MUTANTS IN MICROORGANISMS

The reproduction of the bacterium Escherichia coli is achieved by binary fission, after his genome replication. Complete the following scheme representing the chromosome in various stages

A prototroph bacterium can grow on a minimum media, composed by inorganic minerals and containing an organic source of carbon. Glucose is the most simple source of carbon. Alternative carbon sources can be used by wild bacteria. Some mutants loose this ability.The ability of bacteria to grow on different media is reported in the following table; identify their phenotype.

STRAINMinimum media containing

PHENOTYPEGlucose Galactose Lactose Arabinose

n. 1 + + + +

n. 2 + + - +

n. 3 + - - +

n. 4 + + + -

wt

Lac-

Gal- Lac-

Ara-

A prototroph bacterium can grow on a minimum media, because it can synthesize what it needs. Mutants that aren’t able to grow on a minimum medium (auxotrophs) can be analyzed on media enriched with different kinds of molecules, to determine their phenotype.The ability of bacteria to grow on different media is reported in the following table; identify their phenotype

STRAIN

Complete Media

Minimum media+ glucose and…Phenotype

- - arginine pyrimidine adenine biotin

n. 1 + - + - - -

n. 2 + - - + - -

n. 3 + - - - + -

n. 4 + - - - - +

Arg -

Pyr-

Ade-

Bio-

A mutant bacterium isn’t able to synthesize the prolin amino acid and to metabolize lactose. Indicate which molecules must be added to a minimum media to grow this mutant.

Minimum media + glucose and proline

30°C grow

42°C not grow

An Escherichia coli mutant carries a temperature-sensitive mutation in the polymerase III gene that is necessary for DNA replication. At which temperature can this bacterium grow?

Medium with streptomycin

Synthesize methionine

The wild-type Escherichia coli can’t grow on streptomycin-enriched media. If in a culture of 10 millions cells, there are about 100 streptomycin-resistant mutant cells, how can I select them (i.e. grow only the resistant cells)?

Two Escherichia coli mutants display both the Met- phenotype, i.e. they aren’t able to....

The two mutations are located on different genes. How can you explain this situation?

S1 S2 methionine a b

Genes: met1 met2

No because the pathways of galactose degradation is composed by different genes. If only one of the genes is inactive the galactose is not metabolized.

Two Escherichia coli mutants can’t grow on galactose medium if galactose is the unique source of Carbon.

• Assign the phenotypes at the mutants ……………..• Are you sure that the mutations regard the same gene?

Gal-