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How does a single cell
become a human being?
Cell division

Every time a cell
divides, a copy is
made of all the DNA
in every chromosome
Fertilized
egg
Blastula
Many things happen
•
Number of cells increase by
cell division –
Cells differentiate
•
Cell division → growth



The chromosomes are
copied
The nucleus divides
Result: Each new cell
receives a complete set of
chromosomes.
Cell division → gametes

Producing gametes is
different.


Mitosis
Two kinds of cell division

Mitosis




For growth and repairing or replacing cells
Occurs in body (somatic) cells

Only used for reproduction–for making
sperm and eggs
Occurs only in specialized cells in the
ovary or testes
The egg and sperm are
produced by a special
type of cell division
called meiosis.
The “daughter” cells
only receive half the
normal number of
chromosomes
Human Chromosomes
Our cells have 46
chromosomes

Meiosis

mitosis


Chromosomes in pairs
called diploid (2n)
Sex cells (gametes)
have only 23
chromosomes

called haploid (n)
Homologous Chromosomes


Chromosomes come
in matching pairs
Humans have



Chromosomes


22 pairs of matching
chromosomes
Plus two sex
chromosomes, X and
Y.

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So we have 46
chromosomes
Typically bears thousands of
genes
This is a human karyotype
What happens to chromosomes
when a cell divides?
Chromosomes


Are a combination
of DNA and protein
molecules.
Are not visible in the
nucleus until they
condense before
cell division.



Mitosis:
growth and replacing cells
DNA
replication
Chromosomes
duplicate
(4n)
Each new cell receives a
complete set of chromosomes
How does this happen?
A copy is made of each chromosome before cell division begins
During cell division, the chromosomes are distributed to the
daughter cells
Each daughter cell ends up with a complete set of chromosomes
What’s happening in the
nucleus?
At the beginning of mitosis:
1.
Chromosomes condense
2.
Nuclear membrane
disintegrates
3.
Mitotic spindle forms
Body cell
Diploid
(2n)
Found in the cell nucleus
Genes are located on
chromosomes
Each chromosome contains
one very long DNA molecule
1.
2.
Chromosomes
divide
(2n)
Spindle fibers connect to
each chromosome
They pull the chromosomes
apart
Early in Mitosis
The DNA condenses into visible
chromosomes


Later in mitosis

Each duplicated chromosome appears as 2
identical sister chromatids joined together
The spindle fibers attach to the centromeres


Two new nuclei form and the
cytoplasm divides

Nucleus of each daughter cell
forms




The nuclear membrane
reappears.
The chromosomes uncoil
The spindle disappears
Cytokinesis - the cytoplasm
begins to divide

A new cell membrane forms
Spindle fibers pull
the sister chromatids
apart, towards
opposite ends of the
cell.
Chromosomes form
two new nuclei
Cytokinesis


Is the division of the
cytoplasm
End up with 2 new
daughter cells, each
with a copy of every
chromosome

Identical to the
parent cell
Nuclear division & cytokinesis 0:19
http://www.youtube.com/watch?v=rgLJrvoX_qo&NR=1
Cell division → growth





The chromosomes are copied
They move to opposite ends of
the cell
The nucleus divides
The cytoplasm divides → 2
daughter cells
The Cell Cycle
Mitosis


Events that take place
within the cell between
one cell division and
the next
Two distinct phases:

Each daughter cell receives
a complete set of
chromosomes

Interphase (growth phase)
Interphase (cell growth)
Mitosis (cell division)
Interphase –
Interphase –
a period of normal cell activity

Mitosis

The cell spends
90% of its time in
interphase
Period of cell growth


a period of normal cell activity

Interphase sets the
stage for cell division


The cell makes new
molecules and
organelles
Chromosomes are
duplicated
Note: chromosomes are
not visible in the cell’s
nucleus

The chromosomes
are replicated during
interphase
They are loosely packed
fibers
Interphase (growth phase)
Programmed cell death
Cell cycle control

G1 checkpoint

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G2 checkpoint

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= Cell cycle checkpoints

Are proper growth factors
present?
Is DNA damaged?
Has DNA replicated
properly?
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M checkpoint

Has the spindle
assembled properly?
Meiosis:
What is cancer?
a specialized kind of cell division

Humans are diploid
organisms.

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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
Cells growing out of control
They lose control over cell division

Our cells contain two of
each chromosome (2n).
Our gametes are
haploid, only have one
of each chromosome
How does this happen?
Meiosis –
it only happens in sex cells

Cells divide by meiosis
only to produce
gametes (eggs and
sperm).


Meiosis

DNA is replicated
But there are two cell
divisions

The result?


Takes place only in the
ovaries or testes
Gametes are haploid


Have half the number of
chromosomes
Mitosis

Formation of gametes

Occurs in body cells

Occurs in sex cells

1 cell division

2 divisions

Results in 2 diploid (2n)
genetically identical
cells


Germ-line cell
Results in 4 haploid (n)
genetically different cells
Germ-line
cell
Meiosis plays a key role in
sexual reproduction


End up with just one egg with
all the cytoplasm, plus small
polar bodies.
In males, meiosis
typically produces 4
sperm
DNA
replication
Meiosis → egg production

One diploid cell forms 4
haploid cells
Meiosis → sperm production
Meiosis
Growth and repair of
cells
not one as in mitosis
n
Comparing Mitosis and Meiosis

2n
Meiosis halves the number
of chromosomes
 each gamete is haploid (1n)
In fertilization, the gametes
fuse to form a zygote, which
has two copies of each
chromosome
 diploid or 2n
1n
Gametes
2n
1n
Fertilized cells carry
chromosomes from
BOTH parents
What is crossing over?
Why is SEX important?

sister chromatids
Sexual reproduction has an
enormous impact on how species
evolve – it generates new
genetic combinations

This genetic variability is
produced by

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Crossing over
Random alignment of chromosomes
during meiosis
Random fertilization

1000s of sperm to 1 egg
Crossing-over
Synapsis
of homologues

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Random Alignment of
Chromosomes

Chromatids
after
exchange
Recombinant
daughter
chromosomes
Homologous chromosomes exchange parts
This produces hybrid chromosomes and
Increases genetic variability
Random Fertilization
Chromosome pairs align randomly during
meiosis.

The chromosomes
are “shuffled”
The human egg is
fertilized by one
sperm, leading to
genetic differences
in the offspring.
1000s of sperm to 1 egg
Accidents During Meiosis


Down Syndrome:
An Extra Chromosome 21
Change in
chromosome number
In nondisjunction,

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The members of a
chromosome pair fail to
separate
Produces gametes with
an incorrect number of
chromosomes.
trisomy

Down Syndrome is also called trisomy 21.