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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.
Bacteria has one chromosome and don’t have nucleous.
Mitosis is performed in order to add cells to a population
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 phenotypes.
Minimum media containing
STRAIN
PHENOTYPE
Glucose
Galactose
Lactose
Arabinose
n. 1
+
+
+
+
wt
n. 2
+
+
-
+
Lac-
n. 3
+
-
-
+
Gal- Lac-
n. 4
+
+
+
-
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
analysed on media enriched with different kinds of molecules, to determine their
phenotypes.
The ability of bacteria to grow on different media is reported in the following table; identify
their phenotype
STRAI
N
Complete
Media
n. 1
Minimum media+ glucose and…
Phenotype
--
arginine
pyrimidine
adenine
biotin
+
-
+
-
-
-
Arg -
n. 2
+
-
-
+
-
-
Pyr-
n. 3
+
-
-
-
+
-
Ade-
n. 4
+
-
-
-
-
+
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
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?
30°C
grow
42°C
not grow
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)?
Medium with streptomycin
Two Escherichia coli mutants display both the Met- phenotype, i.e. they aren’t able to....
Synthesize methionine
The two mutations are located on different genes. How can you explain this situation?
S1
S2
a
Genes: met1
methionine
b
met2
Two Escherichia coli mutants can’t grow on galactose medium if galactose is
the unique source of Carbon.
-
Gal
• Assign the phenotypes at the mutants ……………..
• Are you sure that the mutations regard the same gene?
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.