Bacterial Genome

&Variations

• Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria

• Bacteria allow researchers– to investigate molecular genetics in the simplest true

organisms

The Bacterial Genome and Its Replication• The bacterial chromosome

– Is usually a double stranded circular DNA molecule with few associated proteins

– 4.6 million base pairs– ~4,400 genes compared to our 25,000– Proteins cause chromosomes to supercoil in nucleoid region

• In addition to the chromosome– Many bacteria have plasmids, smaller circular DNA molecules that can

replicate independently of the bacterial chromosome

Replicationfork

Origin of replication

Termination of replication

Figure 18.14

• Bacterial cells divide by binary fission– Which is preceded by

replication of the bacterial chromosome

– Replication starts at one replication fork only

– Very quick. E. coli doubling time is 20 minutes

Mutation and Genetic Recombination as Sources of Genetic Variation

• Since bacteria can reproduce rapidly– New mutations can quickly increase a

population’s genetic diversity– Numerous spontaneous mutations ~ 9

million/day in host cell.– Spontaneous mutations do not play a big

role though. More diversity comes from genetic recombination

Vaughn Cooper & Richard Lenski’s experiment testing whether prokaryotes evolve rapidly in response to

environmental change•Established 12 populations of E.coli from a single colony •Followed populations for 3,000 days.•Performed serial dilutions each day on each population to keep cultures fresh.•Growth medium in each culture contained low levels of glucose & other vital resources (to stress out the bacteria)•Samples from each culture were taken often to compare them to the original sample’s bacteria

Results show that despite living in a minimal environment, the bacteria evolved through acquiring beneficial mutations.

Can bacteria acquire genes from another bacterium?• Further genetic diversity– Can arise by recombination of the DNA from two different

bacterial cells

Mutantstrain

arg-- trp+

EXPERIMENT

Figure 18.15

Researchers had two mutant strains, one that could make arginine but not tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar.

Mutantstrain

arg+ trp–

Mixture

Coloniesgrew

Mutantstrain

arg+ trp–

Mutantstrainarg– trp+

No colonies(control)

No colonies(control)

Mixture

Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination.

So how did this happen?

CONCLUSION

Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids.

RESULTS

arg+ trp+ cells

Mechanisms of Gene Transfer and Genetic Recombination in Bacteria

• Three processes bring bacterial DNA from different individuals together– Transformation– Transduction– Conjugation

Transformation• Transformation

– Is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment

– The “Thank you very much” method of genetic recombination– Remember Griffith’s transforming S & R pneumonia bacteria

Transduction• In the process known as Transduction– Phages carry bacterial genes from one host cell to another

Generalized vs Specialized Transduction

Specialized occurs with temperate phages that integrate genome as prophages.Generalized

Conjugation and PlasmidsConjugation aka: bacterial “sex”◦ Is the direct transfer of genetic material between bacterial cells that are

temporarily joined◦ One way transfer: F+ male(donor) with F factor gene to F- female (recipient)

Figure 18.17 Sex pilus or mating bridge 1 m

• Conjugation and transfer of an F plasmid from an F+ donor to an F recipient

Figure 18.18a

A cell carrying an F plasmid is F+ cell) can form a mating bridge with an F– cell and transfer its F plasmid.

A single strand of F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow).

2

DNA replication occurs inboth donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands.

3 The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+.

4

F Plasmid Bacterial chromosome

Bacterial chromosome

F+ cell

F+ cell

F+ cell

Mating bridge

1

Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient

F– cell

How do bacteria “acquire” resistance to antibiotics?

1. Mutation in a chromosomal gene of the bacterium can make it antibiotic resistant– Mutation might alter the intracellular target

protein the antibiotic would have worked on– Mutation might not allow bacteria to take up the

antibiotic into its cell in the first place. Its cell wall may be altered to resist it.

2. “Resistant genes” which code for enzymes that destroys or hinders antibiotic effectiveness

3. Resistance genes are usually located on a plasmid

R plasmids and Antibiotic Resistance

• R plasmids– Confer resistance to various antibiotics