Download Cell Division Powerpoint

Cellular
Division
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When do Cells Divide?
 Cells obtain nutrients and eliminate wastes through
the cell membrane. There must be enough surface
area for all of the exchanges to take place with the
environment.
 It is more difficult for cells with a larger volume to
transport, so cells remain relatively small to survive.
 When a cell reaches its maximum size, the nucleus
initiates cell division.
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Why do Cells Divide?
 Cell division accounts for 3 essential life processes:
 Growth - increase in size
 Differentiation - cells develop specialized shapes
and functions.
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Why do Cells Divide?
 Cell division accounts for 3 essential life processes:
 Repair - heal injuries by cell regeneration at the
injury site
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Why do Cells Divide?
 Cell division accounts for 3 essential life processes:
 Reproduction - making new organisms
Asexual - offspring produced by one parent;
the result of cell division
Sexual - offspring have a combination of
genetic material from two parent organisms;
requires meiosis, a specialized form of cell
division
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How do Cells Divide?
 Phase - a defined period within a cycle of
change
 Cell cycle - the sequence of phases in the life
cycle of a cell
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The Cell
Cycle
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Interphase
• Interphase - growth and preparation
– Occurs between divisions
• 3 stages
– G1 (gap 1) - growth and development
– S (synthesis - chromosomes replicate; the cell
commits to cell division
– G2 (gap 2) - cell synthesizes other organelles
and other materials needed for division
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Interphase
• During interphase, the chromosomes are
in chromatin form - a tangled bundle
• During S phase, each chromosome is
replicated, creating sister chromatids
• As cell division begins (during prophase),
the chromatin coils and condenses to
chromosomes and the sister chromatids
are clearly visible, joined at the
centromere.
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• Chromatin - uncoiled DNA
• Chromosome - neatly coiled DNA
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Cell Division - Mitosis and Cytokinesis
• There are 2 stages in cell division:
– 1. Mitosis
» The cell’s nucleus divides into 2 nuclei.
– 2. Cytokinesis
» The cell’s cytoplasm divides to make 2
daughter cells
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Mitosis
• Mitosis is the splitting of the cell’s nucleus
• It has 4 parts:
– Prophase
– Metaphase
– Anaphase
– Telophase
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Mitosis - Prophase
• Chromosomes condense
• In animal cells, a pair of centrioles
migrates to each side of the cell
• The mitotic spindle forms from the
centrioles
• The nuclear membrane and nucleolus
dissappear
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Mitosis - Prophase
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Mitosis - Metaphase
• Spindle arranges chromosomes along
“metaphase plate”
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Mitosis - Anaphase
• Centromeres divide and sister chromatids
are pulled apart, so that each daughter cell
has an identical set of chromosomes
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Mitosis - Telophase
• Nuclear membrane reforms around each
set of chromosomes
• Two nucleoli form
• Chromosomes uncoil into chromatin
• Cleavage furrow starts to form (for
cytokinesis)
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Cytokinesis
• The cell’s cytoplasm divides to make 2
daughter cells
• The cleavage furrow pinches cell in half
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Four Mitotic Stages
Prophase
Metaphase
Anaphase
Telophase
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Identical Daughter Cells
Two
identical
daughter
cells
Parent Cell
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Mitosis in Animal Cells
• Rat epithelial cells
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Mitosis in Plant Cells
• Plant cells have a cell wall.
• During mitosis, a cell plate forms a new
cell membrane and cell wall between the
two daughter cells.
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TIFF (Unc ompres sed) dec ompres sor
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History
• Mitosis was first observed microscopically
in the 1870s by the German biologist
Walter Flemming
– He coined the term mitosis
– from the Greek word mitos,
– meaning thread
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The Cell Cycle
Interphase may
be up to 90% of
the cell cycle.
The time varies
for different cell
types.
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The Cell Cycle
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Meiosis
Formation of Gametes
(Eggs & Sperm)
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Facts About Meiosis
Produces sex cells called gametes
(eggs & sperm)
Occurs in the testes in males
(Spermatogenesis)
Occurs in the ovaries in females
(Oogenesis)
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Facts About Meiosis
The gametes (eggs and sperm) are
haploid - which means they have 1
set of chromosomes, not 2 like all of
our diploid body cells.
Why we need meiosis --> this is
critical for sexual reproduction.
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The Process of Meiosis
It starts with a diploid cell that
has 46 chromosomes (2N).
The DNA is replicated, so there
are 92 chromosomes. There are 4
copies of each, making a tetrad.
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The Process of Meiosis
The first division, very similar to
mitosis, occurs, producing two cells
with 46 chromosomes each.
A second division occurs, producing
4 haploid daughter cells that each
have 23 chromosomes (N)
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Meiosis: Two Part Cell
Division
Meiosis
I
Diploid
at start
Meiosis
II
Diploid
Haploid
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Meiosis Forms Haploid Gametes
 Meiosis must reduce the chromosome number
by half
 Fertilization then restores the 2n number
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right
number!
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Crossing-Over
Pieces of chromosomes are exchanged,
which produces genetic recombination in
the offpsring
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Crossing-Over
Crossing-over multiplies the number of
different gamete types
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Fertilization
2N = 6
1N = 3
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• Humans
– Haploid number (1N) = 23
– Diploid number (2N) = 46
• Somatic cells are diploid.
• Gametes are haploid.
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