MODULE 4

• Main parts of the cell cycle

  • Interphase
  • M phase
• Interphase phases


  • Gap phase 1 (G1)
  • Synthesis (S)
  • Gap phase 2 (G2)
• M phase components


  • Mitosis (nuclear division)
  • Cytokinesis (cell division)
• Proteins that regulate DNA synthesis, mitotic entry, and mitotic exit are conserved throughout eukaryotic evolution
• In unicellular organisms, division of one cell reproduces the entire organism
• Functions of cell division in multicellular organisms

  • Development from a fertilized cell
  • Growth
  • Repair
• Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division
• DNA (deoxyribonucleic acid) is the hereditary material in humans and almost all other organisms
• Gene


  Basic physical and functional unit of heredity; made up of DNA
• Chromosome


  Made up of DNA tightly coiled many times around proteins (histones) that support its structure
• BRCA1 & BRCA2


  • Genes that produce proteins helping repair damaged DNA
  • Two copies inherited from each parent
  • Sometimes called tumor suppressor genes
  • Harmful variants (mutations) can lead to cancer
  • 5592 nucleotides
• Most of the cell cycle is spent in interphase where the cell grows and engages in diverse metabolic activities
• Nerve and muscle cells do not complete the cell cycle and remain in the G0 stage
• Interphase: GAP 1


  1. Begins at the completion of mitosis and cytokinesis and lasts until the beginning of S phase

  2. Cell chooses to replicate its DNA or enter a quiescent state (the G0 phase)

  3. Cell doubles its organelles, grows in size, accumulates raw materials for DNA synthesis

  4. Late in G1 phase, the cell becomes committed to replicating its DNA
• In eukaryotic cells, the centrosome is comprised of two centrioles surrounded by an electron-dense matrix
• Additional centrosomes lead to chromosome missegregation and abnormal numbers of chromosomes, which are characteristic of malignant cells
• Interphase: S Phase


  1. Replication of the chromosomes (DNA) typically lasts about 6 h

  2. In mammalian cells, the start of S phase (the actual initiation of DNA synthesis) takes place several hours after the cell has committed to carrying out DNA synthesis

  3. Each chromosome replicates exactly once to form a pair of physically linked sister chromatids
• Interphase: S Phase


  1. In animal cells, a pair of centrioles is also duplicated

  2. Proteins associated with DNA are synthesized

  3. Chromosomes enter with 1 chromatid each leave with 2 identical chromatids
• Interphase: S Phase


  • Period when the cell synthesizes the additional histones that will be needed as the cell doubles the number of nucleosomes in its chromosomes
  • Between the two forks, the regions of newly replicated DNA that are already covered by nucleosomal core particles to the same approximate density as the parental strands that have not yet undergone replication
• Interphase: GAP 2


  1. Between DNA replication and onset of mitosis

  2. Cell synthesizes proteins necessary for division

  3. Cell prepares to divide

  4. Some cells can exit the cell cycle from G2 phase, just as they can from G1 phase
• Mitotic (M) Stage


  1. M phase includes: Mitosis, Cytokinesis, Cytoplasm division

  2. Results in two genetically identical daughter cells
• Mitosis


  Divided into five phases: Prophase, Prometaphase, Metaphase, Anaphase, Telophase
• Prophase


  1. Chromosomes condense

  2. Spindle fibers form (spindle fibers are specialized microtubules radiating out from centrioles)

  3. Spindle fibers attach to sister chromatids

  4. Formation of the mitotic spindle
• Prophase


  1. During prophase, as the chromosomes are beginning to condense, the centrosomes move apart from one another as they organize the bundles of microtubules that form the mitotic spindle

  2. This micrograph shows a cultured newt lung cell in early prophase that has been stained with fluorescent antibodies against tubulin, which reveals the distribution of the cell’s microtubules (green)

  3. The microtubules of the developing mitotic spindle are seen to emanate as asters from two sites within the cell

  4. These sites correspond to the locations of the two centrosomes that are moving toward opposite poles at this stage of prophase. The centrosomes are situated above the cell nucleus, which appears as an unstained dark region
• Prophase


  1. ASTER (a radial array of short microtubules) extends from each centrosome

  2. Before segregating its chromosomes, a cell converts them into much shorter, thicker structures by a remarkable process of chromosome compaction (or chromosome condensation), which occurs during early prophase
• Prometaphase


  Some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
• Prophase


  Before segregating its chromosomes, a cell converts them into much shorter, thicker structures by a remarkable process of chromosome compaction (or chromosome condensation), which occurs during early prophase
• Prometaphase


  1. Some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes

  2. Kinetochores are protein complexes associated with centromeres
• Consequence of a missing motor protein on chromosome alignment during prometaphase


  • The chromosomes fail to align at the center of the spindle and instead are found stretched along spindle microtubules and clustered near the poles due to the absence of the motor protein (Kid)
  • Kid normally provides force for moving chromosomes away from the poles
• Metaphase


  The sister chromatids are pulled to the center of the cell, with one kinetochore facing each pole
• The sister chromatids are not split apart from one another until anaphase
• Spindle fibers begin to shorten during Anaphase
• Sister chromatids are pulled to the opposite ends of the cell during Anaphase
• Free spindle fibers lengthen and push poles of the cell apart during Anaphase
• Telophase


  1. Spindle fibers disintegrate

  2. Nuclear envelopes form around both groups of chromosomes

  3. Chromosomes revert to their extended state
• Cytokinesis


  1. This is the division of the cytoplasm

  2. Results in two separate daughter cells with identical nuclei
• Plant cells undergo cytokinesis by forming a cell plate between the two daughter nuclei
• Animal cells undergo cytokinesis through the formation of a cleavage furrow. A ring of microtubules contract, pinching the cell in half
• Cytokinesis


  A ring of microtubules contract, pinching the cell in half
• Cytokinesis


  • Cleavage of an animal cell (SEM)
  • Cell plate formation in a plant cell (TEM)