Module 3. Cell Modifications and Cell Checkpoints

Cell Modifications - are specialized or modifications reacquired by the cell after cell division that helps the cell in performing its specialized function. Each cell, both in plant and animal, has specialized features that allow them to do its job.

Apical Modification - Cell modification found on the apical surface of the cell. It is specialized to carry out functions that occur at these interfaces, including secretion, absorption, and movement of luminal contents.

Examples are the following: \n 1. Cilia and flagella – Cilia are projections, usually short, hairlike structures and a type of organelle seen on the apical surface of epithelial cells. This assists in the movement of material over the epithelial surface in a manner parallel with the surface of the epithelium. Flagella are long, whiplike structure that are formed by microtubules protruding from the cell body of bacteria and some eukaryotic cells. The primary function of a flagellum is that of locomotion, but it also often functions as a sensory organelle, being sensitive to chemicals and temperatures outside the cell. Examples are the neuron axon and sperm cell.

2.  Villi and microvilli – Villi are finger-like projections that arise from the epithelial layer in some organs. They help to increase surface area, allowing faster and more efficient absorption. Microvilli are smaller projections than villi which functions primarily on the efficient absorption of molecules.

3.  Pseudopods – Temporary, irregular lobes formed by amoebas and some other eukaryotic cells. It bulges outward to move the cell or engulf the prey. It primarily consists of actin filaments and may also contain microtubules and intermediate filaments. Pseudopods are used for motility and ingestion.

4.  Extra Cellular Matrix (ECM) – A compound secreted by the cell on its apical surface. The cell wall is the extracellular structure in plant cells that distinguishes them from animal cells. The plant cell wall is made up of molecules secreted by the cell called cellulose, a polysaccharide composed of glucose units. Cellulose assembles into fibers called microfibrils. In animal cells, the major component of extracellular matrix is the protein collagen. Collagen proteins are modified with carbohydrates, and once they are released from the cell, they assemble into long fibers called collagen fibrils. Collagen plays a key role in giving tissues strength and structural integrity. In the extracellular matrix, collagen fibers are interwoven with a class of carbohydrate – bearing proteoglycans, which may be attached to a long polysaccharide backbone. The CM also contains many other types of proteins and carbohydrates. The ECM is directly connected to the cells it surrounds. Integrins are key connector proteins which are embedded in the plasma membrane. Fibronectin also can act as bridges between integrins and other ECM proteins such as collagen. On the other side of the membrane, the integrins are linked to the cytoskeleton.

Basal Modification – Cell modifications found on the basal surface of the cell basement membrane.

1. Desmosomes/Hemidesmosomes – These allow for strong attachment between cells or to a basement membrane. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. Hemidesmosomes are like desmosomes in terms of function, however, they attach the epithelial cell to the basement membrane rather than the adjacent cell.

   Lateral Modifications – Cell modification found on the basal surface of the cell. These are tight junctions, adhering junctions and gap junctions. These structures consist of protein complexes and induce connectivity between adjacent epithelial cells, between cell and ECM. They can contribute to the barrier function of epithelia and control the paracellular transport.

   1. Tight Junctions – They are transmembrane proteins fused on outer plasma membrane. They act as barriers that regulate the movement of water and solutes between epithelial layers.

   2. Adherens Junctions – Protein complexes that occur in cell-to-cell junctions in epithelial and endothelial tissues, usually more basal than tight junctions. It fastens cell to one another.

   3. Gap Junctions – It is also known as communicating junctions. These are specialized intercellular connections between multitude of animal cell types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells.

      PHASES OF THE CELL CYCLE AND THEIR CONTROL POINTS

      Core Concepts:

      • All organisms consist of cells and arise from preexisting cells.
      • Mitosis is the process by which new cells are generated.
      • Meiosis is the process by which gametes are generated for reproduction.
      • The Cell Cycle represents all phases in the life of a cell.
      • DNA replication (S Phase) must precede mitosis so that all daughter cells receive the same complement of chromosomes as the parent cell.
      • The gap phases separate mitosis from S phase. This is the time when molecular signals mediate the switch in cellular activity.
      • Mitosis involves the separation of copied chromosomes into separate cells.
      • Unregulated cell division can lead to cancer.
      • Cell cycle checkpoints normally ensure that DNA replication and mitosis occur only when conditions are favorable and the process is working correctly.
      • Mutations in genes that encode cell cycle proteins can lead to unregulated growth, resulting in tumor formation and ultimately invasion of cancerous cells to other organs.

      The Cell Cycle control system is driven by a built-in clock that can be adjusted by external stimuli (i.e., chemical messages).

      Checkpoint – a critical control point in the cell cycle where “stop” and “go ahead” signals can regulate the cell cycle.

      • Animal cells have built-in stop signals that halt the cell cycles and checkpoints until overridden by go-ahead signals.
      • Three major checkpoints are found in the G1, G2 and M phases of the cell cycle.

      The G1 Checkpoint – the Restriction Point

      • The G1 checkpoint ensures that the cell is large enough to divide and that enough nutrients are available to support the resulting daughter cells.
      • If a cell receives a go-ahead signal at the G1 checkpoint, it will usually continue with the cell cycle.
      • If the cell does not receive the go-ahead signal, it will exit the cell cycle and switch to a non-dividing state called G0.
      • Most cells in the human body are in the G0 phase.

      The G2 Checkpoint – ensures that DNA replication in S phase has been successfully completed.

      The Metaphase Checkpoint – ensures that all of the chromosomes are attached to the mitotic spindle by a kinetochore.

      Kinase – a protein which activates or deactivates another protein by phosphorylating them. Kinases give the go-ahead signals at the G1 and G2 checkpoints. The kinases that drive these checkpoints must themselves be activated.

      • The activating molecule is a cyclin, a protein that derives its name from its cyclically fluctuating concentration in the cell. Because of this requirement, these kinases are called cyclin-dependent kinases or CDKs.
      • Cyclins accumulate during the G1, S and G2 phases of the cell cycle.
      • By the G2 checkpoint, enough cyclin is available to form MPF complexes (aggregations of CDK and cyclin) which initiate mitosis.
      • MPF functions by phosphorylating key proteins in the mitotic sequence.
      • Later in mitosis, MPF switches itself off by initiating a process which leads to the destruction of cyclin.
      • CDK, the non-cyclin part of MPF, persists in the cell as an inactive form until it associates with new cyclin molecules synthesized during the Interphase of the next round of the cell cycle. \n

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