MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

SECTION 1. BIOLOGICAL BASES OF HUMAN LIFE ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.3. Biology of Individual Development

Genetic control of Development

The GENERAL PATTERNS OF ontogeny in Multicellular Organisms are explained by the hypothesis of differential Gene activity. At various Selection/3.html">Stages of development, different structural genes are active, driving cellular differentiation. The core mechanism of Cell Differentiation boils down to the selective blocking and unblocking (inactivation and activation) of individual genes or gene groups. This phenomenon is clearly observable in the giant polytene Chromosomes of Drosophila larvae and other dipterans. Active regions of such chromosomes—puffs—shift along the chromosome length depending on the developmental stage, and their number varies. These regions are sites of active mRNA synthesis, corresponding to genes that switch on at specific stages of The life cycle (Fig. 1.87).

Out of the 40,000 genes in a sea urchin, 30,000 are active at the blastula stage, 12,000–15,000 at the gastrula and larval stages, and 3,000–5,000 in adult animals. In adult human Cells, approximately 90% of genes are "silent". Many of these have fulfilled their function at various Stages of Ontogeny and remain in a blocked state. Genes are not lost during the differentiation of somatic cells; specific gene groups merely lose their activity. Genes that have lost their activity during cell specialization can become active again following nuclear transplantation into an egg cell, as demonstrated by the experiments of the English embryologist J. Gurdon.