1. Genetic transfer and recombination
Microbial Genetics
Genetic transfer and recombination

2. Genetic recombination
Genetic recombination through sexual reproduction is an important means of variation in eukaryotes
Prokaryotes do not have an equivalent process of sexual reproduction
However, prokaryotes do have mechanisms by which DNA can be transferred between strains of the same species, or even between different species
Contributes to a population’s genetic diversity

3. Gene transfer Vertical gene transfer Horizontal gene transfer
Occurs during reproduction. Transfer of genes from an organism to its offspring
Horizontal gene transfer
Transfer of genes from one organism to another within the same generation

4. Horizontal gene transfer
Involves one way transfer from a donor cell to a recipient cell
A recipient cell that incorporates DNA from the donor is called a recombinant
Genes are transferred naturally between bacteria by three mechanisms
Transformation: DNA is transferred as “naked” DNA
Conjugation: DNA is transferred between bacteria that are in contact with each other
Transduction: DNA is transferred by a bacterial virus (bacteriophage)

5. Homologous recombination
DNA introduced into bacteria usually does not have a mechanism to replicate itself
It relies on integration into the genome of the host bacterium in order to survive and be passed on
Incoming DNA
gene A
gene Z
gene C
Host genome
gene A
gene B
gene C
Recombinant
genome
gene A
gene Z
gene C

6. Griffith’s experiment, 1928 Streptococcus pneumoniae

7. “Naked” DNA
When a bacterial cell lyses, it releases its DNA into the environment
CENSORED!

8. Transformation DNA is transferred as naked DNA
DNA breaks into pieces on cell lysis
DNA is taken up by the recipient cell
A region of the recipient DNA is replaced by the donor DNA (recombination)
Unrecombined DNA is degraded

9. Transformation occurs naturally in very few genera of bacteria
Bacillus, Haemophilus, Neisseria, Acinetobacter, and some strains of Streptococcus and Staphylococcus.
The recipient cell must be in a physiological state to take up DNA.
Changes in the bacterial cell wall make it permeable to large DNA molecules
Some bacteria, which are not normally can be made so in laboratory

10. Conjugation
DNA transfer between two bacteria that are in contact with one another
Contact between donor and recipient cells is initiated by sex pili
DNA is transfer through a conjugation bridge or open pore between donor and recipient cell
Mediated by a plasmid, called an F-factor (fertility factor) or a conjugative plasmid

11. Plasmids Small, circular molecules of DNA
Replicate independently of the chromosome
Usually dispensable for growth, but under some conditions provide a selective advantage such as antibiotic resistance or a unique metabolic pathway
Conjugative plasmids: carry genes for conjugation including sex pili

12. Mechanism of conjugation
Donor contacts recipient, attaches using sex pilus
F-factor initiates transfer of a copy of itself
Recipient is converted to a new donor cell

13. Importance of plasmids
Providing a selective advantage
Some Pseudomonas sp. have plasmid encoded enzymes to degrade petroleum allowing them to live in fuel tanks or fuel spills
Enhancing pathogenicity
The E. coli strain causing infant or traveler’s diarrhea carries plasmids for toxin production and bacterial attachment.
Antibiotic resistance
Many antibiotic resistance genes are carried on plasmids which can be rapidly transferred to other bacteria, resulting in widespread resistance to antibiotics and strains that resistant to multiple antibiotics such as methicillin-resistant Staphylococcus aureus or Golden Staph

14. Transduction Mediated by a bacterial virus (bacteriophage or phage)
DNA from the donor is transferred to the recipient inside the phage particle
Two types of transduction
Generalized
Specialized

15. Generalized transduction
Donor cell is infected with a phage
Donor DNA is incorporated into the phage (transducing phage)
The donor cell lyses
The transducing phage infects the recipient cell and injects the donor DNA
DNA integrated into the genome

16. Transposons
Segments of DNA that can move from one region of DNA to another and integrate through non-homologous recombination
Contain information for their own transposition
Transposase enzyme for cutting and resealing DNA
Short terminal repeats which the transposase recognizes as recombination sites
Insertion sequences are the simplest transposons
Complex transposons carry other genes e.g., antibiotic resistance genes

17. Natural history of a transposon
chromosome
transposon
plasmid

18. Genomes of bacteria are elastic
Bacterial genomes are often receiving genetic information from other bacteria through genetic transfer and recombination
In the same way that mutations can be beneficial, neutral or harmful, so is the recombination of incoming DNA
New gene combinations are maintained if they provide the organism with a selective advantage