38
J . B. GURDON
These chromosomes consist of many thousand strands aligned parallel
to one another (Beermann, 1956), and in some species of Chironomus
they are so large that they can be seen with the naked eye. These
chromosomes contain many transverse bands some of which become
greatly distended into so-called puffs or balbiani rings. Direct evidence
that these puffs or rings represent active gene sites has been provided
by the close correlation that exists between the appearance of a balbiani
ring at the end of chromosome I I I and the appearance of some granules
in the basal salivary gland cells of certain species (Beermann, 1961).
Those species and cell types which lack these granules also lack a
balbiani ring at this chromosome position. In the cells of a given tissue,
particular chromosome bands consistently become distended into puffs
at certain stages of development while different bands do so in other
tissues. Some most interesting results of Clever (1961) have shown that
the hormone ecdysone if injected into mid-larvae will induce puffing of
certain chromosome bands which usually show puffing only at the final
larval instar when ecdysone normally induces the pupal moult. This
research on Chironomus has given strong evidence that the puffing of
chromosome bands corresponds to gene activity and that in each tissue
certain genes show phases of activity from which they subsequently
regress. Reversible gene activity of this kind probably takes place in all
cells, but is cytologically detectable only in cells with enormously
distended chromosomes such as those with polytene chromosomes, as
well as in amphibian oocytes (Callan and Lloyd, 1960) and Drosophila
spermatocytes (Meyer et ah, 1961).
2. Induced Enzyme Formation
Another phenomenon which supports the concept of reversible gene
activation is induced enzyme formation. The addition of an amino acid
or other substance to the culture medium of Bacteria often results in
the rapid appearance of an enzyme concerned in the breakdown of (or
synthesis from) this substance. Thus enzyme formation is often stimulated by the presence of its substrate and an enzyme which is being
synthesized may be repressed by the accumulation of its end product
(e.g. Vogel, 1957). Induced enzyme formation has been described in
multicellular animals. Thus, tryptophan pyrrolase can be induced in
rat liver (Knox, 1951 ; Pitot and Cho, 1961), in amphibian liver (Spiegel,
1961; Spiegel and Spiegel, 1964), and in Drosophila larvae (Rizki and
Rizki, 1963). The appearance and disappearance of an enzyme does not
in itself show whether this is controlled by the activation and repression
of a gene ; it might have resulted, for example, from the polymerization
of protein components already present in the cell. However actinomycin
D has recently been much used to find out whether new gene action is
J . B. GURDON
These chromosomes consist of many thousand strands aligned parallel
to one another (Beermann, 1956), and in some species of Chironomus
they are so large that they can be seen with the naked eye. These
chromosomes contain many transverse bands some of which become
greatly distended into so-called puffs or balbiani rings. Direct evidence
that these puffs or rings represent active gene sites has been provided
by the close correlation that exists between the appearance of a balbiani
ring at the end of chromosome I I I and the appearance of some granules
in the basal salivary gland cells of certain species (Beermann, 1961).
Those species and cell types which lack these granules also lack a
balbiani ring at this chromosome position. In the cells of a given tissue,
particular chromosome bands consistently become distended into puffs
at certain stages of development while different bands do so in other
tissues. Some most interesting results of Clever (1961) have shown that
the hormone ecdysone if injected into mid-larvae will induce puffing of
certain chromosome bands which usually show puffing only at the final
larval instar when ecdysone normally induces the pupal moult. This
research on Chironomus has given strong evidence that the puffing of
chromosome bands corresponds to gene activity and that in each tissue
certain genes show phases of activity from which they subsequently
regress. Reversible gene activity of this kind probably takes place in all
cells, but is cytologically detectable only in cells with enormously
distended chromosomes such as those with polytene chromosomes, as
well as in amphibian oocytes (Callan and Lloyd, 1960) and Drosophila
spermatocytes (Meyer et ah, 1961).
2. Induced Enzyme Formation
Another phenomenon which supports the concept of reversible gene
activation is induced enzyme formation. The addition of an amino acid
or other substance to the culture medium of Bacteria often results in
the rapid appearance of an enzyme concerned in the breakdown of (or
synthesis from) this substance. Thus enzyme formation is often stimulated by the presence of its substrate and an enzyme which is being
synthesized may be repressed by the accumulation of its end product
(e.g. Vogel, 1957). Induced enzyme formation has been described in
multicellular animals. Thus, tryptophan pyrrolase can be induced in
rat liver (Knox, 1951 ; Pitot and Cho, 1961), in amphibian liver (Spiegel,
1961; Spiegel and Spiegel, 1964), and in Drosophila larvae (Rizki and
Rizki, 1963). The appearance and disappearance of an enzyme does not
in itself show whether this is controlled by the activation and repression
of a gene ; it might have resulted, for example, from the polymerization
of protein components already present in the cell. However actinomycin
D has recently been much used to find out whether new gene action is