Transcript Cell Division and Fertilization

Introduction to Mitosis
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How many cells?
An adult human is made up of about 100 trillion cells.
That’s 100 000 000 000 000 cells!
Everyone started out as just one single cell.
How does one cell become 100 trillion?
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By cell division – called…..
MITOSIS
Parent cell
Compare to first
cell –DNA is copied
nucleus
Nucleus divides
Cytoplasm divides
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This animation will be
explained in more detail
in the slides to come.
For now, just sit back
and watch it.
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Mitosis
Why is mitosis important?
a) For growth –
How else would you go
from being a single
fertilized egg cell to the
young person you are
today?
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b) To replace old/ damaged cells –
- did you know that we shed our skin
cells about every 35 days. But don't
worry, we do not shed all our skin cells at
the same time like snakes do. In human
beings, only the skin cells that are old
are shed, others are not.
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c) To repair damaged
tissue
-when you cut yourself,
new skin cells will grow
to seal the wound.
These new cells come
from preexisting cells
found near the wound
that divided many, many
times. How? BY
MITOSIS, of course!
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Cell division and genetic information
Cells don’t just split in half when they
divide. If they did, important
information would be lost.
Think back to the “house building”
anology. Carpenters, bricklayers,
electricians all need the house
drawings (blueprints). This
information needs to be copied.
Your cells have special molecules
called DNA that carry the cells’ building
instructions. Your DNA
needs to be copied before
it is transferred to the new cells.
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DNA molecules can only be seen in
The real thing
a microscope when they are
looks like this:
supercoiled (condensed) – like
when you take 100 feet of yarn and
wrap it around and around and
Copied DNA strand
(chromosome)
around so that it forms a small ball
(the ball of yarn is in a condensed
form). A DNA molecule that has
been copied and has supercoiled
2 identical DNA strands are held
(condensed) is called a
together by a centromere
chromosome.
Original DNA strand
(chromosome)
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Find the model of a
chromosome provided by
your teacher. Notice the
coiling? Imagine you
could uncoil that molecule
into one long strand.
What does the elastic in
your model represent?
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Cell division and chromosomes
Chromosomes must be accurately
copied and passed on during
cell division.
This is important to make sure
that no genetic information is lost.
Compare the DNA in the nucleus of this cell
with the DNA of a cell in interphase. Do you
notice how the DNA molecules are shorter
and fatter (more visible). We call them
chromosomes now. Did you notice that not
all chromosomes are the same size?
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How many chromosomes would you
expect to count if this was a human skin
cell?
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What is mitosis?
Mitosis begins with a single cell.
How many chromosomes does
this cell contain?
(answer: 4)
original
cell
First the cell makes a copy
of each chromosome…
…then it divides.
cell
division
Each new cell has a full set
of chromosomes and is
identical to the original cell.
2 new cells
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