Transcript Mitosis – PROPHASE (step1)

Chapter 10 and 11-4
MITOSIS AND MEIOSIS
WHAT IS MITOSIS?
 For growth and to replace old, worn out or damaged cells.
 Occurs in body cells therefore forms body cells.
 Results in 2 identical cells of the original parent cell.
 Maintains chromosome number
 Creates diploid cells (2N) – has two sets of chromosomes
 One division of the nucleus
 Also known as binary fission
 In single-celled organisms, cell division is a form of asexual
reproduction.

CELL GROWTH
There are limits to cell growth
1. DNA “information” overload: The
bigger the cell, the greater the demand
on the DNA.
 Compare a cell to a growing town. The
town library has a limited number of
books. As the town grows, these limited
number of books are in greater demand,
which limits access.
 A growing cell makes greater demands
on its genetic “library.” If the cell gets
too big, the DNA would not be able to
serve the needs of the growing cell.
CELL GROWTH
2. Exchange materials: The bigger the cell, the harder it is to move
materials and wastes in and out of the cell.
- Small cells maintain more efficient transport systems.
- Imagine a cell shaped like a cube. As length of the sides increases, its
volume increases faster than its surface area, decreasing the ratio of
surface area to volume.
- If a cell gets too large, the surface area of the cell is not large enough
to get enough oxygen and nutrients in and waste out.
WHAT IS THE SOLUTION?
The Cell Cycle: Interphase + Cell division
Interphase = cell growth (preparation to divide)
Cell division = dividing the cell
Cell Division = Mitosis + Cytokinesis
Mitosis – the division of the nucleus
Cytokinesis – the division of the cytoplasm
G1 Phase: The cell grows in
size and develops duplicate
organelles.
Checkpoint 1
S Phase:
DNA # is
Checkpoint 2
G2 Phase: gets ready
for mitosis. Finishes
all growth.
doubled.
*Has 2 complete sets
of DNA
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CELL DIVISION – 3 MAJOR STEPS
1. Interphase is the phase in which
cells spend 90% of their time
getting ready to divide.
Has 3 parts:
1. G1
2. S
3. G2
CELL DIVISION – 3 MAJOR STEPS
 G1 phase – The first period of cell growth (size), when the cell
grows its organelles.
 S phase – The period when DNA is duplicated, replicated or
copied.
 Chromosomes
 Why is this stage so important?
 G2 phase – The period when a cell finishes all growth and prepares
for mitosis.
Chromosomes=
DNA and
Histones(protein)
Chromosome
Chromatid
Sister Chromatids
Duplicated Chromosome
Centromere
Where they join
CELL DIVISION – 3 STEPS
2. Mitosis (Cell Division)
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
3. Cytokinesis (Splitting of the cytoplasm)
Results in 2 new identical daughter cells.
INTERPHASE
Centriole copies
are made (G1)
DNA is replicating (S)
Cell Organelles are being replicated.
More Cytoplasm is being made. The
cell is getting ready for MITOSIS (G2)
MITOSIS – PROPHASE
1. Centrioles start moving apart.
X
2. Spindle
fibers form
X
X
X
3. Chromosomes shorten and thicken
into visible chromosomes.
4. Nuclear Membrane begins
breaking down
MITOSIS – METAPHASE (STEP 2)
1.Centrioles are
NOTE:
at opposite
THE NUCLEAR
poles.
MEMBRANE
2.Spindles X XIS NO LONGER
attach to the
PRESENT
chromosomes
And move toward the middle of
the cell.
X
X
>
>
>
>
MITOSIS – ANAPHASE (STEP 3)
>
>
>
>
1.Centrioles begin to pull sister
chromatids to opposite sides of the
cell.
>
>
>
>
MITOSIS – TELOPHASE (STEP 4)
1.Chromosomes continue to be
pulled toward the centrioles.
>
>
>
>
2.Cell Membrane begins to
separate into two cells
3.Nuclear Membranes begin to
reappear.
4. Spindles disappear.
CYTOKINESIS
Cytoplasm splits between the
two identical cells
THE STAGES OF THE CELL CYCLE
The process is VERY SIMILAR in each type of cell.
There are only 2 differences:
1. Plant cells do not have centrioles
2. The cell membrane cannot pinch because of the cell
wall.
3. Instead a cell plate forms between the 2 nuclei
division.
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REGULATING THE CELL CYCLE
Contact inhibition: stop dividing when crowded
Cyclins: Protein regulators
 Checkpoints
 Internal Regulators (timing)
 External Regulators (growth factors)
Uncontrolled Cell Growth
• Cancer: can form masses called tumors
• Unresponsive to internal and external
regulators.
• Deprive healthy cells from nutrients.
• Results in uncontrolled cell growth.
CANCER
 Tumor: uncontrolled cell division
 Benign (cysts, warts) = not too harmful
 Malignant = Cancer = harmful
 Metastasis: cancer cells break off & spread
 Kinds
 Carcinomas: skin & organ linings (breast & lung CA)
 Sarcomas: bone & muscle
 Lymphomas: tissues that make blood cells
 Leukemia: uncontrolled production WBC
 Cancer Effects on Cell Cycle
 Healthy cells stop dividing when crowded
 Cancer cells keep dividing & steal blood supply
normal cell
cancer cell
bloodstream
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MITOSIS
VS.
For growth and to replace
old, worn out or damaged
cells.
Occurs in body cells
therefore forms body cells.
Results in 2 identical cells
from the original parent
cell.
Maintains chromosome
number
 Creates diploid cells (2N)
One division of the nucleus
MEIOSIS
• To make sperm and eggs.
• Occurs in sex cells therefore
forms sex cells or gametes.
• Results in 4 genetically
different cells from the
original parent cell.
• Cuts chromosome number in
half
• Creates haploid cells (1N)
• Two divisions of the nucleus
Uneven division of cytoplasm
Spermatogenesis
Oogenesis
In males:
These cells develop into
4 individual sperm.
In oogenesis:
These cells would have developed
into three small polar bodies
and only one large egg.
THE POLAR BODIES ASSIST
WITH EQUAL CHROMOSOMAL
DIVISION, HOWEVER THE POLAR
BODIES DIE
Fertilization: Union of sperm and Egg
- Zygote- cell resulting from fertilization of egg by sperm
(full number of chromosomes)
Re-establishes the full number (diploid) of
chromosomes in an organism
2 chromosomes
2 chromosomes
4 chromosomes
PURPOSE OF MEIOSIS
 Also known as reduction division
 To produce sex cells or gametes with half the number of
chromosomes.
 Called haploid cells
 Sex Cells – Reproductive cells produced by sex organs
 Sperm and ova
 In meiosis the cell divides twice.
 Meiosis involves 2 cell divisions.
 Meiosis I
 Prophase I
 Metaphase I
 Anaphase I
 Telophase I
 Cytokinesis
 Meiosis II
 Prophase II
 Metaphase II
 Anaphase II
 Telophase II
 Cytokinesis
STEPS OF MEIOSIS
Prophase I (Step one) - Identical steps to Mitosis
prophase
 EXCEPT
 (a) Homologous (matching) chromosomes
pair up, also known as tetrads
 (b) crossing over occurs – exchange genetic
material

Metaphase I - Identical steps to Mitosis Metaphase
 EXCEPT
 (a) Spindle fibers move homologous pairs of
chromosomes to the center of the cell

STEPS OF MEIOSIS
 Anaphase I – identical steps to Mitosis Anaphase
 EXCEPT
 (a) Spindle fibers move Homologous
chromosomes apart
 Telophase I – identical steps to Mitosis Telophase
 EXCEPT
 (a) These 2 new cells now enter Meiosis II.
MEIOSIS II – THE STEPS ARE
IDENTICAL TO THE STEPS OF MITOSIS.
Prophase II
 Metaphase II
 Anaphase II
 Telophase II
 4 cells with half the number
of original chromosomes

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