398
T . c . HSU
in vivo and in vitro, and related subjects. These areas need not be discussed again. However, in that review, cytogenetics of man was merely
touched upon in passing, and little was mentioned concerning the
cell cycle, nucleic acid synthesis, and related topics, which altogether
deserve a progress summary. Several review articles on human cytogenetics alone have recently appeared (Harnden, 1960a; Shaw, 1960;
Miles, 1961; Book, 1961; Ferguson-Smith, 1961).
The correction of human chromosome number and the discovery of
aneuploidy in human populations have incited a riotous enthusiasm to
study human cytology. Many who never had seen a chromosome were
stimulated to enter this area of endeavour. This "gold rush" indeed
yielded some significant facts: (1) human beings, like fruit flies and
Jimson weeds, may also be aneuploid; (2) in man the frequency of
chromosome anomalies is astonishingly high, indicating that his mitotic
and meiotic mechanisms are no more precise than in other organisms;
and (3) the sex-determining system of man, and possibly that of most
mammals, long explained on the basis of findings from Drosophila, are
revised. After all, it is some consolation that we are not entirely like the
flies!
There is a ray of hope that we might some day understand the long,
unsolved problem of the nature of heterochromatin. In a way, this
feeling came as a byproduct of autoradiographic studies on nucleic
acid synthesis. As utilization of isotopes increased its impact in biological
and chemical investigations, autoradiography has become an important
tool in experimental cell research. It furnishes relatively accurate
information regarding cellular activities which hitherto could not be
determined. Cytologists can now, by using tritium-labeled thymidine,
delimit the period of deoxyribonucleic acid (DNA) synthesis. Similarly,
tritium-labeled uridine supplies information concerning the sites of
ribonucleic acid (RNA) synthesis and its movements. Asynchronous
DNA synthesis among chromosomes and chromosome parts may lead
to the solution of the mystery of heterochromatin. It is highly possible
that studies of this sort may eventually assist research in sex chromosomes, sex chromatin and sex determination.
I shall attempt to collect information regarding these areas and shall
also present my own opinions to formulate some working hypotheses,
hoping to provoke arguments and thoughts. However, it is not intended
to make this article an exhaustive review of literature, which is virtually
a physical impossibility. Merely on human cytogenetics, for example,
the review of Miles (1961) lists 140 references, and numerous papers
have appeared since. Since the present review covers wider grounds
than human cytogenetics, I am obliged to delete many references
except, of course, my own. I hasten to add that this does not imply that
T . c . HSU
in vivo and in vitro, and related subjects. These areas need not be discussed again. However, in that review, cytogenetics of man was merely
touched upon in passing, and little was mentioned concerning the
cell cycle, nucleic acid synthesis, and related topics, which altogether
deserve a progress summary. Several review articles on human cytogenetics alone have recently appeared (Harnden, 1960a; Shaw, 1960;
Miles, 1961; Book, 1961; Ferguson-Smith, 1961).
The correction of human chromosome number and the discovery of
aneuploidy in human populations have incited a riotous enthusiasm to
study human cytology. Many who never had seen a chromosome were
stimulated to enter this area of endeavour. This "gold rush" indeed
yielded some significant facts: (1) human beings, like fruit flies and
Jimson weeds, may also be aneuploid; (2) in man the frequency of
chromosome anomalies is astonishingly high, indicating that his mitotic
and meiotic mechanisms are no more precise than in other organisms;
and (3) the sex-determining system of man, and possibly that of most
mammals, long explained on the basis of findings from Drosophila, are
revised. After all, it is some consolation that we are not entirely like the
flies!
There is a ray of hope that we might some day understand the long,
unsolved problem of the nature of heterochromatin. In a way, this
feeling came as a byproduct of autoradiographic studies on nucleic
acid synthesis. As utilization of isotopes increased its impact in biological
and chemical investigations, autoradiography has become an important
tool in experimental cell research. It furnishes relatively accurate
information regarding cellular activities which hitherto could not be
determined. Cytologists can now, by using tritium-labeled thymidine,
delimit the period of deoxyribonucleic acid (DNA) synthesis. Similarly,
tritium-labeled uridine supplies information concerning the sites of
ribonucleic acid (RNA) synthesis and its movements. Asynchronous
DNA synthesis among chromosomes and chromosome parts may lead
to the solution of the mystery of heterochromatin. It is highly possible
that studies of this sort may eventually assist research in sex chromosomes, sex chromatin and sex determination.
I shall attempt to collect information regarding these areas and shall
also present my own opinions to formulate some working hypotheses,
hoping to provoke arguments and thoughts. However, it is not intended
to make this article an exhaustive review of literature, which is virtually
a physical impossibility. Merely on human cytogenetics, for example,
the review of Miles (1961) lists 140 references, and numerous papers
have appeared since. Since the present review covers wider grounds
than human cytogenetics, I am obliged to delete many references
except, of course, my own. I hasten to add that this does not imply that