YT

• An adult human contains an estimated 100 trillion cells
• We start life as a single cell and then grow, develop, and repair tissue damage by relying on cell division in eukaryotic cells
• Cell division in eukaryotic cells

  Series of well orchestrated steps called mitosis
• Our bodies must produce millions of skin cells every day to replace those lost through normal activity
• Each of these cells must have a complete complement of the genetic material prior to cell division
• Cell division

  1. DNA replication

  2. Chromosomes condense in the nucleus

  3. Chromosomes coil up further shortening and condensing

  4. Replicated chromosomes are called sister chromatids
• Interphase


  Part of the entire cell cycle to prepare for cell division, can be divided into 3 distinct phases: G1, S, and G2
• Interphase

  1. G1 phase: organelles and cytoplasmic components replicate

  2. S phase: DNA replicates

  3. G2 phase: enzymes needed for cell division are produced
• Most eukaryotic cells spend a great deal of time in interphase and a very short period of time actually dividing, a process called mitosis
• Mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase

  • Chromosomes condense and become visible as two sister chromatids held together at the centromere
  • Cytoskeleton disassembles as the spindle begins to form
  • In animal cells, centrioles migrate to opposite poles establishing a spindle apparatus
  • Nuclear envelope breaks down
• Metaphase

  • All chromosomes are aligned at the equator of the cell called the metaphase plate
• Anaphase


  • Proteins that hold sister chromatids together degrade, freeing individual chromosomes
  • Free chromosomes are pulled by their kinetochores to opposite poles
• Telophase

  • Cleavage furrow forms in the center of the cell
  • Chromosomes cluster at opposite poles and begin decondensing
  • Nuclear envelope reforms
  • Spindle apparatus disassembles
• Cytokinesis

  1. In animal cells: cleavage furrow extends to completely separate daughter cells

  2. In plant cells: vesicles form an expanding membrane partition called the cell plate
• Cell cycle checkpoints

  • G1/S checkpoint: primary point where cell cycle continues or stops
  • G2/M checkpoint: allows cells that have completed interphase to begin mitosis
  • Spindle checkpoint: ensures all chromosomes have attached to the spindle
• Growth factors, cell size, and nutritional state are contributing factors in cell cycle regulation
• The entire process of cell division can take on average 10 to 20 hours in a typical plant or animal cell
• Skin cells have a high turnover rate and go through mitosis very frequently, while other cells such as adult neurons and muscle cells rarely divide
• The accuracy of mitosis and the consistency of the checkpoints during interphase ensure that most cells in a eukaryotic organism can produce identical copies of themselves
• This process allows for growth and repair to prolong overall physiology as well as life itself