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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 –
Cell division → growth
Cell division plays a role in:


mitosis
Cells differentiate
•
Growth and replacing cells
Producing gametes (egg and sperm)

How are chromosomes passed
on 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.
The “daughter” cells
receive half the normal
number of
chromosomes
A parent cell duplicates its
chromosomes
The nucleus divides
The cytoplasm divides → 2
daughter cells
Each daughter cell receives a
complete 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
Human Chromosomes

Our cells have 46
chromosomes

23 pairs
called diploid (2n)




Homologous Chromosomes

Sex cells (gametes)
have only 23
chromosomes

called haploid (n)





Chromosomes

Chromosomes come
in matching pairs
Humans have
22 pairs of matching
chromosomes
Plus two sex
chromosomes, X and
Y.
So we have 46
chromosomes
This is a human karyotype
Chromosomes
Found in the cell nucleus
Genes are located on
chromosomes
Each chromosome contains
one very long DNA molecule


Typically bears thousands of
genes
What happens to chromosomes
when a cell divides?
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
Body cell
DNA
replication
Diploid
(2n)



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
Chromosomes
duplicate
(4n)
Each new cell receives a
complete set of chromosomes
How does this happen?
Chromosomes
divide
(2n)
What’s happening in the
nucleus?
Early in Mitosis

2.
The spindle fibers attach to the centromeres

Two new nuclei form and the
cytoplasm divides
Later in mitosis
Spindle fibers pull
the sister chromatids
apart, towards
opposite ends of the
cell.

Nucleus of each daughter cell
forms




Chromosomes form
two new nuclei
Cytokinesis


Each duplicated chromosome appears as 2
identical sister chromatids joined together
Spindle fibers connect to
each chromosome
They pull the chromosomes
apart
1.

The DNA condenses into visible
chromosomes

At the beginning of mitosis:
1.
Chromosomes condense
2.
Nuclear membrane
disintegrates
3.
Mitotic spindle forms
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

The nuclear membrane
reappears.
The chromosomes uncoil
The spindle disappears
Cytokinesis - the cytoplasm
begins to divide

A new cell membrane forms
Examples of mitosis
This remarkable process occurs during
growth of the human fetus
The Cell Cycle

Mitosis

Events that take
place within the cell
between one cell
division and the next
Two distinct phases:


Interphase (cell
growth)
Mitosis (cell division)
Interphase (growth phase)
Interphase –
Interphase –
a period of normal cell activity

Mitosis

The cell spends
90% of its time in
interphase
Period of cell growth


The cell makes new
molecules and
organelles
The chromosomes
are replicated during
interphase
a period of normal cell activity

Interphase sets the
stage for cell division


Chromosomes are
duplicated
Note: chromosomes are
not visible in the cell’s
nucleus

They are loosely packed
fibers
Interphase (growth phase)
Cell cycle control

G1 checkpoint



= Cell cycle checkpoints

Are proper growth factors
present?
Is DNA damaged?
G2 checkpoint


Apoptosis
Has DNA replicated
properly?
M checkpoint

Has the spindle
assembled properly?

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
How do different kinds of cells
arise?
What Is Cancer?

Cancer is a disease of
the cell cycle.

Cancer cells do not
respond normally to the
cell cycle checkpoints

All cells contain
the same DNA,
so what makes
them different?
= Cell cycle checkpoints
Different types of cells express
different combinations of genes
Gene activity is regulated

When cells differentiate


Certain genes are turned
on and off.
Cells become specialized

Fertilized
egg
Blastula

Muscle cells make
contractile proteins (actin
and myosin)
Blood cells make
hemoglobin
Genes
Glycolysis
enzymes
+
+
+
Hemoglobin
–
–
+
Myosin
+
–
–
Insulin
–
–
–
Meiosis:
Meiosis –
it only happens in sex cells
a specialized kind of cell division

Humans are diploid
organisms.



Our cells contain two
sets of chromosomes
(2n).
Our gametes are
haploid, only have one
of each chromosome
How does this happen?

Cells divide by meiosis
only for the production
of gametes (eggs and
sperm).


Takes place only in the
ovaries or testes
Gametes are haploid

Have half the number of
chromosomes
Meiosis: making gametes
Comparing Mitosis and Meiosis
n
2n
Mitosis
4n

In males, meiosis typically
produces 4 sperm

Formation of gametes
Occurs in body cells

Occurs in sex cells

1 cell division

2 divisions


Growth and repair of
cells

Chromosomes duplicate once but divide two times
Meiosis plays a key role in
sexual reproduction
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 and Fertilization


Why is SEX important?

Meiosis halves the number
of chromosomes
 each gamete is haploid (1n)
In fertilization, the gametes
fuse to form a zygote, which
contains two copies of each
chromosome
 diploid or 2n
Meiosis produces just one
egg with all the cytoplasm,
plus small polar bodies.
1n
Sexual reproduction has an
enormous impact on how species
evolve – it generates new
genetic combinations
Gametes

2n
This genetic variability is
produced by

1n
Fertilized cells carry
chromosomes from
BOTH parents


Crossing over
Random alignment of chromosomes
during meiosis
Random fertilization

1000s of sperm to 1 egg
Random Alignment of
Chromosomes
What is crossing over?
sister chromatids

Chromosome pairs align randomly during
meiosis.
The chromosomes
are “shuffled”
Crossing-over
Synapsis
of homologues



Chromatids
after
exchange
Recombinant
daughter
chromosomes
Homologous chromosomes exchange parts
This produces hybrid chromosomes and
Increases genetic variability
Accidents During Meiosis
Random Fertilization

The human egg is
fertilized by one
sperm, leading to
genetic differences
in the offspring.


Change in
chromosome number
In nondisjunction,


The members of a
chromosome pair fail to
separate
Produces gametes with
an incorrect number of
chromosomes.
1000s of sperm to 1 egg
Down Syndrome:
An Extra Chromosome 21
Obtaining fetal chromosomes


Amniocentesis

trisomy
Down Syndrome is also called trisomy 21.
Chorionic villi sampling
Remove fetal cells
from the amniotic fluid
or placenta
Check the karyotype to
see if there are any
chromosomal
abnormalities