Bacteria: Genetic Basics

Chromosome: double-stranded, circular DNA
- It is located in a region of the cell called the nucleoid, which is not bounded by a membrane like a eukaryote nucleus.
- E. coli has 4.6 million nucleotide pairs and 4300 genes.
Plasmids: double-stranded, circular DNA molecules which are much smaller than the main chromosome
- Each plasmid has less than a dozen genes.
- Each bacterium generally has several plasmids.

Figure 7.4, page 106, Campbell's Biology, 5th Edition; part of figure 13.11, Purves's Life: The Science of Biology, 7th Edition

Bacterial Replication: Asexual Reproduction

Replication of a circular DNA molecule begins at one origin of replication and then continues around the circle.
After DNA replication, bacteria divide by binary fission.
Typical E. coli can divide every twenty minutes.
- Within 12 hours 10⁷–10⁸ bacteria can be produced!
- Even though spontaneous mutations occur infrequently, one mutation can quickly be passed on to many offspring and contribute greatly to the population's overall genetic diversity.

Figure 18.10, page 331, Campbell's Biology, 5th Edition

Technically, bacteria cannot sexually reproduce because they do not undergo meiosis and fertilization.
However, bacteria can exchange genetic material.

In 1946, Joshua Lederberg and Edward Tatum demonstrated that genetic information is exchanged between bacteria.
Their experimental subject: E. coli
- Wild-type strain
  - Did not need any special amino acids or vitamins to grow.
  - That is, it could grow on a minimal medium (MM), which contains only glucose as a source of carbon.
- Mutant strains
  - arg⁺ trp⁻ strain: can make arginine but cannot make tryptophan
  - arg⁻ trp⁺ strain: can make tryptophan but cannot make arginine
They mixed the two strains together and let them incubate for a while.
While each strain individually could not grow on MM, when the two strains were mixed together, colonies grew on MM.
Conclusion: there must have been genetic recombination between the two strains of bacteria to produce bacteria capable of making both arginine and tryptophan.

Figure 18.11, page 331, Campbell's Biology, 5th Edition

Transformation: the bacterium takes up of foreign DNA from its surroundings.
Transduction: viruses carry bacterial genes from one bacterium to another.
Conjugation: two temporarily joined bacteria directly transfer genetic material.

Figure 13.10(a), Purves's Life: The Science of Biology, 7th Edition

Phage viruses carrying DNA from one bacteria to another
Generalized transduction:
1. A phage virus attacks a bacterium.
2. The host's DNA is hydrolyzed and phage DNA and proteins are made.
  - Occasionally a bacterial DNA fragment is packaged into a phage capsid.
3. The phage with bacterial DNA infects a new bacterium.
4. The DNA from the original bacterium is recombined with the new bacterium's DNA.
Specialized transduction:
1. A prophage lies dormant inside a bacterium.
2. When it is signaled to exit and produce phages, sometimes it takes adjoining bacterial DNA with it.
3. The new phages are produced with bacterial DNA; they exit the original bacterium and infect a new one.
4. The DNA from the original bacterium is recombined with the new bacterium's DNA.

Figure 18.12, page 332, Campbell's Biology, 5th Edition

Direct transferring of DNA from one bacterium to another
The transfer of DNA occurs through a sex pilus (plural pili)—a tube that forms between two bacteria.
Plasmids move from bacterium to bacterium through the sex pili.
- Often they confer antibiotic resistance (these are known as R plasmids).

Figure 18.13, page 333, Campbell's Biology, 5th Edition; figure 13.12, Purves's Life: The Science of Biology, 7th Edition