Cell Cycle Overview

Cell Cycle Overview

• The cell cycle is a series of events preparing a cell to divide into two daughter cells.
• Cell multiplication is crucial for embryonic development and growth/replacement of dead cells.
• The cell cycle is divided into two major events: interphase and mitosis.

Interphase

• A long period of time.
• The cell increases in size.
• The cell replicates its genetic material.
• Subdivided into three phases:
  • G1 phase: synthesis of macromolecules essential for DNA duplication, cell volume restored to normal, nucleoli reestablished, centrioles start to duplicate.
  • S phase: DNA is duplicated, all requisite nucleoproteins (histone) are incorporated into DNA, forming chromatin, the cell now contains twice the normal amount of DNA.
  • G2 phase: cell undergoes preparation for mitosis, the period between DNA synthesis and mitosis, RNA and proteins essential for cell division are synthesized, energy for mitosis is stored, tubulin is synthesized

Mitosis

• A shorter period of time.
• The nucleus divides first, followed by the cytoplasm, creating two daughter cells.
• Cells may permanently cease mitosis (e.g., muscle, nerve cells)
• Cells that leave the cell cycle are said to be in a resting stage (G0 phase).
  • Prophase: Condensing of chromosomes, centrosomes move to opposite poles, nuclear envelope disappears.
  • Metaphase: Chromosomes align at the cell's equator.
  • Anaphase: Sister chromatids separate and move to opposite poles.
  • Telophase: Chromosomes decondense, nuclear envelope reforms, cytokinesis occurs (division of cytoplasm).

Meiosis

• A specialized cell division resulting in the formation of gametes (sperm or egg).
• The chromosome number is reduced from diploid (2n) to haploid (1n).
• Begins at the conclusion of interphase, producing germ cells (ova and sperm).
• Two crucial results of meiosis:
  • Reduction in chromosome number (2n to 1n).
  • Recombination of genes leading to genetic variability and diversity.
  • Meiosis I (reductional division):
    • Separation of homologous chromosome pairs, reducing chromosome number from diploid to haploid.
    • Prophase I: long phase, homologous chromosome pairs approximate each other forming tetrads, crossing over occurs between homologous chromosomes (exchange of genetic material)
    • Metaphase I: homologous chromosome pairs align at the metaphase plate.
    • Anaphase I: Homologous chromosomes separate and move towards opposite poles.
    • Telophase I: two daughter cells are formed each with half the original number of chromosomes
  • Meiosis II (equational division):
    • Similar to mitosis, sister chromatids separate and move to opposite poles.
    • Resulting in four haploid daughter cells, each with unique genetic content.