Genetic recombination needs:

  • Homologous chromosomes; must differ from one another along one at least two spots
  • Mechanism to bring into close proximity of each other
  • Homology allows the DNA to line and recombine precisely.
  • Cut and passing 4 DNA backbones results in 1 recombination event.

Lederberg Experiment

  • E.coli grown in minimal medium; contains water, organic carbon, salts; NO amino acids
  • Clone: large number of genetically identical cells
  • Prototrophs: can synthesize the necessary amino acids
  • Auxotroph: mutant; only grown if amino acid is provided for them in medium
  • Prokaryotes usually have plasmid, one circular chromosome that carries one particular allele for each of their genes
  • Lederberg mixed millions of these mutants E.coli together in minimal medium; colonies still arose. However colonies still arose!!
  • Could be because the mutation (+)(-) turned back to normal, but when these mutant E.coli were put on own they died.
  • Bacterial cells conjugate; don’t fuse. Movement of information is unidirectional.
  • Cells connect to each other through sex pilus to form cytoplamic bridge
  • Conjugation: copy of part of DNA of one cell moves into other cell. Once DNA from one cell enters the other, genetic recombination can happen. SEXUAL RECOMBINATION BETWEEN BACTERIA!

  • Initiated by small circle of DNA ( F factor plasmid) and main chromosomal DNA (circular)
  • F factor- fertility factor- carries genes for its own reproduction; replication origin,
  • Vertical Inheritance: one generation to another
  • F+= contains the F factor, F-= doesn’t contain the F factor
  • F factor will copy itself into another recipient cells during conjugation; no chromosomal DNA if transferred
  • Sometimes F+ factor will incorporate itself into chromosome through a single crossover event.

  • Hfr Donor Cell: cell with integrated F factor into chromosomal DNA, line up with main DNA, recombination event occurs (two circular plasmids fuse into ONE big circle) Hfr- High Frequency Recombination
  • As the F factor/DNA enter cell, single stranded -> double stranded via synthesis (rolling circle)
  • Crossover will result between entering donor cell’s DNA and the recipient’s chromosome; export copies chromosomal genes to another cell
  • Not all of F+ factor, only half of it; very rare for recipient cell to receive of the F factor.
  • Recipient cell will become partial diploid only have 2 copies of those genes that came through the conjugation bridge
  • Enzymes degrade alleles that do not recombine; those that don’t replicate are lost
  • L.Wollman did mapping of E.coli. Certain genes recombine before others when making recombinants, were able to map this location of genes on cell.
  • Discovery of conjugation/ genetic recombination in E.coli; showed that it wasn’t just restricted to eukaryotes.

Transformation

Bacteria takes up DNA that is released into the environment as other cell disintegrate. Linear fragments are taken up from disrupted virulent cells, recombine with chromosomal DNA of the nonvirulent cells.

Transduction

DNA transferred from donor to recipient cells inside the head of an infecting bacterial virus. Bacteriophage. While new phages are being made into cell, they sometimes incorporate fragments of the host cell DNA along with, or instead, of viral DNA.

Recipient will receive bacterial DNA instead of viral DNA.

Generalized transduction: ALL donor genes are equally likely to be transferred

Lytic cell= viral cycle

Specialized Transduction: only transfer genes lying close to the point of insertion of the prophage: phage that integrates into the host chromosomal DNA

Lysogenic cycle

At some point the viral DNA is excised from the bacterial chromosome. Becomes active and enter the lytic cycle. NOT precise; some of the bacterial DNA is carried out with the phage; it become defective and does not kill new cell it infects; just passes on DNA.

Leave a Reply

3 Comments on "Prokaryotic: F factor, Transformation, Transduction"

avatar
Sort by:   newest | oldest | most voted
shiva
Guest

THANKS SIR!

dr bhupendra
Guest

good illustration….

SACHIN KUMAR
Guest

GOOD WEBSITE

wpDiscuz