Genetics Exam 4

Lateral (or horizontal) gene transfer

traits are introduced from unrelated individuals or from different species.

Vertical gene transfer

occurs in sexually reproducing organisms – traits are transferred from parent to offspring.

3 mechanisms for gene transfer in bacteria

1) Transduction, 2) Transformation, 3) Conjugation

Donor bacterium

provides the DNA that is transferred

Recipient bacterium

receives the DNA, which can result in altered phenotype

Conjugation

Direct transfer of DNA from donor cell to connected recipient cell.

•Can occur via a plasmid mediator (F plasmid)

•Can occur via a conjugative transposable element

Can occur via an integrated plasmid

Bacteria Size/Characteristics

All = are prokaryotes (lack a defined nuclear membrane)

•All lack membrane-bounded organelles

•Most have a cell wall of carbohydrate & peptide polymers that surrounds the cell membrane

•Have a single chromosome

Rapid cell division:  1 hour in minimal medium 20 min. in high nutrient conditions

Escherichia coli (E. coli)

•most studied & best understood bacterial species

•Inhabits intestines of warm-blooded animals

Can grow in complete absence of oxygen or in air

•Lab strains are not pathogenic, but other strains can cause variety of intestinal diseases

Pathogenic

causing disease

Strain

a subset of bacteria differing from other bacteria of the same species by some minor, but identifiable difference

•Prototrophic

can grow in minimal media

Prototophs

feed solely on inorganic matter with no specific nutritional requirements.

Core genome

about 1000 genes that are found in all strains of E. Coli

Pangenome

core genome plus all genes that are found in some strains and not others of E. coli (about 15,000 genes)

Insertion sequences (IS elements)

are small transposable elements (like eukaryotic transposable elements)

•Have inverted repeats (IRs) at two ends (features of DNA transposons)

•Carry transposase gene, (initiates transposition by recognizing IRs)

•Can move to other locations in genome / disrupt a gene’s function by insertion into coding regions

cause of many spontaneous mutations

Tn elements

composite transposable elements

•can carry catabolic genes or antibiotic resistance markers flanked by two copies of similar ISs in direct or inverted orientation.

•Have two nearby transposable elements, which carry transposase gene

Flank a gene conferring resistance to antibiotics or toxic metals

Plasmids

small circular dsDNA carry genes important to the host cell

• vary in length, can replicate independently

Episomes

plasmids that can integrate into the bacterial chromosome

Auxotrophs

•unable to reproduce in minimal media.

•Defective in enzymes required to synthesize complex compounds (amino acids, nucleotides)

•Defective in using complex chemicals from the environment

•Ex- breaking down lactose into glucose and galactose

•Defective in proteins essential for growth.

A strain of E. coli is Trp^‒ Lac^‒: This means it cannot utilize lactose sugar or synthesize tryptophan (an amino acid). Which medium would this bacterium grow on?

Trp^‒ = cannot synthesize Trp

Lac^‒ means the bacteria cannot degrade lactose

The medium contains glucose and at least tryptophan amino acid

Most bacterial genomes are monoploid ….. meaning?

they carry one copy of each gene

Selection

establish conditions in which only the desired mutant will grow

•Select for streptomycin resistance (Str^r) by plating on media with streptomycin

Genetic Screen

examine each colony for a particular phenotype (much more work than genetic selection)

You have E. coli that have Lac^– Met^– phenotypes (cells can’t utilize lactose as a carbon source or make their own methionine).

You perform a conjugation experiment in an attempt to restore lactose utilization in your cells. What medium would you use if you are attempting to select for cells that have reverted back to Lac⁺ (note for this question you can assume the recipient cells remain Met^–)?

Only lactose and at least Methionine as an amino acid

Steps of F plasmid mediated conjugation

F+ donor cell with F plasmid extends pilus to F- cell and binds to its cell wall

pilus shortens and cells are pulled together

gene transfer: DNA from F plasmid enters F- Cell, complementary strand is synthesized

both are F+ cells after conjugation and can continue with other F- cells