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Cell Growth and
Division
Mitosis and Meiosis
Cell Growth
When an organism
grows, the number
of cells increase but
the size of each cell
remains small.
Limits to cell growth
1. DNA “overload”: The larger a cell becomes,
the more demands the cell places on its
DNA.
DNA stores the information that
controls how a cell functions
b. When a cell is small, DNA can
meet the cells needs
c. When a cell is large, it still has
only one copy of DNA so it is more
difficult for the cell to perform its
function
a.
Limits to cell growth
2. Exchanging
materials: additionally,
large cells have more trouble
movingsubstances across the cell
membrane.
a. If a cell is too large, it is difficult
to get enough oxygen and
nutrients in and waste products out
b. This is why cells do NOT grow
much larger even if the organism does
grow large
Division of the Cell
1. Before a cell gets too large, it
will divide to form two
“daughter” cells
2. Before a cell divides, it makes
a copy of its DNA for each
daughter cell
Cell Division
1. Cell division in eukaryotes is more complex
than in prokaryotes.
2. There are two stages of eukaryotic cell
division
a. Mitosis: Division of the cell nucleus
b. Cytokinesis: Division of the cell cytoplasm
3. Unicellular organisms reproduce asexually
by mitosis
a. The daughter cells are identical to the
parents cells
4. Mitosis is how a multicellular organism
grows and develops
Chromosomes
1. Chromosomes are made of
condensed chromatin.
2. Chromatin consists of DNA and
the proteins it is wrapped around.
3. The cells of every organism have
a specific number of
chromosomes (humans have 46
chromosomes).
3. Chromosomes are only visible during cell division,
when they are condensed. The rest of the time the
chromatin is spread throughout the nucleus.
4. Before cell division, each chromosome is replicated
(meaning copied).
a. When a chromosome is replicated, it consists of
two identical “sister” chromatids.
b. When a cell divides the chromatids separate, and
one goes to each of the two new cells.
c. Sister chromatids are attached to
each other at the spot called the
centromere.
The Cell Cycle
• When a cell is NOT dividing, it is said to be
in interphase.
• The series of events that a cell goes
through as it grows and divides is called
the cell cycle.
Events of the cell cycle
Interphase, when the cell is NOT dividing, has three
phases: G1, S, and G2.
• 1. G1 phase: period of activity in which cells do
most of their growing.
– a. Cells increase in size
– b. Cells synthesize (make) new proteins and organelles
• 2. S phase: DNA (chromosomes) is replicated
• 3. G2: organelles and molecules required for cell
division are produced
M phase is the phase of cell division.
This includes:
–1.Mitosis, the division of the cell
nucleus, which is made up of four
segments including prophase,
metaphase, anaphase, and telophase.
–2.Cytokinesis, or the division of
cytoplasm.
Stop at 1
minute
G1 phase
M phase
S phase
G2 phase
Mitosis
There are four phases in mitosis:
1.
Prophase
a.
Longest phase in mitosis (take 5060% of total time mitosis requires)
b.
Chromosomes become visible
because they are condensed
c.
Centrioles become visible on opposite
sides of the nucleus
i.
ii.
iii.
The centrioles help organize the spindle, a
structure made of microtubules that helps
separate the chromosomes
Chromosomes attach to the spindle
fibers near the centromere
Plant cells do not have centrioles but do
have mitotic spindles
d.
Nucleolus disappears
e.
Nuclear envelope breaks down
2. Metaphase
a.
Chromosomes line up in the center of
the cell
b. Microtubules connect to the
centromeres
3. Anaphase
a.
b.
c.
d.
Centomeres split and the sister
chromatids separate
The sister chromatids become
individual chromosomes
Chromosomes move and
separate into two groups
near the spindle
Anaphase ends when the
chromosomes stop moving
4. Telophase
a. Chromosomes change from being condensed
to dispersed
b. A nuclear envelope forms around each cluster
of chromosomes
c. Spindle breaks apart
d. Nucleolus is visible in each daughter nucleus
Telophase in the
midbodies of two
daughter cells
Start at 1 minute
Cytokinesis
• Cytokinesis occurs within the cytoplasm of one cell.
• Cell division is complete when the cytoplasm
•
divides (pinches together to create new cells).
In plants, a structure called the cell plate forms
between the two daughter nuclei. The cell plate
develops into a cell membrane and cell wall.
Cytokinesis
• In animal cells, the cell membrane is
drawn inward until the cytoplasm is
pinched into two equal parts. Each part
has a nucleus and cytoplasmic organelles.
The cleavage of
daughter cells is
almost complete; this
is visualized by
microtubule staining
Spindle
forming
Centrioles
Nuclear
envelope
Chromatin
Centromere
Centriole
Chromosomes (paired
chromatids)
Interphase
Prophase
Cytokinesis
Centriole
Telophase
Nuclear envelope
reforming
Spindle
Individual chromosomes
Anaphase
Metaphase
Meiosis
Chromosomes
• You have 23 different
pairs of
chromosomes, for a
total of 46
chromosomes.
• One chromosome in
each pair came from
your mother and one
from your father.
Each chromosome in
a pair is said to be
homologous,
meaning that the
chromosome from
the father has a
corresponding
chromosome from
the mother.
• Cells that contain both sets of homologous
chromosomes are called diploid.
• All of your cells except the sex cells (sperm
and eggs; also called gametes) are diploid.
Gametes are haploid, meaning they contain
only one copy of each chromosome.
• When one sperm and one egg combine their
DNA, there are two versions of each
chromosome.
haploid
haploid
diploid
We use “N” to represent the haploid number of
chromosomes and “2N” to represent the
diploid number of chromosomes.
– 1.
For humans, the haploid number is
23. We write this as N = 23. The
diploid number is 46, which we write
as 2N = 46.
– 2.
In fruit flies, N = 4 and 2N = 8.
Meiosis
Meiosis is the process that divides one diploid
(2N) cell to form four haploid (N) cells.
This process is a reductional division because the
number of chromosomes per cell are cut in half.
Meiosis is how gametes are formed.
There are two divisions that occur in meiosis:
Meiosis I and Meiosis II.
– 1.
– 2.
– 3.
Before meiosis I begins, cells go through
Interphase I.
This involves DNA replication, forming a
duplicate copy of each chromosome.
Each chromosome is made of two sister
chromatids.
centrioles
Nucleus with
duplicated DNA
that is not
condensed.
cell
Meiosis I is similar to mitosis.
a.
Prophase I:
– i.
– ii.
– iii.
Centrioles are visible on opposite
sides of the nucleus
Nucleolus disappears
Nuclear envelope breaks down
spindle
centrioles
DNA condensed as
chromosomes
iv.
Homologous chromosomes pair
1. When a pair of chromosomes aligns a
tetrad is formed.
2. When chromosomes form a tetrad, they
exchange portions of their chromatids
in a process called crossing-over.
3. Crossing-over produces new combinations
of DNA.
tetrad
• b.
Metaphase I:
i.
Spindle fibers attach to the chromosomes at
the centromere
spindle
centromere
centrioles
chromosomes (DNA)
c.
Anaphase I:
i.
Spindle fibers pull the homologous
chromosomes toward opposite ends of the cell.
ii. Chromosomes move and separate into two
groups near the spindle
iii. Anaphase ends when the chromosomes
stop moving
spindle
centrioles
chromosomes
(DNA)
– d.
• i.
•
e.
Telophase I
Nuclear membranes form around
chromosomes
ii.
Daughter nuclei form
Cytokinesis
i.
Cytoplasm divides
Nucleus
centrioles
chromosomes
(DNA)
Nucleus
Meiosis I produces two haploid (N)
daughter cells that have only one copy
of each chromosome. Each
chromosome is made of two sister
chromatids.
Meiosis II
After Meiosis I, cells enter Meiosis II.
No DNA replication occurs between
Meiosis I and Meiosis II.
Meiosis II separates the sister
chromatids.
a.
Prophase II:
i. Centrioles are
visible on opposite
sides of the nucleus
centrioles
ii. Nucleolus
disappears
iii. Nuclear envelope
breaks down
chromosome
b.
Metaphase II:
• i. Chromosomes
align like they do
in mitosis
• ii. Chromosomes
centrioles
spindle
are attached to
the spindle at the
centromere.
chromosome
c. Anaphase II:
Sister
chromatids
separate and
move towards
opposite ends
of the cell.
centrioles
spindle
chromosome
d.
Telophase II:
– i.Nuclear membranes
form around
chromosomes
– ii. Daughter nuclei
form
e.
Cytokinesis:
centrioles
nucleus
– i. Division of the
cytoplasm
f.
Meiosis II
produces four
haploid (N)
daughter cells
chromosome
Gamete formation
In male animals, the haploid gametes are
called sperm.
In female animals, the haploid gametes
are called eggs.
Comparing mitosis and meiosis
• Mitosis produces two genetically identical
diploid (2N) cells. Mitosis allows an
organism’s body to grow and to replace cells.
In organisms that reproduce asexually, new
organisms are produced by mitosis.
• Meiosis produces four genetically different
haploid (N) cells. Meiosis produces gametes
for use in sexual reproduction.