1 2 . G E N E T I C C Y T O L O G Y
431
cinematographic recordings of HeLa cells treated with amethopterin
and relieved by thymidine showed quite a stagger in the time of onset
of mitosis. Second, it is a well-known fact that thymine deficiency may
result in mutagenesis. Thymine deficiency induced by F U D R also
results in severe chromosome damage (Hsu, Humphrey and Somers,
unpublished data). Synchronization of mitosis for many rounds by such
treatments may cause drastic physiological alteration of cell populations. If the cells are for this reason abnormal because of the treatment,
the entire metabolic pattern may have been changed. Results obtained
from such morbid systems are meaningless. Refinements of techniques
are definitely required to ensure success so that cell culture systems
can be used for investigations of more intricate biological and biochemical problems.
I V . H E T E R O C H R O M A T I N A N D H E T E R O P Y K N O S I S
For several decades geneticists and cytologists have published a large
number of papers concerning the nature and function of heterochromatin. Unfortunately, heterochromatin is still a poorly understood area
of genetics. Even the term itself is confusing to many, partly because
it has been used to denote different cytological entities. A brief resume
of this subject will be presented. The reader is referred to the books
of White (1954) and of Swanson (1957) for detailed morphological
accounts, and to papers by Schultz (1947) and Hannah (1951) for
summaries of genetic information.
Heitz, during the late 1920's and early 1930's, especially the classic
paper on Drosophila chromosomes (1933), laid the foundation of the
heterochromatin concept. In mitotic cells, chromosomes regularly
spiralize to form condensed (or pyknotic) bodies from prophase to metaphase, and despiralize from telophase to interphase. However, parts of
chromosomes, e.g. areas near the centromeres, maintain the condensed
state throughout the nuclear cycle. During interphase, these chromosome parts appear to congregate into several deeply staining chromatin
blocks known as chromocenters. These are the heterochromatic
regions which consist of heterochromatin as opposed to euchromatin
which forms the bulk of chromosomes. The persistence of the condensed
state of heterochromatin when euchromatin despiralizes in interphase
is known as heteropyknosis.
In Drosophila, the behavior of the entire Y chromosome in somatic
cells is similar to that of the heterochromatic regions of other chromosomes. Genetically, it is well known that the Drosophila Y chromosome
does not carry genes corresponding to those in the X ; individuals
431
cinematographic recordings of HeLa cells treated with amethopterin
and relieved by thymidine showed quite a stagger in the time of onset
of mitosis. Second, it is a well-known fact that thymine deficiency may
result in mutagenesis. Thymine deficiency induced by F U D R also
results in severe chromosome damage (Hsu, Humphrey and Somers,
unpublished data). Synchronization of mitosis for many rounds by such
treatments may cause drastic physiological alteration of cell populations. If the cells are for this reason abnormal because of the treatment,
the entire metabolic pattern may have been changed. Results obtained
from such morbid systems are meaningless. Refinements of techniques
are definitely required to ensure success so that cell culture systems
can be used for investigations of more intricate biological and biochemical problems.
I V . H E T E R O C H R O M A T I N A N D H E T E R O P Y K N O S I S
For several decades geneticists and cytologists have published a large
number of papers concerning the nature and function of heterochromatin. Unfortunately, heterochromatin is still a poorly understood area
of genetics. Even the term itself is confusing to many, partly because
it has been used to denote different cytological entities. A brief resume
of this subject will be presented. The reader is referred to the books
of White (1954) and of Swanson (1957) for detailed morphological
accounts, and to papers by Schultz (1947) and Hannah (1951) for
summaries of genetic information.
Heitz, during the late 1920's and early 1930's, especially the classic
paper on Drosophila chromosomes (1933), laid the foundation of the
heterochromatin concept. In mitotic cells, chromosomes regularly
spiralize to form condensed (or pyknotic) bodies from prophase to metaphase, and despiralize from telophase to interphase. However, parts of
chromosomes, e.g. areas near the centromeres, maintain the condensed
state throughout the nuclear cycle. During interphase, these chromosome parts appear to congregate into several deeply staining chromatin
blocks known as chromocenters. These are the heterochromatic
regions which consist of heterochromatin as opposed to euchromatin
which forms the bulk of chromosomes. The persistence of the condensed
state of heterochromatin when euchromatin despiralizes in interphase
is known as heteropyknosis.
In Drosophila, the behavior of the entire Y chromosome in somatic
cells is similar to that of the heterochromatic regions of other chromosomes. Genetically, it is well known that the Drosophila Y chromosome
does not carry genes corresponding to those in the X ; individuals