Genetics of Bacteria

Inheritance of characteristics and variability

            Genotype--genetic constitution of cell

            Phenotype--expression of genotype by observable properties


            Phenotypic changes due to environmental changes

                        organisms do not express all their genetic info.

                                    extent of expression depends on the changes in the environment

            Genotypic changes

                        Genes--functional unit of inheritance

                                    specify particular proteins and RNA’s

                        Mutations--changes in nucleotide sequence of genes

                                    spontaneous mutations 1/1 mill to 1/10 bill


                        Types of Mutations:

                                    Point mutations--nucleotide substitutions





                                          Affects on translation--base substitution

1)      missence mutation

altered codon codes for different AA in protein

results in inactive or less active function

2)      nonsense mutation

termination codon develops

results in incomplete protein and its nonfunction

3)      neutral mutation

altered codon codes for same AA

no change in protein function

                                    Frameshift Mutations--addition or loss of nucleotide

                                                lead to nonfunctional proteins, generally

                        Occurrence of Mutations--during DNA replication

                                    Spontaneous mutations

                                                UV light--formation of dimmers (thymine)


                                    Induced mutations--mutagens

                                                mutagenic chemicals

1)      chemical Rx with DNA

alter amino and hydroxyl groups on bases (represses H-bonding)

2)      base analogs

substitutes for bases (similar)



                        Repair of Mutations

                                    Photoreacting mechanisms--UV damage

                                                visible light promotes enzyme activation

                                                            splitting of dimmers

                                    Endonucleases:  Exonucleases/Polymerases:  Ligases

                        Rate of Mutation--probability of mutation at division

                                    number of mutations/cell/division

                        Types of Mutation

1)      increased tolerance (drug resistance)

2)      altered fermentation or ability to produce end products

3)      nutritionally deficient

4)      changed colonial form or pigmentation

5)      change in antigenic nature (surface structure)

6)      resist bacteriophage penetration

7)      change in morphology (flagella, capsules, etc)

Practical implications:

1)      antibiotic resistance

2)      end product production (commercial)

3)      alteration of “typical stock culture”

4)      metabolic research

Reverse mutation--mutant reverted to wild-type

1)      reversal of original mutation

2)      suppression by second mutation at different site on chromosome

Bacterial Recombination (reassortment of genes followed by genetic information exchange)

1)      Conjugation--transfer of genes by physical contact

2)      Transduction--transfer of genes by bacteriophage

3)      Transformation--transfer of cell-free DNA

**Only portions of chromosomes are generally transferred

            donor:  recipient:  merozygote (partial diploid)


Conjugation--large portion or complete chromosome trans.

            Lederberg-Tatum 1946

                        E. coli strains (tryptophan--histidine)

            Sex factors or F factors (fertility)

                        Donor--F+  independent trans. of F factor

                        F+ X F-  transfer of F factor

                                    low frequency of recombinants

                        Hfr  (High frequency recombination)

                                    isolated from F+ strains

                                    F factor rarely transferred during recombination

                                    high frequency recombination

                        Sexduction--Jacob and Wollman

                                    Hfr reverts to F+ and genes from chrom.

                                    F’ cells

            Linear transfer of chromosomes

            Extrachromosomal Genetic Elements

                        Plasmids--autonomous replication

                                    circular pieces of DNA--extra genes

                                    bacteriocinogenic factors--bacteriocins


                                    resistance transfer factors (R factors)

                                                antibiotic resistance

                                                infectious resistance--trans. conj.


            Transformation--Avery, MacLeod, and McCarty 1944

                        Cell-free “naked” DNA transferred (limited gene info.)

                                    Recombination occurs after absorption

                                                donor cell lysis or chemical extraction

                                                lab culture raised in:

                                                            presence of dead cells

                                                            culture filtrates

                                                            cell extracts

                                    After DNA entry--one chromosome strand degraded


                        Properties of recipient cells

                                    Proper conditions for uptake of donor DNA

                                                late logarithmic phase

                                                competent--ECF protein binds DNA frag.

                        Natural transformation occurs following lysis of microbes


            Transduction--gene transfer by bacteriophage

                        Viral DNA (episome) integrates into chromosome prophage

                        Generalized transduction--genes of chrom. or plasmids

                        Specialized transduction--restricted genes of chrom.


Genetic Engineering--Fact or Fiction??

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