Cell Division: Mitosis, Meiosis, and Hormonal Regulation

Mitosis

Interphase: This is the state in which the cell is when it is preparing for division. During this time, the cell duplicates its genetic material, grows, and prepares the structures and proteins needed to carry out mitosis.

Prophase: This is the first phase of mitosis. During this phase, the cell's centrioles duplicate, and each pair moves to opposite poles of the cell. The nuclear membrane disintegrates. Chromosomes condense and become visible as double structures.

Metaphase: This is the second phase of mitosis. During this phase, the chromosomes align along the equatorial plane of the cell. The cytoplasmic spindle appears.

Anaphase: This is the third phase of mitosis. The chromatids separate and are pulled towards the spindle poles by the cytoplasm.

Telophase: This is the last phase of mitosis. Chromosomes reach the poles. The nuclear membrane forms around the chromosomes. Chromosomes decondense and can no longer be distinguished individually. The cell begins to show an indentation in the cell membrane, indicating its imminent division.

Cytokinesis: It is during this period that the cytoplasmic material of the cell is divided equally between the two daughter cells. The cell membrane divides, resulting in two new genetically identical cells, each with half of the cytoplasmic material of the parent cell.

Meiosis

Prophase I: The nuclear membrane disappears. One kinetochore forms per chromosome, not one per chromatid, and the chromosomes attached to spindle fibers begin to move. The sister chromatids remain closely aligned throughout their length, but the homologous chromosomes are no longer attached, and centromeres and kinetochores are separated.

Metaphase I: The homologous chromosomes are aligned in the equatorial plane. The orientation is random, with each paternal counterpart on one side. This means that there is a 50% chance that the daughter cells receive the paternal or maternal counterpart for each chromosome. The spindle microtubules from each centriole attach to their respective kinetochores.

Anaphase I: The chiasmata separate. The spindle microtubules shorten in the region of the kinetochore, pulling the homologous chromosomes to opposite sides of the cell, with the help of motor proteins. Since each homologous chromosome has only one kinetochore, a haploid set (n) forms on each side. In the distribution of homologous chromosomes, for each pair, the maternal chromosome goes to one pole and the paternal to the other. Thus, the number of maternal and paternal chromosomes at each pole varies randomly in each meiosis. For example, in the case of a species 2n = 4, one pole may have two maternal chromosomes and the other two paternal, or each pole may have one maternal and one paternal chromosome.

Telophase I: Each daughter cell now has half the number of chromosomes, but each chromosome consists of a pair of chromatids. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid system. The chromosomes uncoil back into chromatin. Cytokinesis occurs (a parallel process that separates the cell membrane in animal cells or forms a new cell wall in plant cells, ending with the creation of two daughter cells). Interkinesis often follows, resembling a second interphase, but it is not a true interphase, and no DNA replication occurs.

Hormones

• Transported by the circulatory system.
• Affect target cells, where they can exert their function. Target cells have specific receptors, found in the membrane or nucleus.
• Hormone release is regulated through feedback.

Gland Secretions

• The hypothalamus controls the pituitary gland. This is a part of the brain that contains nerve cells (neurotransmitters).
• Synthesized hormones are peptide molecules.
• Pituitary gland: anterior and posterior. The hypothalamus controls hormone release on both sides.
• Hypothalamic hormones control the pituitary gland: releasing or inhibiting hormones.
• The anterior pituitary produces and releases various hormones, which regulate the hormone production of other endocrine glands.
• FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone) stimulate the production of gametes.
• TSH (Thyroid-Stimulating Hormone) stimulates the thyroid gland to release its hormones.
• ACTH (Adrenocorticotropic Hormone) causes the release of hormones in the adrenal cortex.
• The posterior pituitary releases hormones produced by the hypothalamus.
• Two peptide hormones are synthesized in the hypothalamus and are released into the posterior pituitary: ADH (Antidiuretic Hormone) and oxytocin.
• Oxytocin stimulates milk production by causing muscle tissue to contract during infancy, and it also causes uterine muscle contractions during childbirth, helping to expel the fetus from the uterus.

Thyroid and Parathyroid Glands: Influence on Metabolism and Calcium Levels

• Thyroid: Produces two hormones:
  • Thyroxine: Thyroid hormone, contains iodine, increases the metabolic rate of most body cells.
  • Calcitonin