Nucleus and Cell Cycle: M.Sc. in Basic Sciences

Nucleus

• The control center of the cell that controls metabolism and reproduction
• Contains DNA (genetic material) found on chromosomes
• The largest organelle, dark and round, surrounded by a double membrane called the nuclear envelope
• Nuclear pore is a large complex of proteins that allows small molecules and ions to freely pass, or diffuse, into or out of the nucleus
• Main functions include:
  • Protein synthesis, cell division, and differentiation
  • Controlling the synthesis of enzymes involved in cellular metabolism
  • Controlling hereditary traits of the organism

Cell Cycle

• A self-regulated sequence of events that controls cell growth and cell division
• The goal of the cell cycle is to produce two daughter cells, each containing chromosomes identical to those of the parent cell
• Divided into two main types:
  • Mitosis (Asexual reproduction cycle): Process in which the nucleus of a cell divides to create two new nuclei, each containing an identical copy of DNA (somatic cells)
  • Meiosis (Sexual reproduction cycle): Process by which certain sex cells are created

Phases of the Cell Cycle

• Interphase:
  • The cell grows at a steady rate throughout interphase
  • It is the longest part of the cell cycle involving three sub-phases:
    • G1 (gap1) phase: During which the cell is metabolically active and continuously grows but does not replicate its DNA
    • S (synthesis) phase: During which DNA replication takes place
    • G2 (gap 2) phase: During which cell growth continues and proteins are synthesized in preparation for mitosis
• M phase (Mitosis):
  • The process in which the nucleus of a cell divides to create two new nuclei
  • Includes both karyokinesis (division of the nucleus) and cytokinesis (division of the cell)

Cell Cycle Checkpoints

• The restriction checkpoint (or “point of no return”) is the most important checkpoint in the cell cycle
• At this checkpoint, the cell self-evaluates its own replicative potential before deciding to either enter the S phase and the next round of cell division or to retire and leave the cell cycle
• Malfunction of any of the checkpoints at the G1, S, and G2 phases of the cell cycle and the checkpoint at M phase may lead to a mitotic catastrophe

Mitosis

• Lasts about 1 hour
• Phases of Mitosis:
  • Prophase: The replicated chromatin condenses and becomes visible as chromosomes
  • Metaphase: Each chromatid is attached to a spindle fiber, centromeres of duplicated chromosomes are aligned at the equator
  • Anaphase: The centromeres divide, and the sister chromatids of each chromosome are pulled apart and move to the opposite ends of the cell, pulled by spindle fibers attached to the kinetochore regions
  • Telophase: The chromosomes become indistinct chromatin again, the spindle disappears, and the nuclear envelope and nucleolus reappear, daughter cells are forming

Cytokinesis

• The process by which the cytoplasm of the cell splits to form two independent cells
• The separation at the cleavage furrow is achieved by a contractile ring consisting of a very thin array of actin filaments positioned around the perimeter of the cell
• Many granular matrix formed by the Golgi body and endoplasmic reticulum accumulates in the equatorial region, forming a cell plate