The actual building of polypeptides is called translation. Translation involves
the deciphering of nucleic acid information and converting that information into
a language that the proteins can understand.
THE OPERON MODEL
In 1961 Francois Jacob and Jacques Monod formulated the operon modelthat
described how transcription of mRNA is regulated. Transcription of mRNA is
regulated in two ways. These are repression and induction.
Repressioninhibits gene expression and decreases the synthesis of enzymes.
Proteins called repressorsstop the ability of RNA polymerase to initiate tran-
scription from repressed genes. Inductionactivates transcription by producing
inducer, which is the chemical that induces transcription.
Jacob and Monod identified genes in E. colias structural genes, regulatory
genes,and control regions. Collectively these form a functional unit called the
operon. Certain carbohydrates can induce the presence of enzymes needed to
digest those carbohydrates.
For example when lactose is present, E. colisynthesize enzymes needed to
breakdown lactose. Lactose is an inducer molecule. If lactose is absent, a regu-
lator gene produces a repressor protein that binds to a control region called the
operator site, preventing the structural genes from encoding the enzyme for lac-
tose digestion. Lactose binds to the repressor at the operator site when lactose is
present, freeing the operator site. The structural genes are released and produce
their lactose-digesting enzymes.
Mutations: Not a Pretty Copy
Amutationis a permanent change in the DNA base sequence (Table 7-2). Some
mutations have no expressive effect while other mutations have an expressive
effect. When a gene mutates, the enzyme encoded by the gene can become less
active or inactive because the sequence of the enzyme amino acids may have
changed. The change can be harmful or fatal to the cell, or it can be beneficial—
especially if the mutation creates a new metabolic activity.
The most common type of mutation is point mutation, which is also known
as base substitution mutation. Point mutationoccurs when an unexpected base
is substituted for a normal base, causing alteration of the genetic code, which is
then replicated.
CHAPTER 7 Microbial Genetics^125