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

BIOLOGY

SECTION 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.1. Reproduction

Sexual Reproduction

Sexual reproduction occurs with the participation of sex Cells (Gametes). It is widespread across all types of the PLANT AND ANIMAL kingdoms. Many species reproduce exclusively through sexual means, whereas others exhibit an alternation of sexual and asexual reproduction. The universality of sexual reproduction is attributed to the generation of genetically diverse offspring, which are better adapted to changing environmental conditions than the genetically uniform progeny resulting from asexual reproduction. This genetic diversity is achieved through the involvement of two parent organisms in sexual reproduction. The sexual process emerged early in evolution in the form of conjugation and copulation.

Conjugation is a form of the Sexual process in which two organisms temporarily make physical contact to exchange genetic material. It is found in Bacteria and Ciliates. The transfer of genetic material between donor and recipient bacteria occurs through specialized outgrowths (sex pili). Conjugation transmits the multiple drug resistance (R) factor, which confers bacterial resistance to numerous Antibiotics. In Paramecium caudatum (slipper animalcules), for instance, individuals connect via their preoral depressions during conjugation. The macronucleus (large Nucleus) of each Cell dissolves, while the micronucleus (small nucleus) divides twice to form four nuclei. Three of these degenerate, and the fourth undergoes mitosis to produce two nuclei: a stationary one and a migratory one. The cells then exchange migratory nuclei. Within each cell, its own stationary nucleus and the foreign migratory nucleus fuse to form a synkaryon. The synkaryon divides to reconstitute the macronucleus and micronucleus. Following conjugation, the cells separate and resume reproduction by fission.

Copulation is a sexual process in unicellular organisms where two individuals acquire sexual differentiation—transforming into gametes—and completely fuse to form a zygote. Forms of copulation include isogamy, anisogamy, and oogamy. Isogamy involves the fusion of two morphologically identical, yet physiologically distinct gametes. Anisogamy is the fusion of two dissimilar gametes: one larger female gamete (macrogamete) and one smaller male gamete (microgamete), both of which are motile. Oogamy is the fusion of two distinct gametes: a large, non-motile egg cell and a small, motile spermatozoon. In Multicellular Organisms, sexual reproduction relies exclusively on oogamy.

The Development of gametes in multicellular organisms takes place in the Gonads (from the Greek gone meaning seed). There are two types of sex cells: male (spermatozoa) and female (egg cells or ova). Spermatozoa develop in the Testes, while ova develop in the Ovaries. As a rule, eggs and sperm are produced by separate organisms—female and male, or females and males—which differ from one another in A number of traits, a phenomenon known as Sexual Dimorphism. The traits distinguishing one sex from the other are categorized into Primary and secondary. Primary sex characteristics include the gonads, whereas secondary characteristics encompass all other manifestations of sexual dimorphism.

When both male and female sex cells develop within a single individual, the Organism is referred to as a hermaphrodite. Hermaphroditism is characteristic of many lower-level animals in the organic world's evolutionary scale, such as Flatworms, Annelids, and Mollusks. As a pathological condition, true hermaphroditism occurs in humans. It predominantly arises from Embryogenesis disorders despite a normal set of sex Chromosomes (XX or XY) across all somatic cells. Cases of sex-chromosome mosaicism have also been documented, wherein some somatic cells possess an XX chromosome Complement while others carry an XY complement.