Cell Division

Brian Beaty

AP Biology

What are the key roles of cell division?

• Reproduction

• Growth

• Repair

Chromosome Structure

What is the difference between chromatin, chromatids,

chromosome, DNA, Genes

Has anyone ever seen a gene? Can we see the impact of a gene?

Cell Division

• Distributes identical sets of chromosomes to daughter cells

• Genome – the total hereditary endowment of a cell (Amount varies in organisms)

• Human Genome Project

• Exists in two steps: Mitosis (nuclear division) and Cytokinesis (Cell division)

Cell Cycle

Animal Mitosis

Closer Look at Spindle Fibers

Spindle Fiber Formation

• Form in the cytoplasm from microtubules and associated proteins.

• Microtubules of the cytoskeleton are partially disassembled during spindle formation.

• Aggregates of two proteins, å- and ß-tubulin.• Elongate by the adding tubulin subunits at one end. • Assembly of spindle microtubules begins in the

centrosome or (microtubule organizing center)• In animal cells, a pair of centrioles is in the center of the

centrosome

The function of the nonkinetochore microtubules:

• elongate the whole cell along the polar axis during anaphase.

• overlap at the middle of the cell and slide past each other away from the cell's equator, reducing the degree of overlap.

• ATP provides the energy for this endergonic process.

Cytokinesis

Binary Fission

• Evolution of Mitosis

Control Systems

• The distinct events of the cell cycle are directed by a distinct cell cycle control system.

• A checkpoint in the cell cycle is a critical control point where stop and go signals regulate the cycle.

• Three major checkpoints are found in the G1, G2, and M phases.

Control Systems  Rhythmic fluctuations in the abundance and activity

of control molecules pace the cell cycle.• Some molecules are protein kinases that activate or

deactivate other proteins by phosphorylating them.• The levels of these kinases are present in constant

amounts, but these kinases require a second protein, a cyclin, to become activated.

• Levels of cyclin proteins fluctuate cyclically.• The complex of kinases and cyclin forms cyclin-

dependent kinases (Cdks).

density-dependent inhibition

          Growth factors appear to be important in density-dependent inhibition of cell division.

          Cultured cells normally divide until they form a single layer on the inner surface of the culture container.

          If a gap is created, the cells will grow to fill the gap.

          At high densities, the amount of growth factors and nutrients is insufficient to allow continued cell growth.

Anchorage Dependence

          Most animal cells also exhibit anchorage dependence for cell division.

          To divide they must be anchored to a substratum, typically the extracellular matrix of a tissue.

          Control appears to be mediated by connections between the extracellular matrix and plasma membrane proteins and cytoskeletal elements.

• Cancer cells are free of both density-dependent inhibition and anchorage dependence.

Cancer cells have escaped from cell cycle control

• Cancer cells do not stop dividing when growth factors are depleted either because they manufacture their own, have an abnormality in the signaling pathway, or have a problem in the cell cycle control system.

• If and when cancer cells stop dividing, they do so at random points, not at the normal checkpoints in the cell cycle.

• The abnormal behavior of cancer cells begins when a single cell in a tissue undergoes a transformation that converts it from a normal cell to a cancer cell.

          Normally, the immune system recognizes and destroys transformed cells.

          However, cells that evade destruction proliferate to form a tumor, a mass of abnormal cells.

         

• benign tumor• malignant tumor, the cells leave the original site

to impair the functions of one or more organs.• Metastasis - cancer cells often lose attachment to

nearby cells, are carried by the blood and lymph system to other tissues, and start more tumors

Cell Signaling

• Chemical substances are the principal agents of biological regulation and they exert their effects on cells through signaling systems.

• Cell signaling evolved early in the history of life.

• Illustration of early chemical signaling in yeast

• Yeast mating behavior

• In general, the steps by which a chemical signal is converted to a specific cell response is called a signal transduction pathway.

Communicating cells may be close together or far apart

• A chemical signal that communicates between two nearby cells is called a local regulator. Two types of local signaling have been described in animals: paracrine signaling and synaptic signaling.

• In paracrine signaling, one cell secretes the signal into extracellular fluid and the signal acts on a nearby target cell. Examples of signals which act in a paracrine fashion are growth factors, a group of factors which stimulate cells to divide and grow.

• In synaptic signaling, a nerve cell releases a signal (e.g., neurotransmitter) into a synapse, the narrow space between the transmitting cell and a target cell, such as another nerve cell or muscle cell.

• A chemical signal which communicates between cells some distance apart is called a hormone.

• Insulin, for example, may act in a paracrine fashion on adjacent cells (e.g., other insulin cells in the pancreas, acting to inhibit the further release of insulin in a negative feedback manner) and in a hormonal fashion on distant cells (e.g., liver cells, which store carbohydrate as glycogen).

Comparison of local and long distance signaling

Signaling by direct contact

• Cells also may communicate by direct contact. Some plant and animal cell possess junctions though which signals can travel between adjacent cells

• Look at two examples in the following slide

Three stages of signaling response

• In order for a chemical signal to elicit a specific response, the target cell must possess a signaling system for the signal.

• Cells which do not possess the appropriate signaling system do not respond to the signal.

The signaling system of a target cell consists of the following elements:

• Signal reception. The signal binds to a specific cellular protein called a receptor, which is often located on the surface of the cell.

• Signal transduction. The binding of the signal changes the receptor in some way, usually a change in conformation or shape. The change in receptor initiates a process of converting the signal into a specific cellular response; this process is called signal transduction. The transduction system may have one or many steps.

• Cellular response. The transduction system triggers a specific cellular response. The response can be almost any cellular activity, such as activation of an enzyme or altered gene expression.