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How does a single cell
become a human being?
Ch 18 – Cell Division
Fertilized
egg
Blastula
Many things happen
•
Number of cells increase by
cell division –
•
Cell division plays a role in:


mitosis
Cells differentiate
Cell division → more cells
Cell reproduction and growth.
Producing gametes (egg and sperm)

Passing on genes from cell to
cell


Cell division → gametes

Producing gametes
is different.


The egg and sperm
are produced by a
special type of cell
division called
meiosis.
Sexual reproduction
requires fertilization
of an egg by a sperm.
A parent cell duplicates its
chromosomes before it divides.
The two resulting “daughter” cells
are genetically identical–they
each receive a set of
chromosomes.
Two kinds of cell division

Mitosis



For growth and repairing or replacing cells
Occurs in body (somatic) cells
Meiosis


Only used for reproduction–for making
sperm and eggs
Occurs only in germ-line cells in the ovary
or testes
Homologous Chromosomes


Chromosomes come
in matching pairs
Humans have



DNA packing in chromosomes


22 pairs of matching
chromosomes
Plus two sex
chromosomes, X and
Y.

So we have 46
chromosomes
DNA wraps around
proteins called histones
The DNA is packed into
an elaborate, multilevel
system of coiling and
folding.
The long strands of
DNA are condensed
This is a human karyotype
Male or female?
Chromosome duplication
and distribution


Before a cell divides, it
duplicates all of its
chromosomes, resulting
in two copies called
sister chromatids.
When the cell divides,
the sister chromatids
separate from each
other.
The Cell Cycle

Mitosis

Events that take
place within the cell
between one cell
division and the next
Two distinct phases:

Cell division

Interphase (growth)
Mitosis (cell division)
Interphase (growth phase)
Interphase –
a period of normal cell activity
Mitosis


The cell spends
90% of its time in
interphase
Period of cell growth


Interphase (growth phase)
Chromosome are replicated
in interphase
Chromosome
DNA
replication
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The cell makes new
molecules and
organelles
The chromosomes
are replicated during
interphase
Sister chromatids
Centromere
chromosome
Duplicated
chromosome
Early in Mitosis
1.
2.
2.
The DNA condenses into visible
chromosomes

Chromosomes
condense
Mitotic spindle
forms
1.
3.
Early in Mitosis

The spindle microtubules attach to the
centromeres

Centrosomes move
towards opposite ends
of the cell
Act to pull the
chromosomes apart
Each duplicated chromosome appears as 2
identical sister chromatids joined together
Nuclear membrane
disintegrates
The mitotic spindle forms
centrioles
in centrosome
chromosomes
spindle
fibers
aster
Later in mitosis


Separation of the
chromosomes
Spindle fibers pull
the sister chromatids
apart, towards
opposite ends of the
cell.
The spindle fibers attach to the sister chromatids
Finally the nucleus reforms
and cytoplasm divides

Nucleus of daughter cells
forms




The nuclear membrane
reappears.
The chromosomes uncoil
The spindle disappears
Cytokinesis - the cytoplasm
begins to divide

Cytokinesis


Is the division of the
cytoplasm
End up with 2 new
daughter cells, each
with a copy of the
original DNA
A new cell membrane forms
Nuclear division & cytokinesis 0:19
http://www.youtube.com/watch?v=rgLJrvoX_qo&NR=1
Cell cycle control

G1 checkpoint

= Cell cycle checkpoints
Has the spindle
assembled properly?

Cells growing
out of control
They lose
control over cell
division
They lose control over
mitosis
Cancer is a disease of
the cell cycle.

M checkpoint
What Is Cancer?


Has DNA replicated
properly?

Cells growing out of
control

G2 checkpoint



Are proper growth factors
present?
Is DNA damaged?


What Is Cancer?
= Cell cycle checkpoints
Cancer cells do not
respond normally to the
cell cycle checkpoints
What causes Cancer?


Damage to genes that
regulate the cell cycle
Mutations damage DNA

Some result from chemical or
environmental exposure



Cigarette smoke
UV rays
Some viruses also damage
DNA

HPV virus causes cancer of
the cervix
3D Medical Animation - What is Cancer? 1:06
http://www.youtube.com/watch?v=LEpTTolebqo&NR=1&feature=fvwp
Apoptosis


If a cell has an error in its
DNA that cannot be repaired,
it may undergo programmed
cell death (apoptosis)
Apoptosis


removes genetically damaged
cells that could lead to cancer
plays an important role in the
development of the embryo
Meiosis
The basis of sexual reproduction
Meiosis –
it only happens in sex cells
Meiosis: the special cell division that
precedes sexual reproduction

Humans are diploid
organisms.


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Our cells contain two
sets of chromosomes
(2n).
Our gametes are
haploid, having only one
of each chromosome
How does this happen?

Cells divide by meiosis
only for the production of
gametes (eggs and
sperm).


produced only in the gonads
(ovaries or testes).
Almost all cells are diploid
(2n), except for gametes
(haploid, n)
How meiosis halves
chromosome number
Review: Comparing Mitosis
and Meiosis
Mitosis

Meiosis plays a key role in
sexual reproduction

In males, meiosis typically
produces 4 sperm
Growth and repair of
cells

Formation of gametes

Occurs in body cells

Occurs in sex cells

1 cell division

2 divisions

Chromosomes duplicate once, but the cell divides twice
Meiosis
Results in 2 diploid (2n)
genetically identical
cells

Results in 4 haploid (n)
genetically different cells
In females,
Germ-line
cell
DNA
replication

Germ-line cell
Meiosis produces just one
ovum (egg), plus small polar
bodies.
Fertilization –
the union of an egg and a sperm

Meiosis halves the number

of chromosomes
 each gamete is haploid (1n)
In fertilization, the gametes
fuse. Fertilized cells carry
chromosomes from both
parents


Why is SEX important?

1n
new genetic combinations
Gametes

2n

contain two copies of each
chromosome

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Crossing-over
Synapsis
of homologues
Chromatids
after
exchange
Chromosome pairs align randomly during
metaphase of meiosis.
Recombinant
daughter
chromosomes
Homologous chromosomes exchange parts
This changes the genetic information in each
chromosome and
Increases genetic variability
The chromosomes
are shuffled
Accidents During Meiosis
Random Fertilization

The human egg cell
is fertilized randomly
by one sperm,
leading to genetic
differences in the
offspring.

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Change in
chromosome number
In nondisjunction,

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1000s of sperm to 1 egg
1000s of sperm to 1 egg
Random Alignment of
Chromosomes


Crossing over
Random alignment of chromosomes
during metaphase
Random fertilization
diploid (2n)
sister chromatids

This genetic variability is
produced by

1n
What is crossing over?

Sexual reproduction has an
enormous impact on how species
evolve because it generates
The members of a
chromosome pair fail to
separate during
anaphase.
Produces gametes with
an incorrect number of
chromosomes.