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Mitosis & Meiosis
General Biology
Mr. Cobb
Cell Theory

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According to the cell theory, all
cells come from pre-existing cells.
But why do cells reproduce?
Cell Reproduction
 Cells reproduce to replace lost or
damaged cells with new healthy
cells.
 New cells gradually replace dead
cells.
 Cell reproduction is responsible
for your growth.
Reproduction
 Asexual reproduction occurs when
one organism makes an exact
duplicate of itself.
 Sexual reproduction occurs when
the genetic information from two
organisms combine to make a
new, genetically different,
organism.
Genetic Material

Genetic material (DNA or RNA)
can exist in your cells in three
forms:


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Chromatin
Chromosomes
Chromatids
Chromatin

Genetic material
is called
chromatin when
it is in a mass of
very long fibers.
Chromosomes


As the cell prepares to divide, the
genetic material condenses and
winds around itself, forming
chromosomes.
Chromosomes are visible under
powerful microscopes
Chromosomes



Chromosomes are pictured in a
karyotype.
Humans have 23 pairs of
chromosomes.
These pairs are called homologous
pairs of chromosomes.
Diploid


Cells that have homologous pairs
of chromosomes are called
diploid. (2n)
Most organisms are diploid.
Chromosomes
Chromatids


Before the cell divides, the genetic
material is copied. The two
identical pieces of DNA join
together to form “sister
chromatids.”
These chromatids are joined at the
centromere.
Chromosomes
& Chromatids
Genetic Information
Cell Cycle
 In order for cells to reproduce,
they must first go through the cell
cycle, which lasts about 10-20
hours.
 During the cell cycle, the DNA
and organelles are copied, the cell
increases in size, and then divides.
Cell Cycle
 The cell cycle is composed of
several steps:


Interphase
Mitosis
Prophase
 Metaphase
 Anaphase
 Telophase


Cytokinesis
Interphase




Interphase is made up of 3 phases:
G1 – growth and function.
During S-phase, the cell’s DNA is
synthesized (copied). This phase
lasts for 3 – 6 hours.
During G2 phase the cell prepares
to divide (organelles and
cytoplasm increase)
Cell Division

When the cell is actually dividing,
it is called mitosis, followed by
cytokinesis.
Mitosis

During mitosis,
the nucleus and
the duplicated
DNA divide and
are moved into
two daughter
cells.
Cytokinesis


During cytokinesis, the cytoplasm
of the cell divides into two parts.
At the end of cytokinesis, the two
daughter cells completely
separate, forming two identical
cells.
FLIP BOOK – Label flaps like this
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CELL CYCLE (title page)
MITOSIS – Prophase
Metaphase
Anaphase
Telophase/Cytokinesis
Interphase
Mitosis
Cell Cycle
Cell Cycle

The cell cycle is divided into three
parts:



Interphase
Mitosis
Cytokinesis
Interphase


During
interphase, the
cell is very
active.
Organelles and
DNA are
duplicated
Interphase


Individual
chromosomes
are not visible
because they are
loosely packed
chromatin
ChromatIN is
found only
during
INterphase
Mitosis

Mitosis is divided into 4 phases:

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
Prophase
Metaphase
Anaphase
Telophase
Prophase



During prophase,
the chromatin
fibers condense
and become
chromosomes
Sister chromatids
join at the
centromere.
Sister
chromatIDs are
IDentical
Prophase



The nucleolus
disappears.
The nuclear
membrane
disappears
Spindle fibers
form from spindle
Metaphase

All the
chromosomes
line up in the
middle of the
cell (called the
“metaphase
plate”).
Metaphase

Spindle fibers
attach to the
chromosomes at
the centromere.
Anaphase


Spindle fibers
condense, causing
the sister
chromatids to
separate.
Each chromatid is
now a “daughter
chromosome.”
Telophase


Telophase
begins when the
chromosomes
reach the poles
of the spindle.
Processes that
occurred during
prophase are
reversed.
Telophase



The nuclear
envelope and
nucleolus
reappear.
The spindle
disappears
Chromosomes
uncoil
Cytokinesis


During
cytokinesis, the
cytoplasm
separates into the
two daughter
cells.
The two daughter
cells separate
from each other.
Cytokinesis


In animal cells a
“cleavage furrow”
forms when the two
cells begin to
separate.
In plants, a cell
plate forms and
separates the two
cells.
Cell division

The result of the cell cycle is one
cell becoming two identical cells.
Meiosis
Mitosis



Mitosis occurs in “somatic” cells.
Somatic cells are body cells (ex:
muscle cells, brain cells, liver
cells, skin cells, etc.)
But what about sex cells?
Meiosis

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Sex cells divide by a different
process.
This process, called Meiosis,
allows 4 unidentical cells to be
formed from one cell.
Meiosis

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In females, meiosis occurs in egg
cells before birth, and again during
puberty.
In males, meiosis occurs in sperm
cells throughout their lives.
Definitions


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Somatic Cells - Body cells
Sex cells - Sperm & Egg
Homologous chromosomes chromosomes that are similar in
size, shape, and genetic content.
They are NOT identical, just
similar.
Homologous Chromosomes

Homologous
chromosomes
are similar, but
not identical.
Definitions


Diploid - Have both pairs of
homologous chromosomes (2n).
Humans have 46 chromosomes
Haploid - have one pair of
homologous chromosomes. Ex:
Human sex cells have 23
chromosomes
Diploid


Diploid
organisms have
pairs of
chromosomes.
Human somatic
cells are diploid.
Haploid

Haploid cells
have single
chromosomes.
Meiosis

Meiosis is divided into two parts:
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Meiosis I
Meiosis II
Both Meiosis I and II go through
the steps of Mitosis:

Prophase, Metaphase, Anaphase,
Telophase
Meiosis I

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During Meiosis I, DNA is copied
and CROSSING OVER occurs.
Crossing over allows for genetic
variability.
Meiosis I includes: Prophase I,
Metaphase I, Anaphase I,
Telophase I
Prophase I

After the
chromosomes
condense, the
homologous
chromosomes come
together in pairs (the
pairing up of
chromosomes is
called synapsis)
Prophase I
These pairs of
chromosomes
are known as
Sister
Chromatids,
they are alike
but not
identical.
Prophase I

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The four chromatids come together form a
tetrad.
The chromosomes exchange portions of
their DNA. This exchange of DNA is
called crossing over.
Metaphase I

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The centromeres of the
chromosomes line up at the
equator of the cell.
Each tetrad (group of
chromosomes) is attached to a
spindle fiber.
Anaphase I


During anaphase I, the
homologous chromosomes
separate, with one going to each
pole of the cell.
The sister chromatids do not
separate.
Anaphase I

The homologous
chromosomes
separate, NOT
the sister
chromatids.
Telophase I
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During telophase I, the cytoplasm
divides, forming two daughter
cells.
Each daughter cell has half the
number of chromosomes as the
parent, but each chromosome is
double stranded.
Meiosis I Overview
Meiosis II

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After telophase I has ended,
Meiosis II begins. NO G1, S, or
G2 occurs!
The end product of meiosis II is
four haploid daughter cells that
are NOT genetically identical.
Meiosis II

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Meiosis II includes: prophase II,
metaphase II, anaphase II, and
telophase II.
The process of meiosis II is very
similar to mitosis.
Prophase II

Each of the
daughter cells
forms a spindle
and the double
stranded
chromosomes
move toward the
middle of the
spindle.
Metaphase II

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The chromosomes
line up at the
equator of the cell.
Each chromosome
still consists of two
chromatids
Anaphase II

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The centromeres
divide and the
two chromatids
separate.
The single
stranded
chromosomes
move toward the
opposite ends of
the cell.
Telophase II

Both daughter
cells divide,
forming four
haploid cells.
Meiosis II

At the end of meiosis II, the
haploid cells return to their
interphase state– the DNA uncoils
and the nuclear membrane
reappears.
Oogenesis

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Oogenesis is the formation of an egg
cell through meiosis.
During oogenesis, the cytoplasm
divides unevenly. One cell gets the
majority of the cytoplasm, the other
three cells are much smaller.
The larger cell becomes the egg, while
the others are called “polar bodies”
and are disintegrated by the body.
Oogenesis
Spermatogenesis


Spermatogenesis is the formation
of sperm cells through meiosis.
The cytoplasm divides evenly and
four haploid sperm cells are
generated from one diploid cell.
Spermatogenesis
Spermatogenesis