Mitosis and Meiosis Overview

Understanding the Cell Cycle and Mitosis

• Cell Division Purpose: To ensure each new cell (daughter cell) has a complete set of chromosomes after a parent cell divides.

• S Phase: This phase of the cell cycle is where DNA replication occurs and chromosomes are duplicated.

• Mitosis: The process of dividing the replicated chromosomes into two daughter cells. Key phases include:

  • Prophase: Chromosomes condense and become visible; the nuclear membrane disintegrates; spindle fibers begin to form.
  • Metaphase: Chromosomes line up at the cell's equator and are attached to the spindle fibers.
  • Anaphase: Sister chromatids are pulled apart to opposite ends of the cell.
  • Telophase: Chromatids reach the opposite poles; the nuclear membrane begins to reform, resulting in the formation of two nuclei.

• Cytokinesis: Not part of mitosis itself but is the physical division of the cytoplasm, creating two distinct daughter cells.

Chromosome Structure and Terminology

• Diploid (2n): Cells that contain homologous pairs of chromosomes; in humans, diploid number is 46 (23 pairs).

• Haploid (n): Cells that contain a single set of chromosomes; in humans, this number is 23 (sperm and egg cells).

• Homologous Chromosomes: Chromosome pairs, one from each parent, that are similar in shape, size, and genetic content.

• Sister Chromatids: Two identical copies of a chromosome, connected at a region called the centromere.

Key Points on Mitosis

• Mitosis Always Results in:

  • Two identical daughter cells, each with the same chromosomal content as the parent cell.

• Example of Chromosome Number: A cell with 56 chromosomes before mitosis will produce two daughter cells, each with 56 chromosomes.

Transition from Mitosis to Meiosis

• Meiosis Purpose: To produce gametes (sperm and eggs) for sexual reproduction, where the chromosome number is halved.

• Meiosis Steps: Includes two rounds of division:

  • Meiosis I: Separates homologous chromosomes (reduction division), resulting in two haploid cells.
  • Meiosis II: Functions like mitosis, where the sister chromatids separate.

• Genetic Diversity: Achieved through processes such as independent assortment and crossing over during Meiosis I.

Independent Assortment and Crossing Over

• Independent Assortment: The random orientation of homologous chromosome pairs during metaphase I leads to genetic variation.
• Crossing Over: The exchange of genetic material between homologous chromosomes during prophase I can create new allele combinations.

Example: Human Chromosome Numbers in Reproduction

• Human Gametes: Each human gamete carries 23 chromosomes (haploid).
• Fertilization: When a sperm (23 chromosomes) fertilizes an egg (23 chromosomes), the resulting zygote is diploid with 46 chromosomes (2n = 46).

Conclusion

• Both mitosis and meiosis are essential for life - mitosis for growth and repair, and meiosis for sexual reproduction.
• Understanding these processes and their implications is crucial for recognizing how genetic material is passed on and how diversity is achieved in populations.