Theme 4 Module 1 Notes

Cell Division and the Cell Cycle

Module Overview

• Theme 4 focuses on DNA replication and mitosis.
• Module 1 specifically investigates the cell cycle.
• Learning objectives:
  • Recognize cell division as a crucial cellular process.
  • Compare prokaryotic binary fission and eukaryotic mitosis.
  • Examine chromosome movement during mitosis.
  • Understand cell cycle regulation.

Unit 1: Cell Proliferation in Prokaryotes

• Cell division is a regulated process by which a pre-existing cell gives rise to another cell.

• In prokaryotes, cell division equates to reproduction, resulting in a new, single-celled organism.

• Prokaryotic cells contain all essential elements to reproduce.

• They can create exact copies of their genomes and distribute one copy to each daughter cell.

• Prokaryotic cell division necessitates the distribution of identical genetic material.

• Binary fission is a form of asexual reproduction in prokaryotes.

• The process begins when the bacterial chromosome's DNA attaches to the plasma membrane via proteins.

• DNA replication starts at the origin of replication region.

• As replication continues, the cell elongates, and the replicated DNA also anchors to the plasma membrane.

• Elongation proceeds until the two DNA attachment sites are at opposite ends of the cell.

• Once DNA replication is complete and the bacterium doubles in size, constriction occurs at the cell's midpoint.

• This constriction involves synthesis of new cell membrane and cell wall, leading to division into identical daughter cells.

• Binary fission produces identical daughter cells in prokaryotes, whereas eukaryotic cell division involves mitosis, a much more regulated process.

Unit 2: The Eukaryotic Cell Cycle

• In eukaryotes, cell division facilitates the development of a complex multicellular organism from a unicellular fertilized egg.
• Early embryos have stem cells: unspecialized cells able to reproduce indefinitely and differentiate into specialized cells.
• In fully grown organisms, cell division supports continual cell renewal and tissue repair.
• Adult bodies also have stem cells, which replace non-reproducing specialized cells, unlike embryonic stem cells, which can give rise to all cell types.
• Example: Mammalian adult skeletal muscle has minimal cell turnover. Injury activates quiescent satellite stem cells in the muscle tissue's basement membrane, initiating cell division for muscle regeneration.
• Satellite stem cell activation leads to proliferation, differentiation, and fusion of myoblasts (muscle precursor cells).
• Myoblasts commit to forming mature muscle cells (myofibers), which cannot divide.
• Key question: How do some cells remain dormant and then divide, while others become terminally differentiated and lose the ability to divide?

Eukaryotic Cell Division Characteristics

• A primary distinction between prokaryotic and eukaryotic cell division is the larger size of eukaryotic DNA.

• Eukaryotic DNA is organized into linear chromosomes and highly condensed within the nucleus.

• Eukaryotic cell division demands more regulated control within a larger cell cycle.

• The standard eukaryotic cell cycle includes:

  • Interphase: Includes S phase (DNA synthesis) and gap growth phases G1 and G2.
  • M phase: Includes mitosis and cytokinesis.

• During each mitotic cell division, eukaryotic linear chromosomes are replicated and separated into daughter cells.

• Interphase is when cells prepare for cell division, including DNA replication in the nucleus and overall cell size increase.

  • S phase: DNA replication.
  • G1 and G2 phases: Prepare the cell for DNA synthesis and mitosis, respectively.

• The time required for cells to complete the cell cycle varies depending on the cell type.

  • Epithelial cells of the intestine or skin divide frequently.

• Many cells can pause the cell cycle in the G0 phase, between the M and S phases.

• This pause can vary in length from days to over a year.

• Some cells, such as lens cells, nerve cells, and mature muscle cells, enter a permanent G0 phase and become non-dividing.

• Stem cells exemplify cells that can reproduce indefinitely but also have periods of quiescence without cell division.

• Fully differentiated skeletal muscle exhibits little to no cell division, but upon injury, quiescent satellite stem cells activate from the G0 phase and re-enter the cell cycle.

• This activation enables proliferation, differentiation, and maturation of new muscle cell precursors, which fuse and repair muscle tissue with new muscle fibers.

• Once myofibers form, they exit the cell cycle and return to the quiescent G0 phase.

• Before mitosis, interphase allows DNA replication during the S phase and cell growth during the G1 and G2 phases.

• Mitosis consists of five distinct stages that are characterized by chromosomal changes during the cell division process.

• Walther Flemming's work in 1882 identified that the stages of mitosis could be staged based on chromosomal position and features.

• Flemming analyzed stained developing salamander embryos to visualize dividing cell chromosomes.

• Mitosis consists of 5 stages: prophase, prometaphase, metaphase, anaphase, and telophase.

• Prior to mitosis, chromosomes duplicate and condense to ensure daughter cells receive the same genetic information as the parent cell in a short time.

• During most of interphase, each chromosome exists as a long, thin chromatin fiber.

• Exact chromosome copies are created before mitosis during the S-phase through DNA replication.

• DNA sequences replicate from end to end, and the newly synthesized molecule associates with histones and other chromosomal proteins for tight compaction.

• The centromere is fully replicated, but it appears fused due to high compaction.

• Duplicated chromosomes consist of two identical copies called sister chromatids.

• We inherit a paternal and maternal chromosome, resulting in 23 distinct chromosome pairs:

  • 22 pairs of homologous chromosomes (one maternal and one paternal in origin).
  • 1 pair of sex chromosomes.

• Chromosomes compact into the structure shown only during M-phase.

• As the M-phase progresses, the two sister chromatids separate and move into two new cells, involving many changes in chromosome dynamics.

Unit 3: Chromosome Dynamics

• The M-stage of the cell cycle begins at the end of the interphase G2 phase.

• During interphase, individual chromosomes are unidentifiable due to their organization into long chromatin fibers.

• As a cell transitions from G2 to M-phase, the duplicated chromosomes condense, making individual chromosomes visible even with a light microscope.

• Prophase is the first stage of mitosis.

• During prophase, each chromosome appears as identical sister chromatids joined at their centromeres.

• Duplicated cellular microtubule organizing centers, called centrosomes, radiate long microtubules, forming a mitotic spindle.

• The centrosomes position themselves at opposite poles of the cell, and the mitotic spindle separates the chromosomes into the two daughter cells.

• Prometaphase follows prophase.

• A key feature of prometaphase is the fragmentation of the nuclear envelope.

• The breakdown of the nuclear envelope allows microtubules extending from each centrosome to attach to specialized regions on the centromeres of the chromosomes, called kinetochores.

• Kinetochores are specialized protein structures associated with each sister chromatid on either side of the centromere.

• Some microtubules from the centrosome attach directly to the kinetochore regions, and are essential for pulling chromosomes to the cell poles during mitosis.

• Other microtubules radiating from the centrosome are polar microtubules, which interact with each other and push the poles of the cell apart during mitosis.

• Metaphase is the third stage of mitosis.

• It is characterized by the alignment of chromosomes at the center of the cell in a region called the metaphase plate.

• Kinetochore microtubules attach at the kinetochores of each sister chromatid, facilitating alignment at the metaphase plate.

• Metaphase is followed by anaphase, the fourth stage of mitosis.

• During anaphase, kinetochore microtubules shorten, separating the sister chromatids into individual chromosomes that are pulled towards the opposite spindle poles of the cell.

• Simultaneously, polar microtubules push against each other, elongating the cell.

• By the end of anaphase, the two ends of the cell have equivalent and complete sets of chromosomes.

• Telophase, the final stage of mitosis, occurs once chromosomes segregate equally at the two ends of a dividing cell.

• During telophase, two new daughter nuclei form in the cell.

• The nuclear envelope reforms around the chromosomes at the opposite poles of the dividing cell.

• The chromosomes begin to decondense and spindle microtubules depolymerize (break down).

• The division of one nucleus into two genetically identical nuclei marks the end of mitosis.

• Mitosis is followed by the division of the cell into two individual cells.

• Cytokinesis (division of the cytoplasm and therefore of the cell) is usually underway by the end of telophase.

• Result: the two daughter cells are distinctly apparent shortly after mitosis.

• In animal cells, cytokinesis starts with the formation of a contractile ring composed of motor proteins that contract bundles of actin fibers along the midline of the cell.

• The contraction leads to the formation of a defined cleavage furrow, separating the cell into two distinct and separate daughter cells.

• The stages of mitosis are similar across all eukaryotic cells, but differences in cytokinesis exist depending on the dividing cell type.

• Cytokinesis differs between plant and animal cells.

• Plant cells lay down a newly developed cell wall along a cell plate region in the middle of the dividing cell due to the presence of a cell wall.

• Cytokinesis is complete in plant cells once the forming cell wall fuses with the original cell wall.

Unit 4: Controlling Progression

• Cell division is important during developmental growth, maintenance, and repair. What controls the cell cycle of individual cells?

• Early research in the 1970s explored molecules that could potentially regulate the cell cycle.

• Researchers hypothesized the existence of a mitosis promoting factor that allows for the transition from the G2 to M phase of the cell cycle.

• In the 1980s, Tim Hunt measured the protein level changes of dividing sea urchin embryos.

• Hunt added radioactively labelled amino acids (methionine) to the sea urchin eggs, expecting the radiolabeled methionine to incorporate into newly synthesized proteins.

• Samples of the rapidly dividing embryos were taken every 10 minutes, and protein changes were visualized using gel electrophoresis.

• Most protein bands on the gel darkened as cell division and embryonic development progressed.

• One protein band oscillated in intensity.

• This protein increased and decreased with each subsequent cell division; Hunt named this protein cyclin due to its cyclic nature.

• While its function was unknown, researchers suspected that cyclin was involved with regulating cell cycle progression.

• Follow-up work by Hunt and his colleagues found that the mitosis promoting factor consists of a cyclin protein and a cyclin-dependent kinase (CDK) protein.

• Together, they control progression of the cell cycle.

• Hunt shared the Nobel Prize in Medicine in 2001 for his work with cyclin proteins.

• Kinases are enzymes that activate or inactivate other proteins by phosphorylating key amino acids on the target proteins.

• Many kinases that regulate the cell cycle remain at a constant concentration, but are inactive until activated by binding to cyclin proteins.

• The kinases are called cyclin-dependent kinases because their activity depends on binding to cyclins.

• The cyclin-CDK complex triggers changes during cell cycle events by phosphorylating target proteins that promote cell division.

• Cyclin-dependent kinase activity rises and falls with concentration changes in its activating cyclin protein.

• Different types of cyclin-CDK complexes regulate each stage of the cell cycle.

• Cyclin-CDK regulation occurs at three steps of the eukaryotic cell cycle:

  • G1/S cyclin-CDK complex: Needed for the G1 to S phase transition and prepares the cell for DNA replication (e.g., increasing histone protein expression).
  • S-cyclin-CDK complex: Initiates DNA synthesis.
  • M cyclin-CDK complex: Initiates mitosis, facilitating nuclear membrane breakdown and regulating microtubule assembly in the mitotic spindles via phosphorylation of key structural proteins.

• Multiple checkpoints are key to regulating the cell cycle.

• Cell cycle checkpoints are a form of cellular surveillance that can block cyclin-CDK activity if something goes wrong during cell cycle progression.

• A checkpoint can pause cell division until preparation for the next stage of the cell cycle is complete, or if there is damage to repair.

• The three major checkpoints:

  • DNA damage checkpoint at the end of G1 phase.
  • DNA replication checkpoint at the end of the G2 phase.
  • Spindle assembly checkpoint before anaphase during mitosis.

• Cellular monitoring ensures:

  • Only undamaged DNA enters the S phase for replication (G1 checkpoint).
  • Cells enter mitosis only when all DNA is replicated (G2 checkpoint).
  • Cells complete mitosis only if all chromosomes are attached to a microtubule from the mitotic spindle (M phase checkpoint).

• DNA damage checkpoint:

  • Genes that inhibit cell cycle progression are normally turned off.
  • When DNA is damaged (e.g., double-stranded breaks), specific protein kinases phosphorylate p53, a protein that can inhibit the cell cycle when turned on.
  • p53 is normally present in low levels, and is mostly exported from the nucleus and degraded.
  • Upon phosphorylation, p53 accumulates in the nucleus and acts as a transcription factor to turn on genes that will inhibit the cell cycle.
  • This leads to the production of a CDK inhibitor protein that can bind to and block the activity of the G1-S cyclin-CDK complex, arresting the cell cycle in the G1 phase.
  • The resulting pause gives the cell an opportunity to repair the damaged DNA.

• Spindle assembly checkpoint:

  • Starting at prometaphase, regulatory proteins associated with the spindle assembly checkpoint monitor the attachment of sister chromatids to microtubules of the mitotic spindle at their kinetochore regions.
  • Unattached kinetochores create a “wait” signal, recruiting spindle-assembly checkpoint proteins.
  • These proteins are activated by a lack of tension in the centromere area, and only allow for the progression of metaphase and entry into anaphase when each sister chromatid is attached to a kinetochore microtubule.
  • Once this occurs, the spindle checkpoint proteins are removed from the centromere region, and separase, a specialized enzyme, breaks sister chromatid attachments.

Module Summary

• Prokaryotes reproduce by binary fission, while eukaryotic cells produce identical daughter cells through mitosis as part of a larger cell cycle.
• The cell's DNA replicates so that each daughter cell receives identical genetic information.
• The eukaryotic cell cycle is regulated by cyclin-CDK complexes and checkpoints that regulate overall cell division.
• Cell cycle control mechanisms contribute to chromosome dynamics observed in dividing cells.