Information Technology Reference
In-Depth Information
DNA
mRNA
PROTEIN
Transcription
Translation
Replication
Information
Storage
Information
Transfer
Effector
Molecule
Figure 1.2
The central dogma of molecular biology.
terminator (Figure 1.3). Transcription may be regulated by effector molecules
known as
activators
and
repressors
, which interact with the promoter and in-
crease or decrease the level of transcription. This allows effective control over
the expression of proteins, avoiding the production of unnecessary compounds.
An
operon
is a set of functionally related genes with a common promoter.
An example of this is the
lac
operon, which contains three structural genes that
allow
E. coli
to utilize the sugar lactose. When
E. coli
is grown on the common
carbon source glucose, the product of the
lacI
gene represses the transcription
of the
lacZYA
operon (Figure 1.4). However, if lactose is supplied together
with glucose, a lactose by-product is produced that interacts with the repressor
molecule, preventing it from repressing the
lacZYA
operon. This de-repression
does not initiate transcription, since it would be inefficient to utilize lactose if
the more common sugar glucose were still available. The operon is positively
regulated by the CAP-cAMP (catabolite activator protein: cyclic adenosine
monophosphate) complex, whose level increases as the amount of available
glucose decreases. Therefore, if lactose were present as the sole carbon source,
the
lacI
repression would be relaxed and the high CAP-cAMP levels would
activate transcription, leading to the synthesis of the
lacZYA
gene products
(Figure 1.5). Thus, the promoter is under the control of two sugars, and the
lacZYA
operon is only transcribed when lactose is present and glucose is absent.
It is clear, therefore, that we may view the
lac
operon in terms of a genetic switch
that is under the control of two sugars, lactose and glucose.
PROMOTER
STRUCTURAL GENE(S)
TERMINATOR
Figure 1.3
Major regions found within a bacterial operon.
