Strange and Unexpected Biology

“Male” Bacteria and the F Factor

• In conjugation, transfer of genetic material goes in only one direction, from the donor to the recipient.
• The donor is “male” and has a sex pilus, which forms a temporary bridge between the two bacteria.
• “Maleness” is due to the presence of a special piece of DNA called the F factor (F is for “fertility”).
  • F⁺ cell: has the F plasmid (“male”)
  • F⁻ cell: does not have the F plasmid (“female”)
  • The F factor can be either a segment of a bacterial chromosome or a plasmid.

Figures 18.14(a) (modified) and 18.13, pages 334 and 333, Campbell's Biology, 5th Edition

Integrating the F Plasmid

• The F plasmid can be integrated into the bacterial chromosome.
  • A plasmid that can do this is called an episome.
    • Epi: upon
    • Soma: body
  • The bacterium is then called an Hfr cell, instead of a F⁺ cell.
• Compare:
  • Episome: plasmid is integrated into a bacterial chromosome.
  • Prophage: viral DNA is integrated into a bacterial chromosome.

Figure 18.14(b), page 334, Campbell's Biology, 5th Edition

The F Factor, Conjugation, and Genetic Recombination

• The F plasmid contains 25 genes, most of which are used to produce the sex pilus.
• “Maleness” is contagious!
  • The F⁺ plasmid is transferred to a F⁻ bacterium.
  • This facilitates future genetic recombination.

Parts of figure 18.14, page 334, Campbell's Biology, 5th Edition

Transposons

• A transposon is also known as a transposable genetic element.
  • It is a piece of DNA that can move from one location to another in a cell's genome.
  • Unlike a prophage or an episome, a transposon can never exist independently.
• Transposons can move:
  • Within the main bacterial chromosome
  • From a plasmid to the main chromosome (or vice versa)
  • From plasmid to plasmid
• They are called “jumping genes,” which is a bit misleading:
  • They can “jump” from one place to another: “cut and paste.”
  • They can also insert a copy into a new location without losing the original copy: “copy and paste.”
• Barbara McClintock discovered transposons in corn kernel color in the 1940s.
  • Her work was ignored for years.
  • She finally won the Nobel Prize at age 81!

From the National Library of Medicine.

Transposon Biology

• There are two main types of transposons:
  • Insertion sequences
  • Composite transposons

Insertion Sequences

• Consist only of the DNA necessary for transposition:
  • One gene for the enzyme transposase.
  • Inverted repeats at the two ends:
    • Noncoding sequences about 20–40 nucleotides long
    • Upside-down and backwards versions of each other

Figures 18.15 and 18.16, pages 335 and 336, Campbell's Biology, 5th Edition

Composite Transposons

• These transposons actually include extra genes, like antibiotic resistance.
• The extra genes are sandwiched between two insertion sequences.

Figure 18.17, page 336, Campbell's Biology, 5th Edition

Transposons in Humans

• Nearly half of the human genome is estimated to be made of transposons (~42%)!!
• Most of these retrotransposons no longer move around.
• Want to know more about these crazy transposons? Click Here.