Transcript Mitosis and Cell Division

Focus Areas: Mitosis
 Analysis: How does the cell go about the
process of dividing?
 Visualization: You should visualize the
process?
 Recall: Parts of the Cell
 Personal Responsibility: Make sure you use
the microscope! Do you own work!
Mitosis and Cell Division
 Mitosis, process in which a cell’s nucleus
replicates and divides in preparation for division of
the cell.
 Mitosis results in two cells that are genetically
identical, a necessary condition for the normal
functioning of virtually all cells.
 Mitosis is vital for growth; for repair and
replacement of damaged or worn out cells; and for
asexual reproduction, or reproduction without
eggs and sperm.
Mitosis
 All multicellular animals, plants, fungi, and
protists, which begin life as single cells, carry out
mitosis to develop into complex organisms
containing billions of cells.
 Mitosis continues in full-grown organisms as a means
of maintaining the organism—replacing dying skin
cells, for example, or repairing damaged muscle
cells.
Mitosis
 In the cells of the adult human body, mitosis occurs
about 25 million times per second.
 Multicellular organisms such as sea stars, sea
anemones, fungi, and certain plants rely on mitosis
for asexual reproduction at particular stages in their
life cycles.
 Mitosis is the sole mode of reproduction for many
single-celled organisms. (We will study bacteria
next.)
Mitosis
 The life cycle of eukaryotic cells, or cells
containing a nucleus, is a continuous process
typically divided into three phases for ease of
understanding:
 interphase,
 mitosis, and
 cytokinesis.
Interphase
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Interphase includes three stages, referred to
as G1, S and G2.
In G1, a newly formed cell synthesizes materials
needed for cell growth.
In the S stage, deoxyribonucleic acid (DNA), the
genetic material of the cell, is replicated.
At this stage, DNA consists of long, thin
strands called chromatin.
Interphase
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As each strand is replicated, it is linked to its
duplicate by a structure known as a centromere.
When the S stage is complete, the cell enters a
brief stage known as G2.
In the G2 stage specialized enzymes correct any
errors in the newly synthesized DNA, and
proteins involved with the next phase, mitosis,
are synthesized.
Chromosome
MITOSIS: Prophase
 Mitosis occurs in four steps.
 In prophase the replicated, linked DNA strands
slowly wrap around proteins that in turn coil and
condense into two short, thick, rodlike structures
called chromatids, attached by the centromere.
Chromosome
MITOSIS: Prophase
 Two structures called centrioles, both located on one
side of the nucleus, separate and move toward
opposite poles of the cell.
 As the centrioles move apart, they begin to radiate
thin, hollow, proteins called microtubules or spindle
fibers.
 The spindle fibers arrange themselves in the shape
of a football, that spans the cell, with the widest part
at the center of the cell and the narrower ends at
opposite poles.
MITOSIS: Prophase
 As the spindle forms, the nuclear membrane breaks
down into tiny sacs or vesicles that are dispersed in
the cytoplasm.
 Final disintegration of the membrane marks the
beginning of metaphase.
Interphase/Prophase: Animal
Interphase/Prophase: Plant
Prophase
Mitosis: Metaphase
 In metaphase, the spindle fibers attach to the
chromatids near the centromeres, and tug and
push the chromatids so that they line up in the
equatorial plane (middle) of the cell halfway
between the poles.
 Like two individuals standing back to back at the
equator, one chromatid faces one pole of the cell,
and its linked partner faces the opposite pole.
Metaphase: Animal
Metaphase: Plant
Metaphase:
Mitosis: Anaphase
 This precise orientation is necessary in order for the
next step, anaphase, to occur.
 Anaphase begins when the centromeres split,
separating the identical chromatids into single
chromosomes, which then move along the
spindle fibers to opposite poles of the cell.
Anaphase: Animal
Anaphase: Plant
Anaphase:
Mitosis: Telophase
 As these two identical groups of single
chromosomes gather at opposite poles of the
cell, telophase begins.
 A new nuclear membrane forms around each
new group of chromosomes.
 The spindle fibers break down and the newly
formed chromosomes begin to unwind.
Mitosis: Telophase
 If viewed under a light microscope, the chromosomes
appear to fade away.
 They exist, however, in the form of chromatin, the
extended, thin strands of DNA too fine to be seen
except with electron microscopes.
 Mitosis accomplishes replication and division of
the nucleus, but the cell has yet to divide.
Telophase: Animal
Telophase: Plant
Telophase
CYTOKINESIS
 The final phase of the cell cycle is known as
cytokinesis.
 The timing of cytokinesis varies depending on the cell
type.
 It can begin in anaphase and finish in telophase;
or it can follow telophase.
Mitosis
Cytokinesis
CYTOKINESIS
 In cytokinesis, the cell’s cytoplasm separates in
half, with each half containing one nucleus.
 Animals and plants accomplish cytokinesis in slightly
different ways.
 In animals, the cell membrane pinches in, creating
a cleavage furrow, until the mother cell is pinched in
half.
CYTOKINESIS
 In plants, cellulose and other materials that make
up the cell wallare transported to the midline of
the cell and a new cell wall is constructed.
Mitosis
Cytokinesis
CYTOKINESIS
 The process of DNA replication, the precise
alignment of the chromosomes in mitosis, and the
successful separation of identical chromatids in
anaphase results in two new cells that are genetically
identical.
 The new cells enter interphase, and the cell cycle
begins again.
 "Mitosis," Microsoft® Encarta® Encyclopedia 99. ©
1993-1998 Microsoft Corporation. All rights reserved.
Plant or Animal? What Phases?
BSCS, 5th Edition
What Phases?
BSCS, 5th Edition
What Phases?
BSCS, 5th Edition
What Phases?
BSCS, 5th Edition
CONTROL OF CELL DIVISION
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In multicellular organisms, cell division must be
carefully regulated to ensure that growth of the
organism is coordinated, replacement of dead cells
takes place in an orderly fashion, and repair of injured
cells is initiated when needed.
 Cell division must also be halted when growth and
repair are completed.
CONTROL OF CELL DIVISION
 Cell division is controlled by a variety of factors.
 One of the most important controls is carried out by
molecules called growth factors.
 Growth factors first come into play late in the G1stage
of interphase.
 Cells cannot pass from G1 to the S stage unless
growth factors bind to the plasma membrane.
CONTROL OF CELL DIVISION
 The binding of growth factors triggers a cascade of
biochemical activity that propels the cell into the S
stage.
 If the cell does not enter the S stage, it exits from the
cell cycle into the G0 stage, a period of normal
metabolic activity where other control mechanisms
prevent it from dividing.
CONTROL OF CELL DIVISION
 Most of the cells in the adult human body remain in
the G0 stage throughout life.
 Certain cells, such as bone, muscle, or liver cells,
can return to the cell cycle and divide if they are
injured.
 Injuries release growth factors that override the
controls over the non-dividing state.
CONTROL OF CELL DIVISION
 Once a cell is committed to dividing, still other growth
factors ensure that steps in mitosis are carried out
accurately.
 At the end of the G2 stage, mitotic (or maturation)
promoting factor (MPF) triggers prophase, and enzymes
condense DNA into chromosomes, break down the
nuclear membrane, and form the spindle.
 A complex interplay of other growth factors carries the
cell through the remaining steps of mitosis and
cytokinesis.
CONTROL OF CELL DIVISION
 Scientists have identified over 50 different growth
factors.
 Some are very specific, and react only with certain
cells.
 Nerve growth factor, for example, stimulates the
growth of nerve cells during embryonic development,
but has no effect on other cells.
 Others, such as epidermal growth factor, control
division in a variety of cells.
CONTROL OF CELL DIVISION
 Understanding the production of growth factors and
their precise mode of activity pose significant
research challenges.
 As scientists learn more about the mechanisms for
normal cell division, they gain insight into the causes
of the unregulated cell growth that leads to cancer.
 "Mitosis," Microsoft® Encarta® Encyclopedia 99. ©
1993-1998 Microsoft Corporation. All rights reserved.