Lecture on Meiosis by Amoeba Sisters

Introduction to Meiosis

Meiosis is a process contributing to genetic variety.
Unlike mitosis, meiosis creates gametes (sperm and egg cells) rather than body cells.
Humans typically have 46 chromosomes but sperm and egg cells contain 23 chromosomes each.
When a sperm and egg cell combine, they form a zygote with 46 chromosomes.
Meiosis is referred to as a "reduction division" because it reduces chromosome number from 46 to 23.

Interphase occurs before meiosis, as it does before mitosis.
During interphase:
- The cell grows.
- DNA is replicated.
- Cell processes are carried out.
Chromosomes duplicate from 46 to 92 chromatids, but are counted as 46 chromosomes due to centromeres.

Prophase I:
- Chromosomes condense, thicken, and align with homologous pairs.
- Crossing over occurs, where chromosomes exchange genetic information, leading to genetic variation.
Metaphase I:
- Chromosomes align in pairs in the middle of the cell.
Anaphase I:
- Homologous chromosomes are pulled away to opposite poles by spindle fibers.
Telophase I and Cytokinesis:
- Formation of two new nuclei and two new cells.
- Cytoplasm divides, completing meiosis I.

Prophase II:
- Similar to prophase I but without homologous pairs or crossing over.
Metaphase II:
- Chromosomes align in a single file line in the center of the cell.
Anaphase II:
- Chromatids are pulled apart by spindle fibers to opposite poles.
Telophase II and Cytokinesis:
- Nuclei reform in each of the two cells.
- Each cell divides, resulting in four non-identical gametes.

Independent assortment and crossing over ensure genetic variety in gametes.
Resulting sperm cells (in males) and egg cells (in females) from meiosis are genetically unique.
This diversity explains physical and genetic differences between siblings with the same parents.

Nondisjunction is an error in meiosis where chromosomes don't separate properly, leading to genetic disorders.
It's an area of ongoing scientific research.