ULTRASTRUCTURE OF THE AMPHIBIAN EGG
193
pool within the nuclear sap and thus serve to direct synthesis of gene
products. Experimental evidence for this concept has come from autoradiographic studies of uridine-H
3
incorporation. It was shown in these
experiments that incorporation follows a sequential pattern, starting at
the thin end of the giant loop and proceeding around the loop (Gall
and Callan, 1962). I t was also shown (Callan, 1963) that radioactive
RNA is detectable within hours after subcutaneous injection in the
oocyte nucleus but does not appear in the cytoplasm until 4 days afterward. Thereafter, nuclear radioactivity decreases, whereas that in the
cytoplasm increases. These results support the concept that the nucleus
is the sole source of cytoplasmic RNA (Prescott, 1960).
That the synthesis of RNA in the chromosome is a DNA-dependent
process, judged by its sensitivity to inhibition by actinomycin D, was
reported by Izawa et al. (1963a). Moreover, they showed that the latter
compound not only inhibits RNA synthesis, but also leads to a disappearance of the chromosomal loops. This, and other evidence, suggests
that the morphology of the active chromosomal site is not only closely
related to its capacity to synthesize RNA, but is dependent on it.
Chemical analysis of the nucleus of amphibian oocytes was also investigated by Izawa et al. (1963b). They found that the DNA content of
lampbrush chromosomes is about four times that of chromosomes in
nuclei of other cell types in the newt. The nucleoplasm of the oocyte
contains as much DNA as is present in the chromosomes. Some of the
nucleoplasmic DNA is located in nucleoli. The protein : DNA and
RNA : DNA ratios in lampbrush chromosomes are very much higher
than in other chromosomes. The unusual characteristics of the composition of lampbrush chromosomes are related to the fact that these
chromosomes, unlike others, are active at all loci. This is in marked
contrast to interphase chromosomes, in which a few loci are active
whereas most are relatively inactive (Beermann, 1952; Allfrey and
Mirsky, 1962), and also in striking contrast to mitotic chromosomes,
which appear to be metabolically inert, although they are being moved
about (Prescott and Bender, 1962).
Callan (1963) feels that the gene products produced by the loops are
in a morphological form whose dimensions are below the level of resolution with the light microscope. This may be true in the case of typical
loops whose matrix material seems to consist of fine fibers that project
out from the loop axis (Fig. 23). On the other hand, in granular loops,
the gene activity may be represented by the accumulation of spherical
masses located at the ends of the loop fibers (Fig. 24). These masses are
probably shed into the nuclear sap and appear there in the form of free
bodies. Since such bodies contribute toward the total nuclear population,
we shall now turn our attention to a discussion of these elements.
193
pool within the nuclear sap and thus serve to direct synthesis of gene
products. Experimental evidence for this concept has come from autoradiographic studies of uridine-H
3
incorporation. It was shown in these
experiments that incorporation follows a sequential pattern, starting at
the thin end of the giant loop and proceeding around the loop (Gall
and Callan, 1962). I t was also shown (Callan, 1963) that radioactive
RNA is detectable within hours after subcutaneous injection in the
oocyte nucleus but does not appear in the cytoplasm until 4 days afterward. Thereafter, nuclear radioactivity decreases, whereas that in the
cytoplasm increases. These results support the concept that the nucleus
is the sole source of cytoplasmic RNA (Prescott, 1960).
That the synthesis of RNA in the chromosome is a DNA-dependent
process, judged by its sensitivity to inhibition by actinomycin D, was
reported by Izawa et al. (1963a). Moreover, they showed that the latter
compound not only inhibits RNA synthesis, but also leads to a disappearance of the chromosomal loops. This, and other evidence, suggests
that the morphology of the active chromosomal site is not only closely
related to its capacity to synthesize RNA, but is dependent on it.
Chemical analysis of the nucleus of amphibian oocytes was also investigated by Izawa et al. (1963b). They found that the DNA content of
lampbrush chromosomes is about four times that of chromosomes in
nuclei of other cell types in the newt. The nucleoplasm of the oocyte
contains as much DNA as is present in the chromosomes. Some of the
nucleoplasmic DNA is located in nucleoli. The protein : DNA and
RNA : DNA ratios in lampbrush chromosomes are very much higher
than in other chromosomes. The unusual characteristics of the composition of lampbrush chromosomes are related to the fact that these
chromosomes, unlike others, are active at all loci. This is in marked
contrast to interphase chromosomes, in which a few loci are active
whereas most are relatively inactive (Beermann, 1952; Allfrey and
Mirsky, 1962), and also in striking contrast to mitotic chromosomes,
which appear to be metabolically inert, although they are being moved
about (Prescott and Bender, 1962).
Callan (1963) feels that the gene products produced by the loops are
in a morphological form whose dimensions are below the level of resolution with the light microscope. This may be true in the case of typical
loops whose matrix material seems to consist of fine fibers that project
out from the loop axis (Fig. 23). On the other hand, in granular loops,
the gene activity may be represented by the accumulation of spherical
masses located at the ends of the loop fibers (Fig. 24). These masses are
probably shed into the nuclear sap and appear there in the form of free
bodies. Since such bodies contribute toward the total nuclear population,
we shall now turn our attention to a discussion of these elements.