Ch 8

Introduction

• Bacteria are prokaryotes. (haploid)

• Bacteriophages

    – Viruses that use bacteria as their host.

• Study of bacteria and bacteriophages is essential in gathering data in areas of genetics.

    – Molecular genetics

    – Recombination phenomena

        -can’t be used to find genetic mapping in the way of a diploid

    – Gene structure

• Bacteria and viruses are useful organisms for genetic studies.

     – Short reproductive life cycles.

        - temperate phage: prophage => lysogenic → Lytic life cycle or just lytic

▪ Billions of genetically identical bacteria or phages are produced in a short period of time.

    – Can be studied in pure cultures.

▪ Bacteria, mutants, and viruses can be isolated for study.

    – Knowledge of bacteria and their plasmids led to widespread use in DNA cloning and recombinant DNA studies.

Bacteria Mutate Spontaneously and Are Easily Cultured

Bacteria

• Pure cultures of bacteria can give rise to mutant cells that exhibit heritable variation.

    – Cells can be isolated and established independently from the parent strain, using selection techniques.

    – Most bacteria have a single chromosome, as do viruses—making them haploid.

    – Thus, all mutations are expressed directly.

• Bacteria can be grown in a liquid culture medium petri dish on a semisolid agar surface.

• Minimal media

    – Contains organic carbon source (glucose or lactose) and a variety of ions 2 2 4(Na, K, Mg, Ca, and NH).+ + + + +

    – To grow on this medium, bacteria must be able to synthesize all essential organic compounds.

    – Prototrophs can do this.

    – Auxotrophs can not.

        - Due to mutation, they have lost the ability to synthesize one or more essential compounds— must be provided in the medium to grow.(enriched media)

Bacterial Growth

• To study bacterial growth quantitatively:

    – Inoculum of bacteria placed in liquid culture.

    – Cells grown in liquid media were transferred to a semisolid medium petri dish and incubated.

    – Colonies were produced following many divisions.

Serial Dilutions

• Serial dilutions

    – Successive dilutions are used to study bacteria quantitatively when the colonies on a petri dish are too great to count. (when in log phase)

• Each colony arose from a single bacterium.

    – The number of colonies multiplied by the dilution factor represents the number of bacteria in each milliliter of initial inoculum.

Results of the serial dilution technique and subsequent culture of bacteria. Each dilution varies by a factor of 10. Each colony is derived from a single bacterial cell.

8.2 Genetic Recombination Occurs in Bacteria

Genetic Recombination

• Genetic recombination in bacteria provided the basis for the development of chromosome mapping methodology. • Genetic recombination in bacteria.

    – Replacement of one or more genes present in the chromosome of one cell along with the chromosome of a genetically distinct cell.

    – In eukaryotes, this is called crossing over.

    – In both organisms, genetic information is transferred; the genotype is altered.

Transfer of Genetic Information in Bacteria

• Genetic information can be transferred from one bacterium to another, resulting in an altered genotype.

• Three processes

    – Conjugation

    – Transformation

    – Transduction

Genetic Transfer in Generations

• Vertical gene transfer

    – Transfer of genetic information between members of the same species.

• Horizontal gene transfer

    – Transfer of

        - Genetic information between unrelated cells— related but distinct bacterial species.

        - Antibiotic resistance genes to another species.

        - Major factor in speciation in bacteria.