Biology Reference
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OD 540
lac: T gen = 2 h
0.5
glc: T gen = 1 h
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Time (h)
Figure 5 Biphasic growth of E. coli on a medium of glucose plus lactose.
The optical density (OD 540 ) (on a log scale) of a batch culture is depicted as a function of
time. Difference in slopes reflects the difference in growth rate. In the initial phases of growth,
E. coli grows more rapidly, consuming glucose. In later phases of growth, when glucose has been
consumed, E. coli grows more slowly, relying on lactose; the generation time (T gen ) on lactose is
two times longer that that on glucose.
By 1953, Jacob and Monod had two interesting mutants of E. coli . The
'wild type' (lac + ) is one in which we see the characteristic biphasic growth
described above. It turns out that the two mutants involved genes with very
different functions. It was a remarkable achievement to recognize the duality in
the mechanism. The first mutant is a constitutive one (lac ), in which lactose
is always consumed by the organism. The second mutant is uninducible;
that is to say, the uninducible mutant is one in which -galactosidase is
never synthesized, and so even if lactose is consumed it cannot be used in
metabolism. This could be seen experimentally by indicators which could
distinguish the presence of the lac mutants by different colors in popula-
tions. The lac + was either pink or white, depending on the specific medium
on which the strains were grown. The lac was red. The presence of the
lac + and lac mutant forms required two different mechanisms within the
operator: one was involved with stimulation and the other with repression. The
constitutive lac mutant is one in which a repressor is lost or dysfunctional
and so one in which lactose is always consumed by the organism. The unin-
ducible mutant is one in which activation is impossible, and so lactose is never
consumed. As there are two genetic variants which are different from the 'wild
type', we know that the operator in 'wild type' organisms has at least two
components; otherwise, there would not be two different ways in which the
mechanism could be 'broken'. This still left it unclear what these two components
controlled. (We now know one controls the synthesis of -galactosidase and the
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