Cell Division - APBioScholars
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Transcript Cell Division - APBioScholars
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.