Mitosis is the process of cell division that produces identical daughter cells from a parent cell. It is the primary mechanism by which organisms grow.

The Cell Cycle and Mitosis

The cell cycle consists of interphase (where the cell grows and DNA replicates) and the mitotic phase (M-phase), which includes:

• Prophase: Chromosomes condense and become visible; the nuclear membrane breaks down
• Metaphase: Chromosomes line up at the cell equator
• Anaphase: Sister chromatids separate and move to opposite poles
• Telophase: Nuclear membranes reform around the two new nuclei
• Cytokinesis: The cytoplasm divides, forming two separate cells

How Mitosis Leads to Growth

During growth, cells undergo repeated mitosis, producing more cells. These cells then enlarge and differentiate to form tissues and organs. In mammals, mitosis occurs rapidly in embryonic stages and continues in specific tissues (bone marrow, skin, intestinal lining) throughout life. In plants, mitosis occurs in meristematic tissues (root tips, shoot apical meristem) throughout the plant's life.

Human growth occurs in two main periods: pre-natal (in the womb) and post-natal (after birth).

Post-Natal Growth Stages

i. Infancy (Birth to 2 years)

• The neonatal stage (0-5 months): Babies develop basic reflexes, can see about 20cm, and rely on breastfeeding for immunity
• Older baby stage (6-12 months): Babies learn to sit, crawl, and begin teething
• Toddler stage (1-3 years): Brain reaches 90% development, milk teeth appear, and children begin walking independently

ii. Childhood (3 to 11 years)

• Early childhood (3-6 years): Rapid growth, development of motor skills, curiosity increases
• Late childhood (7-9 years): Children become active, take on responsibilities, and form peer groups
• Pre-adolescence (10-11 years): Growth accelerates, appetite increases, and secondary sexual characteristics begin to emerge

iii. Adolescence (12 to 18 years)

This is the period when sexual maturity is reached. Puberty involves hormonal changes that trigger secondary sexual characteristics:

• In boys: Broad shoulders, muscle growth, beard growth, deepening voice, development of sex organs
• In girls: Breast development, widening hips, growth of pubic hair, onset of menstruation

iv. Adulthood

Body reaches maximum size and functioning capacity. Cells continue to divide for tissue repair but overall growth stops.

v. Old Age (Senescence)

Gradual decline in body functions, reduced cell division, and eventual death.

Factors Affecting Human Growth

• Nutrition: Adequate proteins, vitamins, and minerals are essential
• Hormones: Growth hormone, thyroid hormone, and sex hormones regulate growth
• Genetics: Determine potential growth potential
• Health status: Illnesses can impair growth

Seed Germination

Seed germination is the process by which a seed develops into a new plant. It involves the following stages:

1. Imbibition: The seed absorbs water, swelling and activating enzymes
2. Activation: Enzymes become active and begin breaking down stored food in the cotyledons
3. Respiration: The embryo uses released energy for growth
4. Radicle emergence: The root (radicle) emerges first, anchoring the seed
5. Shoot emergence: The shoot (plumule) grows upward, developing leaves

Types of Seed Germination

• Epigeal germination: Cotyledons are pushed above the soil (e.g., beans, sunflower). The hypocotyl elongates, pulling the cotyledons above ground.
• Hypogeal germination: Cotyledons remain below the soil (e.g., maize, rice). The epicotyl elongates, pushing the shoot above ground.

Factors Affecting Plant Growth

1. Water: Essential for seed germination and transporting nutrients
2. Oxygen: Required for respiration to release energy
3. Temperature: Affects enzyme activity; most plants grow best at 20-30°C
4. Light: Needed for photosynthesis after leaves develop
5. Minerals: Macronutrients (N, P, K, Ca, Mg) and micronutrients (Fe, Zn, Cu) support growth

Plant Growth Regions

Plants grow in specific regions called meristems:

• Root apical meristem: Produces new root cells
• Shoot apical meristem: Produces new shoot cells
• Lateral meristem: Responsible for secondary growth (thickness)

Understanding growth mechanisms is important in Tanzanian daily life. For example, when cultivating crops like beans or maize, farmers apply this knowledge by ensuring proper planting depth (to support hypogeal or epigeal germination), adequate watering, and appropriate timing of fertilizer application based on the plant's growth stages. In human health, knowing the stages of child development helps parents in Tanzania recognize when their children need additional nutrition during growth spurts, particularly during adolescence when rapid physical changes require increased protein and mineral intake.