Chapter 6 Vocab

What are bacteria?

• prokaryotes

  • eubacteria and archaea

Bacteriophages

viruses that use bacteria as their host

Why are bacteria and bacteriophages essential model organisms in genetics?

• Short reproductive life cycles

  • Short reproductive life cycles

  • 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

Why can pure cultures of bacteria give rise to mutant cells

spontaneous mutations

Pure cultures

• cells can be isolated and established independently from parent strain, using selection techniques

  • most bacteria have a single chromosome, as do viruses-making them haploid - makes studying mutations in them easier

  • only one copy of each gene, os all mutations are expressed directly (no need to worry about dominant/recessive)

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

Serial dilutions

• successive dilutions used to study bacteria quantitatively when colonies on petri dish are too great to count

Why are single bacterial colonies useful?

genetically identical

minimal medium

• organic carbon source and inorganic ions

  • to grow on this medium, bacteria must be able to synthesize all essential organic compounds

complete medium

additional complex compounds are added as supplements to minimal medium

phototroph (wild type)

• can grow on minimal medium

  • can synthesize all essential organic compounds

Auxotroph (mutant)

• needs complete medium

• has lost ability, via mutation, to synthesize essential compounds

types of genetic transfer processes

• conjugation

• transformation

  • transduction

transformation

uptake of free DNA

conjugation

• direct transfer of DNA from one bacterium to another

• unidirectional - one bacterium donates DNA to another

• not a lot of recombination

transduction

transfer of bacterial DNA by a bacteriophage

vertical gene transfer

transfer of genetic information between members of same species

horizontal gene transfer

• transfer of genetic information between related but distinct species

• plays a significant role in evolution of bacteria

sex pili

hair-like appendage found on the surface of certain bacteria that acts as a connector to transfer genetic material between cells

F factor (fertility factor)

• F⁺ serve as DNA donors

• F^- cells are the recipients

• F⁺ cells contain fertility factor (genes required for conjugation) on a plasmid

Hfr (high-frequency recombination

• special class of F⁺ cells where the F factor is integrated into the bacteria’s chromosome

• Hfr cells can also donate genetic information to F^- cells

• recipient does not become F⁺ usually - the entire bacterial chromosome must be transferred before the F factor, and the process is usually interrupted before completion

interrupted mating technique

• a culture with a mix of Hfr and F^- strains is incubated, then put in a blender

• blending interrupts conjugation, terminating the transfer of the chromosome

• demonstrated that specific genes in Hfr strain are transferred./ecombined sooner than others

time mapping

• the chromosome of an Hfr cell is transferred linearly

• gene roder and distance between genes can be predicted based on the time required to transfer them

• this serves as the basis for the genetic map of E. coli

origin

the point where the transfer begins

F’ cells

• F factor can lose its integrated status

• cells revert to F’ state - F factor is on a separate plasmid again but may have taken some chromosomal genes with it

• F’ behaves like F⁺ by initiating conjugation with F’ cells

• transfer of an F’ to an F^- cell results in partially diploid cells (merozygote)

merozygote

partially diploid

plasmids

• double-stranded closed circles of DNA separate from the bacterial chromosome

• contains one or more genes and uses the same replication enzymes as host

  • plasmids are distributed to daughter cells along with host chromosome

RTF: Resistance Transfer Factor

RTF encodes genetic information essential to transferring plasmid between bacteria

R-determinants

• R-determinants confer resistance to antibiotics or toxins like mercurys

• several R-determinants may be present-conferring multiple resistance to several antibiotics

Col plamids (Col E1)

• Derived from E. coli

• not transmissible to other cells by conjugation

• encode colicins - highly toxic proteins

• colicins kill bacterial strains that do not harbor same plasmid

  • bacteria that carry plasmid are called colicinogenic

Transformation

• small pieces of extracellular DNA are taken up by bacterial cell

  • integrated stably into the chromosome

two steps of transformation

• entry of foreign DNA into recipient cell

• recombination between foreign DNA and homologous region in recipient chromosome

Two outcomes of transformation

• completion of both steps required for genetic recombination

  • first step alone results in additional foreign DNA to cytoplasm, but not chromosome

heteroduplex

when the recombinant region contains one host strand and one mutant strand - binary fission results in one transformed daughter cell and one untransformed daughter cell

competence

• only cells with a particular physiological state of competence take up DNA

• often triggered by stressful conditions (survival mechanism)

• bacteria can be artificially made competent in lab with treatments like osmotic stress

Cotransformation

• simultaneous transfer of genes

• genes that are close enough together can be carried on single segment of 10,000-20,000 nucleotide pairs and transferred together during transformation

  • frequency of two unlined genes transforming simultaneously is lower than that of linked genes - can be used for mapping

Bacteriophages

• bacteria eater

• viruses that have bacteria as their host

• during reproduction, phages can be involved in a type of genetic recombination called transduction

  • bacteriophages themselves also undergo recombination

Bacteriophage T4

• E. coli is host

  • belongs to group of related bacterial viruses called T-even phages

Phage structure

• head of virus: DNA contained in icosahedral protein coat

• tail: contractile sheath surrounding central core

  • tail fibers contain binding sites that recognize bacterial outer surface

Life Cycle of Phage T4

1. Initiated when phage binds to bacterial host cell

2. Phage DNA is injected into the bacterium

3. DNA is replicated to produce additional copies

4. DNA is also transcribed & translated to produce viral proteins

5. mature phages are assembled from DNA &. proteins

6. bacteria cell ruptured by the enzyme lysozyme

7. mature phages are released from host cell

plaque assay

• performing serial dilutions of virally infected bacteria - then pouring onto agar plates

• occurs whenever a single virus initially infected on bacterium in culture

• plaque represents clones of single infecting bacteriophage

  • by counting number of plaques ion plates, number of phages in original culture determined

Plaques

areas clear of bacteria on a culture plate (lawn)

Lysogeny

• viral infections do not always immediately result in lysis of host cell

• virus DNA can integrate into the host’s DNA and lay dormant

• viral DNA is replicated each time the host DNA is replicated, passed to daughter cells

• viral DNA integrated into a bacterial chromosome is called prophage

  • a bacterium harboring a prophage has been lysogenized

temperate phages

can either lyse cell or behave as prophage

virulent phages

can only lyse cell

Transduction

• bacterial recombination mediated by bacteriophages

• virus acts as an accidental container for bacterial DNA and brings bacterial DNA into another bacteria

Cotransduction

• two genes that are close eonugh can be transduced simultaneously

• two independent transduction events may occur if genes are not close enough

  • can be used for mapping

Viral Recombination

viral recombination can occur if two viruses infect the same host cell