Transcript Cell Growth & Reproduction II

Cell Reproduction
Chapter 8.2
Part 2
The discovery of chromosomes
Chromosomes carry the genetic material
that is copied and passed down through
generations.
 They appear in the nucleus just before
cell division, and disappear soon after.
 They look like long stringy structures, and
they become dark when stained.
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The structure of eukaryotic
chromosomes
 Chromatin are long
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strands of DNA wrapped
around proteins called
histones.
Chromosomes exist in
the form of chromatin for
most of the cell’s lifetime.
Chromatin looks like
beads on a string.
Before cell division can
occur, chromatin must
coil up and become
tightly packed (see figure
8.10 pg.205)
Chromosome Structure
The Cell Cycle
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The Cell Cycle is the sequence of growth
and division of a cell.
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Divided into two periods;
Period of growth
2. Period of division
1.
Cell Cycle - Interphase
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Interphase is a cell growth
phase where the cell increases
in size, carries on metabolism,
and duplicates chromosomes
prior to division.
Interphase is divided into 3
parts:
G1 – Cell grows & protein
production is high.
S – DNA Synthesis – the cell
copies it’s chromosomes during
this phase.
G2 –A second, shorter growth
period – mitochondria and
other organelles are
manufactured.
“Sister Chromatids”
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Each duplicated
chromosome is made up
of two halves called
Sister Chromatids.
They are exact copies of
each other formed when
DNA is copied during
interphase.
Sister chromatids are
held together by a
centromere.
Mitosis begins with Prophase
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Prophase – the first
and longest phase of
mitosis.
Chromatin coils up
into visible
chromosomes.
The nucleus begins to
disappear – nuclear
envelope and
nucleolus dissolve.
Prophase cont.
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Centrioles, small dark cylindrical structures
made of microtubules, begin to move to opposite
ends of the cell (in Animal Cells only!)
Spindle, a football-shaped, cage like structure
consisting of thin fibers made of microtubules,
begins to form.
Plant cells do not have centrioles, only spindles!
Stage 2: Metaphase
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Chromosomes become attached to the spindle fibers by
the centromeres.
Chromosomes are pulled into a line across the equator
of the cell by the spindle fibers.
Each sister chromatid is attached to a single spindle
fiber, and the fibers extend to opposite poles of the cell.
This is to ensure that each new cell gets an identical and
complete set of genetic information!
Stage 3: Anaphase
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Anaphase begins with the
separation of sister
chromatids.
The centromeres split
apart, and the sister
chromatids separate from
each other.
Microtubules in the spindle
fibers shorten and pull the
chromatids towards
opposite ends of the cell.
Final Phase: Telophase
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Begins when chromatids
reach opposite poles of the
cell.
Like Prophase in reverse  Chromosomes
unwind.
 Spindle breaks down.
 Nucleolus reappears.
 New nuclear membrane
forms around each group of
chromosomes.
 A new double membrane
begins to form between the
two new nuclei.
Cytokinesis
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After telophase, the cell’s cytoplasm divides in a process
called cytokinesis.
Animals – plasma membrane pinches along the equator
until the cell is separated into two new cells. (fig 8.15)
Plants – a cell plate forms across the equator, and new
cell membranes and cell walls form on either side of the
plate until two new cells are created.
Results of Mitosis
Guarantees genetic continuity – two new
cells (Daughter Cells) with identical
chromosome sets as the parent cell.
 Daughter cells will also divide to create
four more identical copies, etc…
 In multicellular organisms, this process
results in growth and reproduction of cells
which work together as tissues, which
make up organs, which are a part of organ
systems, and so on…
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