Genetics with Elements of Breeding - M.P. Myhun - 2008

CHAPTER II. Material Foundations and Molecular Mechanisms of Heredity

2.1. The Role of the Nucleus and Chromosomes in Heredity

It is well known that the basic Structure OF THE Cell—the protoplast—is divided into The Nucleus and the Cytoplasm. Both of these components play a vital role in heredity. However, the nucleus holds the primary role because it houses the Chromosomes, which in turn contain DNA and genes. Nuclear size depends on cell size, and for each cell type, There is a constant nucleocytoplasmic ratio. If this ratio is altered, The Cell either divides or dies. The leading role of the nucleus in heredity has been confirmed by both indirect and direct evidence. One such piece of direct evidence was provided by H. Hämmerling. He took two species of single-celled green Algae, Acetabularia, which differed in the shape of their caps. He excised the rhizoids—where the nucleus is located—and grafted them onto the stalks of the other species. It turned out that the cap shape is determined by the species identity of the nucleus rather than the cytoplasm. The Role of the nucleus in heredity was further demonstrated by B.L. Astaurov using silkworms. Unfertilized eggs were exposed to X-ray irradiation. Under these conditions, the egg nuclei were destroyed, yet the eggs did not lose their capacity for Fertilization. Upon penetration by sperm Cells, two of them would fuse to form a zygote, which then developed into individuals with a purely paternal phenotype. W. Sutton drew attention to the striking parallelism in The behavior of Mendelian hereditary factors and chromosomes, and in 1903, he brought these two concepts together. The formulation of the chromosomal theory of heredity belongs to the American geneticist T.H. Morgan and his school. In the mid-1920s, the principles of the linear arrangement of genes in chromosomes and the initial version of the Gene theory were established.

Chromosomes and the genes localized within them are the primary material bearers of heredity.

A chromosome is a self-reproducing element of the Cell Nucleus. The capacity for self-reproduction, which is based on METABOLISM/36.html">DNA Replication and occurs on a template basis according to THE PRINCIPLE OF complementarity, is one of the Fundamental properties of chromosomes. The number of chromosomes in all somatic cells of an Organism is double—diploid. It arises from the fusion of two Germ Cells, each of which contains a single—haploid—set of chromosomes. In the diploid set, chromosomes are represented in pairs. With the exception of sex chromosomes, each chromosome corresponds in shape and size to another—one inherited from the mother and the other from the father; such chromosomes are called homologous.

The main constituents of chromosomes are Nucleic Acids and Proteins. Nucleic acids are of critical importance in transmitting hereditary traits from generation to generation. It is through these properties that the regular inheritance of characteristics occurs. The chromosomal theory represents a further Development of the nuclear theory of heredity, and its validity is confirmed by the most advanced achievements of biochemical and Molecular Genetics.

However, not only the nucleus but also cytoplasmic structures (such as Mitochondria, Plastids, etc.) play an important role in heredity. The DNA contained within these structures harbors genes (plasmagenes) that serve as carriers of cytoplasmic heredity. At the same time, proteins encoded by nuclear genes are synthesized in the cytoplasm with the participation of Ribosomes, ensuring the phenotypic expression of genomic information. Consequently, a close interaction exists between the nucleus and the cytoplasm in hereditary processes.