Cell Structures, Mitosis, and Meiosis

Cell Structures and Their Functions

Mitochondria: These have a double membrane. The outer membrane is flat, and the inner is folded, forming cristae. Cellular respiration takes place inside.

Ribosomes: Made up of two subunits, ribosomes synthesize cell proteins.

Golgi Apparatus: Stacks of flattened membrane sacs called cisternae, which form vesicles. It processes and packages substances.

Endoplasmic Reticulum: A network of flattened membrane sacs and tubes. It transports proteins and sends them to the Golgi apparatus.

Lysosomes and Other Vesicles: Lysosomes are vesicles that carry digestive enzymes and carry out cellular digestion. Other vesicles store or transport substances.

Vacuoles: Large vacuoles full of sap (a liquid with water, mineral salts, sugars, and proteins) are characteristic of plant cells.

Chloroplasts: These green organelles perform photosynthesis.

Centrioles: Two hollow cylinders found only in animal and protozoa cells. They control the movement of the chromosomes during mitosis and meiosis and take part in the formation of cilia and flagella.

Cilia and Flagella: Extensions of the plasma membrane. Short and numerous filaments are cilia; long, singular filaments are flagella. They help the cell to move.

Cell Wall: Surrounds the cell membrane. It protects and gives rigidity to the cells.

Phases of Mitosis (Division of the Nucleus)

Prophase: The nuclear membrane and the nucleolus break down. Centrioles separate and move to opposite poles of the cell, filaments grow between them, and form the mitotic spindle.

Metaphase: Chromosomes reach maximum condensation. They arrange themselves in the center of the cell, perpendicular to the filaments of the mitotic spindle. This line is known as the equatorial plate.

Anaphase: The sister chromatids of each chromosome separate as they are pulled apart by the filaments of the spindle. Each chromatid moves to one end of the cell.

Telophase: Daughter chromosomes complete their migration to the two ends of the cell. Once there, they decondense and change back into the bundle of chromatin. The fibers of the spindle disappear, and the two nucleoli and nuclear membranes are made from the fragments left over from prophase. At the end of the phase, the cell has two nuclei.

First Meiotic Division

Prophase I: The fibers of chromatin, which were duplicated in interphase, condense, and the chromosomes become visible. Each one is composed of two chromatids. Homologous chromosomes pair up (synapsis) and exchange fragments (crossover), which will mean that the daughter cells are genetically different from the mother cell.

Metaphase I: The pairs of homologous chromosomes align along the equatorial plane.

Anaphase I: The fibers of the meiotic spindle become shorter and separate the homologous chromosomes. Each homologous chromosome migrates to a different end of the cell.

Telophase I: In this phase, the homologous chromosomes decondense, and the nuclear membrane is formed. The meiotic spindle disappears. Finally, the cytoplasm divides, and two cells form, which have half the number of chromosomes as the initial cell.

Second Meiotic Division

Prophase II: Without going through interphase, the spindle is formed again, the nuclear membrane breaks down, and the chromosomes condense again.

Metaphase II: Chromosomes align along the middle of the cell and form the equatorial plate.

Anaphase II: The two chromatids of each chromosome separate, pulled apart by the fibers of the spindle. Each chromatid moves towards one end of the cell.

Telophase II: The chromosomes decondense, and the chromatin is formed. The nuclear membrane is created, and the spindle disappears.

Cytokinesis: Finally, the division of the cytoplasm takes place, and the organelles are distributed between the daughter cells. The end result is that four n daughter cells are formed.