylating agent
Used as antineoplastic drugs
Base analogs
Chemicals that are structurally similar to the nitrogenous bases but
have slightly altered base pairing properties
Base analogs include:
2-aminopurine which incorporates in the place of adenine but
binds with cytosine
5-bromouracil which incorporates in the place of thymine but
binds with guanine

12. Intercalating agents
Molecules that insert themselves between adjacent
bases and create space between bases
Ethidium bromide is common intercalating agent
Potential carcinogen
Used extensively in biochemical/molecular biological
research

13. Transposition
Common procedure used to induce mutation in the
laboratory
A transposon inserts into a gene (insertion mutation)

14. Radiation
Ultraviolet light
Causes covalent bonding between adjacent thymine bases forming
thymine dimers which distort DNA
X rays
Cause breaks and alterations in DNA. Breaks that occur on both strands
are often lethal

15. Mutations and Their Consequences
Mutations provide the organism with a way to
respond to environmental stresses
Environment selects for cells suited to survive
Environment does not cause mutation

16. WESTERN BLOT

17. WESTERN BLOT

18. WESTERN BLOT

21. WESTERN BLOT

22. NORTHERN BLOT

23. NORTHERN BLOT

24. NORTHERN BLOT

25. NORTHERN BLOT

26. SOUTHERN BLOT

27. Southern blot

29. Mechanisms of Gene Transfer
Genes are naturally transferred between bacteria
DNA-mediated transformation
Transduction
Conjugation

30. Gene exchange in bacteria
1. Donor DNA is transferred and accepted by the
recipient cell
Three mechanisms
DNA-mediated transformation
Transduction
Conjugation
2. Donor DNA is integrated into the recipient cell’s
chromosome

31. No integration
Integration

32. DNA-mediated transformation (transformation)
The transfer of naked DNA from one bacterium to
another
Discovered by Fredrick Griffith in 1928 while working with
Streptococcus pneumoniae
Griffith realized S. pneumoniae existed in two forms
Encapsulated, virulent form (smooth in appearance)
Nonencapsulated, avirulent form (Rough in appearance)
Griffith hypothesized that injections with the smooth
strain could protect mice from pneumonia
Griffith injected mice with the two different strains

33. Griffith’s Results

34. Natural transformation occurs when bacterial
cells are “competent”
Competence is a condition in which bacterial cells
are capable of taking up and integrating large
fragments of DNA into their chromosome
Competence usually occurs naturally during the late log,
early stationary phase

35. Dying cells rupture during the stationary and
death phases. The chromosome breaks into
small pieces and explodes through the ruptured
cell wall
Recipient cells absorb pieces of “naked” DNA
The naked DNA is integrated into the recipient
cell’s chromosome
Naked DNA integrates at a homologous site on the recipient’s
chromosome

36. Entry of the DNA
Only single strands enter
Integration of the donor DNA
Donor DNA is integrated at a
homologous site
Enzymes cleave recipient DNA
Donor DNA replaces recipient DNA
via recombination
Cell multiplication
Transformed cells multiply under selective
conditions in which non transformed cells
will not grow

37. Transduction
Bacterial DNA that is transferred from donor to
recipient via a bacterial virus (bacteriophage)
Two types of transduction
Generalized
Any gene from the donor can be transferred
Specialized
Only specific genes can be transferred

38. Transduction

39. Conjugation
Conjugation is mediated by a plasmid
R plasmids
F plasmids
Conjugation requires direct contact between cells
Cells must be of opposite mating types
Donor cells carry a plasmid that codes for fertility factor or “F
factor”
This cell is designated F+
Recipient cell does not carry a plasmid
This cell is designated F-

42. Conjugation

43. High frequency of
recombination – Hfr strains