Fundamental Principles of Cell Biology and Division

Principles of Cell Theory

• The cell is the structural unit of living things.
• The cell is the functional unit of living things.
• Every cell comes from another existing cell.
• The cell is the genetic unit of living things.

Levels of Biological Organization

These correspond to different degrees of complexity in which matter is organized. They are grouped into abiotic and biotic levels.

Classification of Cell Types

• Prokaryotes: These lack a nucleus (coreless).
• Eukaryotes: These contain a nucleus. They can be classified as animal or plant cells.

Essential Cell Organelles

Endoplasmic Reticulum: Composed of flattened sacs and tubular conduits. It includes the smooth reticulum (involved in lipid synthesis) and the rough reticulum (containing ribosomes for protein synthesis).

Golgi Apparatus: Consists of flattened and stacked tanks. it processes substances from the ER and transports them via vesicles.

Ribosomes: Particles made of RNA and proteins. They are responsible for protein synthesis.

Mitochondria: Spherical or elongated organelles. They are the site of cellular respiration and energy production.

Lysosomes: Membrane vesicles originating from the Golgi apparatus. They perform hydrolysis (breaking down complex molecules using enzymes).

Vacuoles: Membranous vesicles that store different products such as water, reserves, or pigments.

Characteristic Organelles of Plant Cells

• Cell Wall: Protects the cell and maintains its regular shape.
• Plasmodesmata: Channels that connect plant cells to each other.
• Chloroplasts: Oval organelles with a double membrane where photosynthesis takes place.
• Large Vacuoles: Typically larger than those found in animal cells.

Characteristic Organelles of Animal Cells

Animal cells contain centrioles and generally have fewer, smaller vacuoles.

The Nucleus

Its appearance differs depending on the current state of the cell:

• When the cell is not dividing (Interphase): It shows the chromatin (DNA-protein complex).
• When the cell divides: Chromosomes are observed.

Chromosomes and Genetic Information

Chromosomes are only visible during the division of the nucleus. Their number is fixed for a given species. The complete set of chromosomes of a species is known as the karyotype.

Humans have:

• 2 heterochromosomes (sex chromosomes) involved in sex determination: XY in males and XX in females.
• 44 autosomes.

Somatic cells have a diploid chromosome number (2n), while gametes have a haploid number (n).

The Cell Cycle

The cell cycle comprises two main phases:

• Interphase: The non-division phase. The cell grows and the DNA is duplicated.
• Division Phase (M): This includes the division of the nucleus (mitosis) and cytoplasmic division (cytokinesis).

Mitosis is further divided into four stages: prophase, metaphase, anaphase, and telophase.

Meiosis and Gamete Formation

Meiosis comprises two successive divisions:

• Meiosis I: An exchange of genetic information occurs between homologous chromosomes. At the end, two haploid cells originate.
• Meiosis II: This results in the development of four haploid daughter cells called gametes.

This entire process of gamete production is known as Gametogenesis.