Chapter 6 - Genetic Analysis & Mapping

Benefits of bacteria for geneticists

Genome simplicity
Mostly haploid genomes (no dominant/recessive interactions)
Short generation time
Large progeny numbers
Easily & inexpensively maintained & propagated
Numerous heritable differences

Characteristics of bacterial genome

Small size
DNA associated with histone proteins & supercoiled
Circular
No introns
One chromosome (usually)
No mitochondria

Characteristics of eukaryotic genomes

Large size
DNA associated with histone octamer, coiled into chromatin
Linear
Introns within genes
Multiple chromosomes
Mitochondria present

Plasmids

Circular 'extrachromosomal' DNA in bacteria containing non-essential genes not necessary for life

Plasmid DNA

Sometimes incorporated into the bacterial genome

Fertility (F) plasmids

Contain genes that assist with transfer of that plasmid to another host bacterial cell

Resistance (R) plasmids

Contain genes that confer antibiotic resistance

Three types of genetic transfer between bacteria

Conjugation
Transformation
Transduction

Conjugation

Transfer of replicated DNA from a donor to a recipient cell, may only involve transfer of plasmid DNA

Transformation

Uptake of DNA from the environment, donor bacteria fragments are incorporated into the recipient genome

Transduction

Transfer of DNA from one bacterium to another by a viral vector, happens when a virus accidentally incorporated donor bacterial DNA

Conjugation with donor F+ strand

Does not transfer bacterial genome, only the F plasmid

F plasmid

Can become incorporated into the bacterial genome

High frequency recombination (Hfr)

F plasmid converts bacteria into this

Conjugation pair

Usually breaks apart before the entire donor genome is transferred

Multiple Hfr strains

Can be used to map all genes in a species, F factor may integrate into different locations on a bacterial chromosome, Integrated F factor can be in either direction

F+ bacterial chromosome

Only transfers the bacterial genome

Transfer Hfr and bacterial genome

Plasmid incorporated into circular genome

F'

includes genes from bacterial chromosome in the F plasmid

Competent cell

A cell capable of being transformed

Co-transformation of multiple genes

Indicated that they are closely linked, provides information on the genetic map of the donor and recipient

Transduction

Transfer of genetic material from a donor to a recipient cell by bacteriophage (bacterial virus cell)

Transductant

Donor DNA is integrated into recipient's chromosome form

Bacteriophage's two types of life cycles

Lytic & lysogenic cycle

Lytic cycle

The phage genome does not get incorporated into the bacterial genome
Donor genes can be incorporated into the phage
DNA can be inserted into a recipient cell to produce a transductant when recombination integrates the DNA into the recipient genome

Lysogenic cycle

Phage DNA is incorporated
Resulting prophage may undergo multiple cell divisions before the phage genome is excised

Fine-structure mapping w lambda phage

Shows recombination between genes

Normal plaque development

Prevented when the bacteria is coinfected by phage strains with different mutations
Strains fail to complement (plaques don't form or look weird

Functional wildtype allele

Normal form/function
Formed by recombination within defective genes

Lateral/horizontal gene transfer

Transfer of genetic material between individual bacteria or archaea and other organisms of same or different species

~12%

Amount of pervasive genes in average bacterial genome

Biased gene transfer

Involves genes related to pathogenicity & antibiotic resistance
Examples among bacteria and between bacteria & eukaryotes (mitochondria & chloroplasts)