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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 cyclindependent 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.