Genetics: Genetics Lecture 16, 10/6 - Infectious Heredity, Bacteria, Mapping

Chapter 9 overheads available here.http://docs.google.com/Presentation?id=dhqwrndc_393g3pmdwfm

Chapter 6 overheads available here. http://docs.google.com/Presentation?id=dhqwrndc_384d93nzm8c

Infectious heredity: a trait that is passed to a eukaryotic cell by living in symbiosis with a microorganism

o Killer phenotype → these cells secrete a compound which is toxic to any cell not having the kller phenotype
• Two things necessary for killer phenotype

o Paramecium can undergo conjugation (Sexual reproduction) → Genetic exchange occurs during conjugation. Also during conjugation cytoplasmic exchange sometimes occurs.
• Cytoplasmic exchange is necessary to pass on the killer phenotype
• For a Heterozygote that undergoes autogamy (asexual reproduction) it will produce two offspring → One homozygous dominant and one homozygous recessive.

o Gene products in the egg which influence the phenotype. This gene product influences the early development of your offspring.
o EX. Shell coiling in the snail (linnea peregra)

• Two types of coils: dextral - right handed coil (dominant trait) and sinistral: left handed coil (recessive trait)
• Snails are hermaphrodites. They can undergo sexual reproduction or self fertilization.
• The offspring’s phenotype is directly based on the genotype of the mother. Not based on the offspring’s genotype. An offspring from a mom with a dominant D will be dextral. A mom with a recessive d will be sinistral.

o Complete media – media which contains all of the necessary components for growth. Also referred to as rich media.
o Minimal media – contains basic chemical requirements for growth.
o Auxotrophic mutant – any strain which fails to grow on minimal media. Typically has a mutation in a gene necessary for making some component.

o Lag phase: immediately after entereing the culture, period of little or not growth. Adjustment time
o Log phase: cells begin to grow rapidly. Doubling of the population every 30 to 60 minutes
o Stationary phase: begins when some component of your media becomes limiting. Populations level off and there is little to no new growth

o In 1946 the first report was issued that bacterial cells can undergo conjugation: one bacterial cell sending information to a second.
o Experiments which showed conjugation could occur (34:20)

• Start with Two strains in complete media, both auxotrophs (see pic). They are auxotrophic for different genes.
• Plated each strain on minimal media as a control. Neither strain grow on minimal media. In a different culture the two strains were mixed the two strains together and incubated overnight.
• Took the mix and plated on minimal media – produced some prototrophs. This occurred at a rate of 1 in every 10^7 colonies.
• This indicates that some exchange of information has occurred.
• Bacteria have the ability to undergo unidirectional transfer.
• F+ cell is a cell that can donate information. F- cell receives information. Following the transfer the F+ cell becomes F- and vice versa.

o Does transfer require physical contact between the cells? DAVIS U-TUBE experiment (40:45)

• Take auxotrophic strains and place them in a u tube. Inbetween the strains place a glass filter - Allows media to be exchanged BUT Cells cannot pass across the filter
• Plate media from both sides of the filter on to minimal media – found no growth from either side.
• Indicates that- physical contact is required for gene transfer. This becomes the basis for mapping bacterial genes.

• 1950 – scientist working with an F+ strain treating it with nitrogen mustard. Found a strain which underwent recombination more frequently . . . 1 in every 10^4 cells. Hfr strains – high frequency recombination strains.

• Hfr strain with four mutations
• F- strain without theses mutations
• 1.) Mix the Hfr strain and the F- strain then (2.)Give them 10 min for conjugation. (3) Pull out some cells, blend your cells. Why? Conjugation is facilitated through a sexpilus long projection from one cell to another through which DNA can pass.
• plate these cells to look for the passage of the mutation
• repeat at 15 minutes steps 2 and 3
• at 20 minutes repeat steps 2 and 3
• at 30 minutes repeat steps 2 and 3
• based on your data (results of Hfr mapping)

Genetics