Genetic transfer and recombination Microbial Genetics Genetic transfer and recombination

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

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

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)

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

Griffith’s experiment, 1928 Streptococcus pneumoniae

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

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

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

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

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

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

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

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

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

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

Natural history of a transposon chromosome transposon plasmid

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