CHROMOSOME. — Characteristic formations that derive from the nuclear substances of animal and plant cells when these enter reproductive activity. While the nucleus of a resting cell (a term here embedded to indicate the temporary or definitive absence of karyokinetic activity) is characterized by a karyoplasm that, with ordinary histological and cytological investigative methods, appears more or less homogeneous, in the cell undergoing karyokinetic crisis it resolves into a complex of granules that lead to the identification of small bodies well stainable with basic dyes, having a spherical, elongated, or fusiform shape, and these are the chromosomes; their size varies according to the living species, as does their number, but within the scope of individuals of the same species, number, shape, and size are rigorously constant. There thus exist living organisms with two or four chromosomes (ascaris), with 12 chromosomes (pine), with 48 chromosomes (man), up to several hundred (radiolarians). The number of chromosomes that a cell possesses as its complement remains constant in all daughter cells originating from it because, at every division occurring in the cell, the chromosomes undergo a longitudinal splitting; thus there is a temporary doubling of them until cytoplasmic division or cytokinesis leads to the formation of two daughter cells, each of which will have as its complement half of the chromosomes formed following the longitudinal splitting. Only in germ cells is there a reduction by half in the number of chromosomes.
This chromosomal arrangement of the cell is called the diploid complement and is indicated by 2n. If, for example, one considers an egg of an animal species and assumes that this egg has a diploid complement of 4 chromosomes, all the cells that will originate from the egg cell and that, taken together, will constitute the tissues and organs of the adult individual, will maintain the constant number of 4 chromosomes characteristic of the species in question. But in an adult individual, besides the cells constituting the tissues and organs of vegetative and relational life (which are called somatic line cells), there also exist—culminating in the sexual maturity of the individual—cells deputed to the maintenance of the species, and these elements are called germ line cells.
Maternal and paternal germ cells transmit to the offspring the traits of the parents, and in the grandchild the traits of the grandparents are found again; there is something stable that cannot be altered in broad outline and that necessarily must be present in the paternal and maternal cells that gave rise to the offspring. The idea of stability inherent in hereditary traits, the conception of the immutability of hereditary traits on the one hand, and on the other the constancy of the number and shape of the chromosomes of living organisms and the fact that every individual has a chromosomal complement that is half of maternal origin and half of paternal origin, led, from Sutton onward, to consider chromosomes as the bearers of that *quid* which determines in an individual the traits.
In the chromosomes would thus have their topographical and functional seat those entities of submicroscopic and molecular order capable of determining the traits of an individual and transmissible from generation to generation. To these entities the name "genes" has been given. Work, especially from the American school of Morgan, has led to results of enormous importance. It has been possible to verify experimentally—and the material of choice for this research has been *Drosophila*, the common vinegar fly—that along the chromosomes of germ cells the genes are aligned that determine a large number of traits of this animal species. Genes function both by their constitution and by their reciprocal position along the chromosome. If, through experimental intervention, segments of chromosomes or entire chromosomes are destroyed or if a sector of a chromosome is caused to shift as a result of breaking and reattachment in a different position, the individual that develops from germ cells that have undergone these processes is altered in its traits and usually shows organs or systems that are rudimentary or aberrant, or morphological and functional characteristics that are different. To these modifications that appear and are transmitted hereditarily the name "mutations" has been given. It has also been seen that the sex of an individual is in certain cases determined by a particular chromosome that has taken the name of heterochromosome or sex chromosome, and that some traits that are inherited together with sex have their genes in the sex chromosome. It must however be remembered that today many authors rightly give importance also to the cytoplasm for the transmission of hereditary traits; thus one speaks of cytoplasmic genes or "plasmagenes" (Casparì).
Recent studies on submicroscopic structures have also had as a seductive field of investigation the constitution of chromosomes and genes. These investigations have mainly aimed to inquire into the molecular structure of the chromosome and to study its chemical composition and, together with this, the constitution of genes, also with the aim of explaining the reason for the cyclical appearance and disappearance of
CHROMOSOME — Spiral structure of chromosomes during the first division of the pollen grain of Angiosperms: a-c *Allium nutans*, d. *Gasteria depressa* (Liliaceae): c. anaphase, the others in metaphase (according to Gellert).
Germ line. Once mature, the germ cells will unite, a male cell or spermatozoon with a female one or egg, to give rise to a new individual, thus ensuring the perpetuation of the species. It is clear that if both the egg and the spermatozoon maintained the diploid complement of chromosomes, for example 4, the zygote, or fertilized egg, would have 8 chromosomes and all the cells deriving from the egg, that is, the entire organism, would have doubled the number of chromosomes of the parents. Thus, for every generation of offspring there would be a doubling of the number of chromosomes of the parents, and in a short time this number would become enormous; instead, there exists a ratio between the quantity of nuclear substance and the quantity of cytoplasmic substance in every cell (nucleus-plasma ratio) that has limits of oscillation beyond which life is not possible. A regulatory mechanism therefore intervenes in germ cells by which the sexual elements, following a process called meiosis (Farmer and Moore), reduce their number of chromosomes by half of the diploid number, thus achieving the haploid complement indicated by n. With fertilization, that is, with the union of two cells having a haploid complement, a cell with a diploid complement would originate. There exist individuals in whose cells there is naturally or can be experimentally induced an increase in the number of chromosomes, within however restricted limits; such a number is generally a multiple of the diploid complement, and the individuals take the name of polyploids. This phenomenon falls within the field of mutations, those hereditary variations that have their seat in the chromosomal complement.
It has been said that chromosomes are visible only in cells undergoing karyokinetic crises; they can rarely be identified in the resting nucleus of cells, except in rare exceptions (giant chromosomes of the salivary glands of dipterans, plumose chromosomes in growing oocytes of urodeles, etc.). This observation assumed enormous importance when genetic studies brought chromosomes to the forefront. It is well known that every individual inherits from its parents the traits that, taken together, constitute the individual itself. The environment and its influence can induce certain variations in the individual, but these always fluctuate within a range whose amplitude is determined by hereditary traits. The studies of the Augustinian G. Mendel demonstrated that the cells
chromosomes. Each chromosome contains one or more spiralized fibrils called chromonemata; along these spirals are arranged serially stainable elements known as chromomeres. The chromonemata have a fibrillar structure, probably polypeptidic, while ultraviolet examination reveals in the chromonemata a protein mass. Other fundamental constituents of chromosomes are nucleic acids, the study of which, following the research of T. Caspersson and J. Brachet, has assumed great importance. Chromosomes contain predominantly deoxyribonucleic or thymonucleic acid and small quantities of ribonucleic acid. It is believed that nucleic acids form, at regular intervals, saline compounds with the protein fiber of the chromonema.
The stainable substances of the nucleus are generically termed chromatin, a term that does not imply a defined chemical concept. It comprises the two aforementioned forms of nucleic acids. Giant chromosomes consist of two distinct portions: in euchromatin there is a regular structure of disks rich in thymonucleic acid alternating with achromatic regions. This regular structure is lost in the more condensed and more stainable heterochromatin, which is composed of both ribonucleic and thymonucleic acids. Ribonucleic acid constitutes part of the nucleolus and takes part in the synthesis of many protein substances.
During karyokinesis, nucleic acids undergo transformations, demonstrating that they have a readily modifiable structure.
Nucleic acids were considered the fundamental substance of chromosomes, but they have a relatively uniform chemical structure that poorly accords with genetic theories on the constitution of genes.
Modern developments in genetics—i.e., the science concerned with the heredity of traits in living forms—have brought to the forefront the study of chromosomes, small bodies that are resolved from the nucleus of cells in which genes are localized. These genes, through a complex probabilistic interplay, are responsible for the transmission of traits from parent to offspring. Today, an even more complex mechanism is envisaged for the interrelations between chromosomal genes and genes of cytoplasmic nature. However, the study of these organelles has not ceased, and although their structure is now fairly well understood, research continues in search of a solution to one of the most peculiar aspects of living beings: the hereditary transmission of traits (v. EREDITARIETÀ; PROTOPLASMA).