BacteriaCh 18

Bacteria

• Genome –double stranded circular DNA

DNA packed into nucleoid

Reproduction:

-Binary Fission

- short generation times

•http://www.youtube.com/watch?v=gEwzDydciWc

How do bacteria vary genetically?

• Mutations• Genetic recombination• Horizontal gene transfer – transfer of genes through means other than sexual reproduction• Types: Transformation, Transduction, Conjugation

Transformation

• Free DNA is taken up into cell and expressed•What famous experiment illustrated this?

Transduction

• DNA is transferred from one bacterium to another by a virus (bacteriophage).

• Bacteriophages accidently pick up DNA from bacteria they infect, and transfer it to other bacteria

Conjugation• Direct transfer of material between

bacterial cells through direct cell to cell contact or a bridge-like structure between cells.

• Donor cell attaches to recipient cell by using sex pili, which grabs onto recipient cell and then draws in two cells together, forming a bridge between 2 cells

F factor• F factor (fertility factor) is required to produce

a pili & donate DNA during conjugation• The F factor in E. coli is made up of 25 genes,

which help produce the pili• It can exist as a plasmid or a segment of DNA

in the bacterial chromosome• Cells with the F factor are F+, cells without

are F-

F plasmid conjugation (from F+ to F-)

If only the F factor is transferred, this is NOT recombination – since chromosomal genes have not been transferred.

Conjugation with Hfr bacteria

• F factor can also be integrated into bacterial chromosome

• Such a bacterial cell is now called Hfr cell (high frequency of recombination)

• When chromosomal DNA from Hfr cell enters a F- cell, homologous regions of the Hfr chromosome & F- chromosome can align, so segments of DNA are exchanged. This results in recombination.

• A Hfr cell can initiate conjugation with another cell, and tries to transfer entire bacterial chromosome, with F factor last

• Transfer is not complete, so part of bacterial chromosome is now in new cell

• New DNA can recombine with host cell DNA

• http://www.youtube.com/watch?v=7stZk6TesKk