318
J . S E E D
A . L O C A T I O N
In 1924, Feulgen and Rossenbeck described the adaptation of
Schiff
5
s test for aldehydes as a histochemical test for the presence of
D N A : the tissues had first to be subjected to acid hydrolysis in order
to expose the aldehyde groups. Subsequently, from the results obtained
by the use of this method, it became apparent that the majority of the
cellular D N A is contained within the nucleus during interphase and
within the chromosomes during mitosis. Later, Caspersson (1941)
demonstrated with the u.v. microscope that the chromosomes absorb
u.v. strongly and he concluded that the absorption was caused by the
presence of nucleic acids having an absorption maximum at 2,600 A.
The work of Morgan (1919) had already provided evidence that the
genes, the units of heredity, were carried on the chromosomes and, with
this information, it was reasonable to suppose that at least one of the
two major constituents of the chromosomes, i.e. the D N A or the protein,
must be the genetic material. For some time it was generally assumed
that the genetic information was carried entirely by the protein
component, as the nucleic acid structure was thought to be too simple
to act as a repository of such complex information. However, the demonstration by Avery, McLeod and McCarty (1944) that purified D N A
from one strain of Pneumococcus could permanently transform the characteristics of another strain (that is, from the rough non-encapsulated
variant to the smooth, fully-encapsulated form) demonstrated the
importance of D N A as genetic material.
Indirect evidence for the genetic role of D N A also came from the
experiments of Boivin, Vendrely and Vendrely (1948), who showed
that the amount of D N A per cell was approximately constant for the
cells of any one organism, as would be expected for a genetic material.
Similar results were later obtained by Mirsky and Ris (1949), Davidson,
Leslie, Smellie and Thomson (1950), Swift (1950a, b) and by Leslie
and Davidson (1951) who used embryo-chick cells in culture. Within the
same organism, the amount of D N A per cell is found to be proportional
to the number of sets of chromosomes present; thus a sperm cell has
only one half the D N A content found in the nuclei of various somatic
cells, and certain tumours known to contain four sets of chromosomes
(Hauschka, 1952) have twice the D N A content observed in a normal
diploid cell.
It should be noted that not all organisms have D N A as their genetic
material: certain viruses, e.g. tobacco mosaic, contain no D N A and
in this case R N A has been shown to be the genetically significant part
(Gierer and Schramm, 1956; Fraenkel-Conrat, 1956).
J . S E E D
A . L O C A T I O N
In 1924, Feulgen and Rossenbeck described the adaptation of
Schiff
5
s test for aldehydes as a histochemical test for the presence of
D N A : the tissues had first to be subjected to acid hydrolysis in order
to expose the aldehyde groups. Subsequently, from the results obtained
by the use of this method, it became apparent that the majority of the
cellular D N A is contained within the nucleus during interphase and
within the chromosomes during mitosis. Later, Caspersson (1941)
demonstrated with the u.v. microscope that the chromosomes absorb
u.v. strongly and he concluded that the absorption was caused by the
presence of nucleic acids having an absorption maximum at 2,600 A.
The work of Morgan (1919) had already provided evidence that the
genes, the units of heredity, were carried on the chromosomes and, with
this information, it was reasonable to suppose that at least one of the
two major constituents of the chromosomes, i.e. the D N A or the protein,
must be the genetic material. For some time it was generally assumed
that the genetic information was carried entirely by the protein
component, as the nucleic acid structure was thought to be too simple
to act as a repository of such complex information. However, the demonstration by Avery, McLeod and McCarty (1944) that purified D N A
from one strain of Pneumococcus could permanently transform the characteristics of another strain (that is, from the rough non-encapsulated
variant to the smooth, fully-encapsulated form) demonstrated the
importance of D N A as genetic material.
Indirect evidence for the genetic role of D N A also came from the
experiments of Boivin, Vendrely and Vendrely (1948), who showed
that the amount of D N A per cell was approximately constant for the
cells of any one organism, as would be expected for a genetic material.
Similar results were later obtained by Mirsky and Ris (1949), Davidson,
Leslie, Smellie and Thomson (1950), Swift (1950a, b) and by Leslie
and Davidson (1951) who used embryo-chick cells in culture. Within the
same organism, the amount of D N A per cell is found to be proportional
to the number of sets of chromosomes present; thus a sperm cell has
only one half the D N A content found in the nuclei of various somatic
cells, and certain tumours known to contain four sets of chromosomes
(Hauschka, 1952) have twice the D N A content observed in a normal
diploid cell.
It should be noted that not all organisms have D N A as their genetic
material: certain viruses, e.g. tobacco mosaic, contain no D N A and
in this case R N A has been shown to be the genetically significant part
(Gierer and Schramm, 1956; Fraenkel-Conrat, 1956).