Chromosome

CHROMOSOME. - Characteristic formations that are resolved from the nuclear substances of animal and plant cells when these enter into reproductive activity. While the nucleus of a resting cell (an inexact term that indicates the momentary or definitive lack of karyokinetic activity) is characterized by a karyoplasm, which by the common means of histological and cytological investigation proves to be more or less homogeneous, in the cell in karyokinetic crisis it is resolved into a complex of granulations that lead to the identification of small bodies well stainable with basic dyes, having a spherical or elongated or spindle shape, and which are the chromosomes; their size varies according to the living species, as does their number, but within individuals of the same species, number, shape and size are rigorously constant. There thus exist living beings 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 has per complement remains constant in all the daughter cells originating from it, because at every division that occurs in the cell the chromosomes undergo a longitudinal splitting; there is thus a momentary doubling of them until the cytoplasmic division or cytokinesis leads to the formation of two daughter cells, each of which will have as its complement one half of the chromosomes formed as a result of the longitudinal splitting. Only in germ cells is there a reduction to half the number of chromosomes.

This chromosomal arrangement of the cell takes the name of diploid complement and is indicated by 2 n. If, for example, one considers an egg of an animal species and posits that this egg has a diploid complement of 4 chromosomes, all the cells that will originate from the egg cell and that in their totality will constitute the tissues and organs of the adult individual will maintain constant the number of 4 chromosomes characteristic of the species considered. But in an adult individual, besides the cells constituting the tissues and organs of vegetative and relational life and which take the name of cells of the somatic line, there exist — and their differentiation culminates with the sexual maturity of the individual — cells deputed to the maintenance of the species, and these elements take the name of cells of the line

(Ist. from the vire)
CHROMOSOME - Giant chromosomes of Chironomus Thummi in vivo showing the characteristic structure with alternately colored bands (chromomeres) and colorless ones (according to Bauer).

germinal. The germinal cells, once mature, will unite, a male cell or spermatozoon with a female one or egg, to give rise to a new individual, thus ensuring the perpetuity of the species. It is clear that if both the egg and the spermatozoon kept the diploid complement of chromosomes, for example 4, the zygote, or fertilized egg, would have 8 chromosomes, and all the cells derived from the egg, that is the whole 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 be enormous; instead there exists a relationship between the quantity of nuclear substance and the quantity of cytoplasmic substance in every cell (nucleo-plasmatic ratio) which has limits of oscillation, on either side of which life is not possible. Therefore a regulatory mechanism intervenes in the germinal cells, by which the sexual elements, following a process called meiosis (Farmer and Moore), reduce their number of chromosomes to half the diploid number, thus realizing the haploid complement which is denoted by n. With fertilization, that is with the union of two cells having a haploid complement, a cell with a diploid complement will originate. There exist individuals in whose cells an increase in the number of chromosomes occurs naturally or can be experimentally induced, though within narrow limits; this number is generally a multiple of the diploid complement, and the individuals are called polyploids. This phenomenon falls within the field of mutations, of those hereditary variations that have their seat in the chromosomal complement.

It has been said that the chromosomes are visible only in cells in karyokinetic crisis; they can rarely be identified in the resting nucleus of cells, except for rare exceptions (giant chromosomes of the salivary glands of diptera, "feathery chromosomes" in the growing oocytes of urodeles, etc.). This observation took on enormous importance when genetic studies brought the chromosomes into a position of primary importance. It is known that every individual inherits from its parents the characters that in their totality constitute the individual itself. The environment and its influence can produce in the individual certain variations, but these always oscillate within a range whose breadth is determined by the hereditary characters. The studies of the Augustinian G. Mendel demonstrated that the maternal and paternal germinal cells transmit to the offspring the characters of the parents and that in the offspring of the offspring the characters of the grandparents are found again; there is something stable, which cannot be modified in its broad outlines and which must necessarily be found in the paternal and maternal cell that gave origin to the offspring individual. The idea of stability inherent in hereditary characters, the conception of immutability of the hereditary characters themselves on the one hand, and on the other the constancy of the number and form of the chromosomes of living beings and the fact that every individual has a chromosomal complement that is half of maternal origin and half of paternal origin have led the majority of geneticists, from Sutton onward, to consider the chromosomes as the carriers of that quid which determines in an individual its characters.

In the chromosomes, therefore, would have their topographic and functional seat those entities of submicroscopic and molecular order capable of determining the characters of an individual and transmissible from generation to generation. To these entities the name of "genes" has been given. The work especially of 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 the drosophila, the common vinegar fly, that along the chromosomes of the germinal cells the genes that determine a great number of characters of this animal species are found aligned. The genes function both by virtue of their constitution and by virtue of their mutual position along the chromosome. If by experimental intervention either segments of chromosomes or entire chromosomes are destroyed, or a shift of a sector of a chromosome is induced, following breakage and reattachment in a different position, the individual that develops from germinal cells that have undergone these processes turns out altered in its characters and usually shows rudimentary or aberrant organs or apparatuses or different morphological and functional characteristics. To these modifications that appear and that are transmitted hereditarily the name of 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 certain characters that are inherited together with the sex have their genes in the sex chromosome. It must be remembered, however, that today many authors give importance, and rightly so, also to the cytoplasm for the transmission of hereditary characters; thus one speaks of cytoplasmic genes or "plasmagenes" (Caspari).

The recent studies on submicroscopic structures have had as a seductive field of investigation also the constitution of the chromosomes and of the genes. These researches have predominantly had the purpose of investigating the molecular structure of the chromosome and of studying its chemical composition and together the constitution of the genes, also with the purpose of explaining the reason for the cyclical appearance and disappearance of the

(after L. Geitler, Chromosomenbau, Berlin 1933)
CHROMOSOME - Diagram of the structure of a somatic c. The distal region of the right arm is heterochromatic (heterochromatin), spiral chromonema with chromomeres (after Heitz, from Geitler).

c. Each c. contains one or more spiralized fibrils called chromonemes; along these spirals are arranged in series stainable elements that take the name of chromomeres. The chromonemes have a fibrillar structure, probably polypeptidic, while examination under ultraviolet light reveals a protein mass in the chromonemes. Other fundamental constituents of the c. are the nucleic acids, the study of which, after the research of T. Caspersson and J. Brachet, has assumed great importance. In the c. there is found predominantly deoxyribonucleic or thymonucleic acid and small quantities of ribonucleic acid. It is held that the nucleic acids form, at regular intervals, salt compounds with the protein fiber of the chromonema.

The stainable substances of the nucleus are called by the generic term chromatin, which does not imply a defined chemical concept. It comprises the two aforementioned forms of nucleic acids. The giant c. are constituted by two distinct portions: in the euchromatin there exists 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 acid and thymonucleic acid. Ribonucleic acid constitutes part of the nucleus and takes part in the synthesis of many protein substances.

During karyokinesis the nucleic acids undergo transformations, demonstrating that they have an easily modifiable structure.

The nucleic acids were considered as the fundamental substance of the c., but they have a relatively uniform chemical structure that accords poorly with the genetic theories on the constitution of the genes.

The modern developments of genetics, that is, of the science that deals with the heredity of characters in living forms, have brought to the forefront the study of the c., small bodies that are resolved from the nucleus of the cells in which the genes are localized, responsible, through a complex probabilistic interplay, for the transmission of characters from parent to offspring. Today one glimpses an even more complex mechanism due to the interrelations between chromosomal genes and genes of a cytoplasmic nature. Nevertheless the study of these organelles has not come to a halt, and although their structure is by now fairly well known, one still seeks in them the solution of one of the most peculiar aspects of living beings: the hereditary transmission of characters (v. EREDITARIETÀ; PROTOPLASMA).

Enrico Urbani