1 2 . G E N E T I C C Y T O L O G Y
415
Attempts to apply B U D R to human cell cultures have failed to yield
substantial information, however. Perhaps different cell types require
different number of cell cycles to respond to B U D R effect. In blood
cultures fixed on the third day, most cells have gone through only one
cell cycle. Cytologists will have to keep an eye on the developments in
other fields to provide valuable leads for their own enterprises. The
asynchronous D N A synthesis reported in many plant and animal
species may be one of the clues to human genetics (see p. 436).
Sex determination forms one of the most palatable subjects in
mammalian cytogenetics. What is not clear is the interplay between
the genes in the X chromosome and those in the Y. Why is the Y
chromosome more male-determining in man than in Drosophila? Must
the Y be present in order to develop the male gonad? What is the nature
of the sex chromatin? From the evolutionary viewpoint, one may ask
questions similar to the one asked by White (1960): "Are there no
mammal species with X O males—and if not, why not?" From the
viewpoint of developmental biology, the recent discoveries on sex
anomalies and chromosomes opened some entirely new concepts on
the physiology of sex. In some aspects, such as sex determination,
certain animals offer a better system than man, and experiments can
be made to elucidate a number of perplexing phenomena. The golden
hamster (Mesocricetus auratus) appears to be useful for such studies
(Ohno and Weiler, 1961), because the X chromosomes of this species
are the largest of the entire complement.
I I I .
D N A
S Y N T H E S I S A N D T H E C E L L C Y C L E
In the older textbooks of biology nuclei not in mitosis are termed
"resting nuclei". Biologists knew that they were not resting, but the
term was used to differentiate them from mitotic stages. It is, nevertheless, a misnomer. Some replaced it with the term "metabolic nuclei",
which is equally inappropriate, for it implies that mitotic cells are
metabolically inactive. It is generally accepted now to use the somewhat noncommittal term "interphase" to denote the period in which
the cells are not in division stages. This term will be used in the present
article.
Cytologists have long suspected that new chromosomes are made
during interphase. The mitotic events clearly show that the sister
chromatids, which divide to opposite daughter cells, are discernible
at earliest prophase. This could only mean that synthesis of new
chromosomes is completed prior to prophase or during interphase
between successive mitoses.
415
Attempts to apply B U D R to human cell cultures have failed to yield
substantial information, however. Perhaps different cell types require
different number of cell cycles to respond to B U D R effect. In blood
cultures fixed on the third day, most cells have gone through only one
cell cycle. Cytologists will have to keep an eye on the developments in
other fields to provide valuable leads for their own enterprises. The
asynchronous D N A synthesis reported in many plant and animal
species may be one of the clues to human genetics (see p. 436).
Sex determination forms one of the most palatable subjects in
mammalian cytogenetics. What is not clear is the interplay between
the genes in the X chromosome and those in the Y. Why is the Y
chromosome more male-determining in man than in Drosophila? Must
the Y be present in order to develop the male gonad? What is the nature
of the sex chromatin? From the evolutionary viewpoint, one may ask
questions similar to the one asked by White (1960): "Are there no
mammal species with X O males—and if not, why not?" From the
viewpoint of developmental biology, the recent discoveries on sex
anomalies and chromosomes opened some entirely new concepts on
the physiology of sex. In some aspects, such as sex determination,
certain animals offer a better system than man, and experiments can
be made to elucidate a number of perplexing phenomena. The golden
hamster (Mesocricetus auratus) appears to be useful for such studies
(Ohno and Weiler, 1961), because the X chromosomes of this species
are the largest of the entire complement.
I I I .
D N A
S Y N T H E S I S A N D T H E C E L L C Y C L E
In the older textbooks of biology nuclei not in mitosis are termed
"resting nuclei". Biologists knew that they were not resting, but the
term was used to differentiate them from mitotic stages. It is, nevertheless, a misnomer. Some replaced it with the term "metabolic nuclei",
which is equally inappropriate, for it implies that mitotic cells are
metabolically inactive. It is generally accepted now to use the somewhat noncommittal term "interphase" to denote the period in which
the cells are not in division stages. This term will be used in the present
article.
Cytologists have long suspected that new chromosomes are made
during interphase. The mitotic events clearly show that the sister
chromatids, which divide to opposite daughter cells, are discernible
at earliest prophase. This could only mean that synthesis of new
chromosomes is completed prior to prophase or during interphase
between successive mitoses.