Transcript 2. Cell Division

Cell Division
Ch 10
Sections 10-2 & 10-3
10-2 Cell Division
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In Eukaryotes (Cells with a nucleus) cellular
division occurs in 2 stages:
1.
2.
Mitosis= 1st stage where the cell’s nuclues
divides
Cytokinesis= 2nd stage where the cytoplasm
divides
Chromosomes
 Chromosomes: Found in eukaryotic cells
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Pass genetic information from one generation
to the next
Made up of DNA
The cells of every organism have a specific #
of chromosomes EX: Fruit flies= 8
chromosomes, Humans= 46 chromosomes,
carrot cells= 18 chromosomes
Each chromosome is composed of two
chromatids
Chromatids
 Chromatids: 2 identical strands that make up
1 chromosome

During cell division, sister chromatids separate
from each other

One chromatid then goes to each of the
2 new cells
The Cell Cycle
 The Cell cycle= A series of events that cells
go through as the grow and divide
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Cells grow
Prepare for division
Divide to form 2 daughter cells
Each daughter cell then begins the cycle
again
Events of the Cell Cycle
 Events of the cell cycle can be divided into
two major stages:
1. Interphase= Can be quite long. Divided into 3
parts
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G1= Cells grow and increase their size
S= Chromosomes replicate, DNA is synthesized
G2= Produces organelles required for cell division
2. Cell Division= AKA Mitosis

Takes place quickly
Mitosis
 Mitosis= Process of cellular division.
Divided into 4 phases:
1.
2.
3.
4.
Prophase
Metaphase
Anaphase
Telophase
Prophase
 Prophase= 1st and longest phase of mitosis (50-
60%) of total time required to complete mitosis
 Chromosomes become visible
 Centrioles (2 tiny structures located in the
cytoplasm) separate and take opposite sides of
nucleus
 Condensed chromosomes attach to the spindle
 Chromosomes coil more tightly
 Nuclear envelope breaks down & nucleolus
disappears
Metaphase
 Metaphase= 2nd phase of mitosis. Lasts only
a few minutes

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Chromosomes line up in center of the cell
Microtubules connect centromere of each
chromosome to the 2 poles of the spindle
Anaphase
 Anaphase= 3rd phase of mitosis
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Sister chromatids split & separate becoming
individual chromosomes
Chromosomes move into two groups near
poles of the spindle
Anaphase ends when chromosomes stop
moving
Telophase
 Telophase= 4th and final phase of mitosis
 Chromosomes begin to disappear into tangle
of dense material
 Nuclear envelope reforms around each cluster
of chromosomes
 Spindles break apart
 Nucleolus becomes visible in each daughter
nucleus
 Mitosis is complete
**CELL DIVISION HOWEVER IS NOT
COMPLETE**
Cytokinesis
 Cytokinesis= Division of the cytoplasm itself
and is the final phase of cellular division
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Usually occurs during the same time as
telophase
In animal cells= cytoplasm is drawn inward
and pinched into two equal parts
In plant cells= Cell plate forms causing a cell
wall to form between the 2 divided nuclei
10-3 Regulating the Cell Cycle
 Controls on Cell Division: Cell growth can be
turned on and off depending on the situation

EX: if cells growing on a Petri dish are
removed, the cells surrounding that now
empty space will start dividing, filling in the
area where the cells were removed
Cell Cycle Regulators
 Cell Cycle Regulators= Help maintain homeostasis
(relatively stable environment) in the body.
 Cyclins= A family of closely related proteins that
regulate the timing of the cell cycle in eukaryotic cells
 Internal Regulators= Proteins that respond to
events inside cell

Allow the cell cycle to proceed only when certain
processes have happened inside the cell.
 EX: Several proteins make sure cell doesn’t start
mitosis until all chromosomes have been replicated.
Cell Cycle Regulators Con’t…
 External Regulators= Proteins that respond to
events outside the cell.

Direct cells to speed up or slow down the cell
cycle.
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EX: Growth regulators are important during
embryonic development & wound healing
EX: molecules on neighboring cells can cause
cell cycles to speed up or slow down keeping
tissues of the body from disrupting each other.
Uncontrolled Cell Growth
 The consequences of uncontrolled cell
growth in multicellular organisms can be very
severe.
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Cancer cells DO NOT respond to the signals
that regulate the growth of most cells
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They divide uncontrollably forming tumors
Some fail to respond to internal regulator proteins
Some fail to respond to external regulator
proteins
Causes of cancer can be: smoking, radiation
exposure, and even some viral infections