Cellular Processes: Osmosis, Mitosis, and Meiosis

Osmosis and Passive Transport

Osmosis is the movement of particles from a high concentration to a low concentration. Specifically, it is the movement of a solvent, such as distilled water, through a semipermeable membrane. This process occurs when two solutions with different concentrations of solutes are present in the same recipient.

Mitosis: Somatic Cell Division

Mitosis creates identical cells through cell division. This process is essential for growth and produces body cells, also known as somatic cells.

Phases of Mitosis

• Prophase: Chromosomes become visible and begin condensing.
• Metaphase: The nuclear membrane breaks down, and the chromosomes line up in the middle of the cell.
• Anaphase: The chromatids separate and move toward opposite sides of the cell, heading toward the centrioles.
• Telophase: Chromosomes are separated on opposite sides. A nucleus forms on each side, eventually leading to the formation of new cells.
• Cytokinesis: This stage finishes dividing the cytoplasm to complete the division into two distinct cells.

Meiosis: Genetic Variety and Gametes

Meiosis contributes to genetic variety. Unlike mitosis, it does not produce body cells; instead, it creates sperm and egg cells, known as gametes. While humans have 46 chromosomes in their somatic cells, sperm and egg cells contain only 23 chromosomes. When these haploid cells combine, the resulting baby will have the full 46 chromosomes.

The Interphase Stage

Before meiosis begins, the cell enters interphase. During this stage, the cell grows, replicates its DNA, and carries out essential cell processes. When DNA replicates, the cell starts with 46 chromatids; after duplication, there are 46 chromosomes consisting of 92 chromatids. We still count them as 46 chromosomes because we count the centromeres.

The Two Stages of Meiosis

In meiosis, the PMAT (Prophase, Metaphase, Anaphase, Telophase) cycle occurs twice. Phases are numbered to distinguish between the first and second divisions.

Meiosis I

• Prophase 1: Chromosomes condense, thicken, and line up with their homologous pairs. Crossing over occurs, where they transfer and exchange genetic information.
• Metaphase 1: Chromosomes line up in pairs in the middle of the cell rather than in a single line.
• Anaphase 1: Chromosomes are pulled away by the spindle fibers.
• Telophase 1: Meiosis I ends with two new cells, followed by cytokinesis to split the cytoplasm.

Meiosis II

• Prophase 2: This phase is similar to Prophase 1, but without the crossing over process.
• Metaphase 2: This time, the chromosomes align in a single line in the middle of the cell.
• Anaphase 2: Chromatids are pulled apart by the spindle fibers.
• Telophase 2: Nuclei reform as the two cells divide into four. Cytokinesis finally splits the entire cytoplasm to complete the process.