BIOLOGY Lecture Notes - Golden Pages 2003

4. REPRODUCTION

Reproduction is the ability of All living organisms to produce a new generation similar to them in most characteristics. The Molecular Basis of all types of reproduction is The process of Replication (duplication of DNA molecules). This ensures the duplication of genetic material, which is evenly distributed into daughter Cells during division. The genetic material brings into The Cell the program for the individual development of that given cell or Organism. The cytological basis is Cell Division, which is grounded in mitosis or Meiosis. The modes of reproduction in Living organisms are highly diverse. Based on the presence or absence of Gametes, reproduction is of two types: 1 — asexual, 2 — sexual.

Asexual reproduction is a type of reproduction that does not involve the formation and fusion of gametes.

Characteristics of asexual reproduction:

a) only one parent is involved in the reproduction process;

b) no formation and fusion of gametes take place;

c) reproduction is based exclusively on mitotic division;

d) new individuals can develop from a somatic part of the parent organism (Vegetative Reproduction);

e) the newly produced individuals are genetically identical;

f) the process ensures a rapid increase in the number of individuals;

g) the unit of reproduction can be the entire parent body, a body part, or a single somatic cell;

h) it is characteristic of lower organisms.

Types of asexual reproduction. A distinction is made between reproduction based on the division of a single cell into two, reproduction by a group of cells — spore formation in Fungi, mosses, and ferns — and reproduction by a part of the organism (vegetative propagation in plants and somatic Embryogenesis in Flatworms and Annelids).

Sexual reproduction is a type of reproduction that involves the formation and fusion of sex cells called gametes. The main processes involved in sexual reproduction that maintain a constant chromosome number from generation to generation are gametogenesis and Fertilization.

Characteristics of sexual reproduction:

a) usually two parents (male and female) participate in the process, with the exception of hermaphrodites;

b) characterized by the formation and fusion of gametes;

c) includes meiosis as the mode of division of gametogonia during gametogenesis, and mitosis during zygote development;

d) characterized by high genetic Variability: offspring differ genetically from their parents because variations arise due to new Gene combinations during chromosome Crossing Over, random chromosome assortment, and random gamete fusion;

e) units of reproduction are haploid gametes;

f) characteristic of highly organized organisms;

g) The rate of reproduction is low.

Biological Significance of reproduction:

1. Every species of living organisms consists of individuals, each of which has a limited lifespan. Only the ability of individuals to reproduce ensures the long-term (millions of years) existence of a given species.

2. Any multicellular organism consists of A large number of cells that vary in Structure and function. The lifespan of virtually all cells is shorter than that of the organism itself. Only the constant renewal of cells, supported by their division, ensures the long-term survival of individuals.

Cell types depending on their purpose and ability to divide:

1. Somatic cells, which form the entire variety of Tissues and corresponding Organs and body parts (up to 200 distinct types).

The Nucleus of such cells divides solely by mitosis! During division, a diploid set of Chromosomes (46 in humans) is formed from the Chromatin of the interphase nucleus.

All daughter cells are genetically identical and possess 46 chromosomes.

The purpose of somatic cells is to ensure survival and The formation of Germ Cells.

2. Primary germ cells (spermatogonia, oogonia), which form during embryogenesis and colonize the Gonads within the epithelium of the sex glands. Only from these cells can mature germ cells develop. They are capable of dividing mitotically (the multiplication phase of gametogenesis) and meiotically (the gamete maturation phase). Gametogonia possess a diploid set of chromosomes (46). Purpose: formation of gametes.

3. Mature germ cells—gametes—formed As a result of the meiotic division of gametogonia. Spermatozoa and egg cells possess a haploid set of chromosomes—23—and are capable of division. Purpose: to fuse with a gamete of the opposite sex and form a zygote.

Oogenesis is the complex of processes resulting in the formation of mature haploid egg cells from diploid reproductive cells (oogonia) in the Ovaries of the female body.

It consists of 3 phases:

1. Multiplication phase. This takes place during the period of intrauterine development. The primary germ Cells of the ovarian germinal epithelium undergo rapid mitotic division to form groups of diploid maternal egg cells—oogonia. Their genetic formula corresponds to 2n2c prior to the S phase and 2n4c after it, where n represents the haploid set of chromosomes and c represents The amount of DNA.

2. Growth phase. During this time, a single oogonium grows and transforms into a diploid primary oocyte, while the surrounding oogonia form a nutritive follicular epithelium around it. This structure is called the primary follicle. The following processes take place during the growth phase:

1) an increase in the size of the oogonia (approximately 200-fold in humans) as a result of yolk accumulation (vitellogenesis);

2) a significant increase in the size of the nucleus;

3) the formation of a thin vitelline membrane around the oocyte;

4) an increase in the number of Mitochondria, ER components, and the Golgi apparatus;

5) METABOLISM/36.html">DNA replication occurs. The genetic formula of the primary oocyte corresponds to 2n4c.

3. Maturation phase. Characterized by meiosis. During this period, the diploid and fully grown primary oocyte undergoes meiosis I (reductional division) to form two unequal haploid cells. The smaller cell is called the first polar body and contains a small amount of Cytoplasm. The larger cell is called the secondary oocyte. Both are haploid, and each possesses half the chromosome set. The genetic formula of secondary oocytes corresponds to 1n2c. The secondary oocyte then undergoes meiosis II (equational division), once again forming two unequal haploid cells. The smaller one is called the secondary polar body and contains very little cytoplasm, while the larger one differentiates into an ovum (egg cell). Simultaneously, the first polar body may also divide into two. The genetic formula of the egg cell and polar bodies is 1n1c. In most mammals, ovulation occurs at the secondary oocyte stage after the completion of the first meiotic division. In humans, ovulation occurs when the egg cell is at the metaphase stage of the second meiotic division. Meiosis II can only be completed after fertilization.

Spermatogenesis is the process of forming haploid microscopic male gametes (spermatozoa) from diploid reproductive cells (spermatogonia) present in the Testes. Spermatogenesis in males occurs over many years following Puberty. Spermatogenesis is divided into 2 stages: formation of spermatids; spermiogenesis.

The formation of spermatids is divided into 3 phases:

1. Multiplication phase: rapid mitotic division of germ cells (primary sex cells) present in the germinal epithelium of the seminiferous tubules of the testes. This process ensures the formation of spermatogonia.

2. Growth phase: characterized by spermatocytogenesis, during which the diploid spermatogonium increases in size (approximately 2-fold) and forms a diploid primary spermatocyte.

3. Maturation phase: characterized by meiosis. The primary spermatocyte undergoes meiosis (reductional division) and forms two identical haploid cells called secondary spermatocytes. Then, meiosis II (equational division) occurs immediately, and from each secondary spermatocyte, 2 equivalent haploid spermatids are formed.

Spermiogenesis is the transformation of non-motile, rounded spermatids into functional and motile spermatozoa. It includes the following changes:

a) the nucleus becomes condensed, small, and localizes in the anterior part of the cell;

b) the Golgi complex forms the acrosome;

c) the spermiid centrioles form the neck of the spermatozoon;

d) the distal centriole initiates the growth of microtubules that form the axial filament of the sperm tail (axoneme);

e) mitochondria form spiral rings around the proximal part of the axoneme;

f) most of the cytoplasm is discarded, but a small amount remains surrounding the neck and tail of the spermatozoon;

g) mature spermatozoa are released into the lumen of the seminiferous tubules.

The two testes of a young man produce approximately 120 million spermatozoa per day. There are two types of sperm: 50% gynospermia, carrying the X chromosome, and 50% androspermia, carrying the Y chromosome. Spermatogenesis is a prolonged process that takes approximately 74 days to complete in humans.