12. G E N E T I C C Y T O L O G Y
425
C. C H R O M O S O M E S T R U C T U R E
An old cytological problem also has been subjected to renewed
interest in the light of modern techniques such as autoradiography. The
problem is whether each chromatid is subdivided into half-chromatids
or is a single unit (cf. Ris, 1957). That is to say, each chromosome seen
in metaphase is composed of two discernible chromatids, but according
to some cytologists, each chromatid is composed of two microscopically
undiscernible but existing subunits or half-chromatids. Thus, from
anaphase to the end of the G x period, each chromosome is already
composed of two units, the two half-chromatids. If this is so, how does
a chromosome synthesize its D N A during the S period? The work of
Taylor and others, using tritiated thymidine and autoradiography, has
provided some interesting clues which will be summarized.
If each chromosome consists of a single chromatid prior to the S
phase, and tritiated thymidine is available during the synthetic period,
the new chromatid should bear label while the old one should not.
In roots of plants with large chromosomes such as Bellevalia romana
(2w = 8), application of H
3 - T D R for labeling and colchicine for arresting
mitosis and separating chromatids of individual chromosomes in
metaphase to achieve greatest clarity, Taylor (1958a) found that both
chromatids of each chromosome were labeled. This suggests rather
strongly that the original chromatid was not a single unit. However,
this observation alone does not conclusively settle the issue. As Taylor
(1959) pointed out tersely: "each chromosome either could have divided
and built a new half, or all the D N A could have been destroyed and
replaced by new D N A " . If a chromatid does contain two halves, the
labeling of both chromatids would mean that the two half-chromatids
individually synthesized D N A when radioactive thymidine was supplied. Thus, the labeled chromatid seen at metaphase should actually
contain an unlabeled half which is the original half-chromatid. T o
demonstrate this, Taylor developed experiments which permitted the
cells to undergo another division cycle, this time in the absence of
tritium. The metaphase of the second mitotic division told the story.
The chromosomes were now half labeled and half unlabeled. Since the
DNA of the labeled chromatid was not destroyed during interphase
and was passed on to the daughter cells, the original unlabeled D N A
must have likewise been conserved. During the synthesis period of the
second cycle, each chromatid must have two halves, one labeled and
the other not labeled (the original). Now they both synthesized D N A
without label. The resulting two chromatids were, therefore, different
in their labeling. The one which was labeled would remain labeled but
it contained a sister that was not labeled. The original unlabeled half
425
C. C H R O M O S O M E S T R U C T U R E
An old cytological problem also has been subjected to renewed
interest in the light of modern techniques such as autoradiography. The
problem is whether each chromatid is subdivided into half-chromatids
or is a single unit (cf. Ris, 1957). That is to say, each chromosome seen
in metaphase is composed of two discernible chromatids, but according
to some cytologists, each chromatid is composed of two microscopically
undiscernible but existing subunits or half-chromatids. Thus, from
anaphase to the end of the G x period, each chromosome is already
composed of two units, the two half-chromatids. If this is so, how does
a chromosome synthesize its D N A during the S period? The work of
Taylor and others, using tritiated thymidine and autoradiography, has
provided some interesting clues which will be summarized.
If each chromosome consists of a single chromatid prior to the S
phase, and tritiated thymidine is available during the synthetic period,
the new chromatid should bear label while the old one should not.
In roots of plants with large chromosomes such as Bellevalia romana
(2w = 8), application of H
3 - T D R for labeling and colchicine for arresting
mitosis and separating chromatids of individual chromosomes in
metaphase to achieve greatest clarity, Taylor (1958a) found that both
chromatids of each chromosome were labeled. This suggests rather
strongly that the original chromatid was not a single unit. However,
this observation alone does not conclusively settle the issue. As Taylor
(1959) pointed out tersely: "each chromosome either could have divided
and built a new half, or all the D N A could have been destroyed and
replaced by new D N A " . If a chromatid does contain two halves, the
labeling of both chromatids would mean that the two half-chromatids
individually synthesized D N A when radioactive thymidine was supplied. Thus, the labeled chromatid seen at metaphase should actually
contain an unlabeled half which is the original half-chromatid. T o
demonstrate this, Taylor developed experiments which permitted the
cells to undergo another division cycle, this time in the absence of
tritium. The metaphase of the second mitotic division told the story.
The chromosomes were now half labeled and half unlabeled. Since the
DNA of the labeled chromatid was not destroyed during interphase
and was passed on to the daughter cells, the original unlabeled D N A
must have likewise been conserved. During the synthesis period of the
second cycle, each chromatid must have two halves, one labeled and
the other not labeled (the original). Now they both synthesized D N A
without label. The resulting two chromatids were, therefore, different
in their labeling. The one which was labeled would remain labeled but
it contained a sister that was not labeled. The original unlabeled half