Download Interphase Prophase

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The “dance” that chromosomes go through when
cells divide is known as. . .
Prophase
Chromosomes become visible as strands; the nuclear
membrane usually disappears.
Interphase
When a cell isn’t dividing, the nucleus usually just looks
grainy, and chromosomes are present but not visible.
At the same time, two tiny organelles (the centrosomes)
produce a set of fibers called the mitotic spindle. These
are hard to see with ordinary microscopy, but special
fluorescence techniques show them clearly (spindle fibers
in green, chromosomes in blue)
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Metaphase
Chromosomes line up in center, along metaphase plate,
moved into place by the spindle fibers.
Anaphase
Chromosomes break apart at the centromeres, and the
separate chromatids move to opposite sides of the cell
The spindle fibers are again hard to see with regular light
microscopy, but here's a fluorescent picture. . .
and another fluorescent micrograph. . .
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Telophase. . . Chromosomes cluster into new nuclei and lose
individuality; new nuclear membranes form.
. . . and Cytokinesis
Usually, the cell itself divides at this time—a process
called cytokinesis. (Occasionally this doesn't happen, and
then you have a cell with two nuclei.)
And here's the spindle fibers again. . .
Cytokinesis (division
of the whole cell) is
different in plant cells,
which can't "pinch
apart" because of the
cell wall. Instead, a
new cell wall (faintly
visible here) forms to
divide the cell.
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Chromatids
We now divide a cell's life into the cell cycle. Interphase is
divided into G1, or growth (the cell grows), S, or synthesis
(the cell creates new chromatids) and G2 (the cell grows
some more and prepares for mitosis).
• At the start of mitosis, each chromosome in a cell
consists of two chromatids.
• At the end, each chromosome consists of only one
chromatid.
• But if you watch the daughter cells until they
divide again. . . each daughter cell will start
mitosis with two chromatids per chromosome.
• Somewhere during interphase, each chromosome
must somehow make a new chromatid.
Significance
Thought mitosis was bad? It gets worse. . .
• Mitosis happens in. . .
– reproduction of single-celled eukaryotes
– development (from a fertilized egg to an adult)
– physical growth
– maintenance
• Example: Every day, 50-70 million cells in your
body die, and must be replaced by new cells
– wound repair
– cancer (uncontrolled mitosis) 4
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Chromosomes
from a normal
human female.
There are 46
chromosomes,
which can be
sorted by size,
centromere
position, and
band pattern
into 23 pairs,
numbered 1-22
and X. (More
later on what X
means—just
know now that
there are 23
pairs.)
Meiosis
• To understand meiosis, you need some more
background on chromosomes. . .
• Chromosomes in a cell are not all identical
– Some are long, some are short
– Some have the centromere near to the actual center
(centric) and some have the centromere "off-center",
nearer to one end (acentric)
– Chromosomes differ in the pattern of bands on each one
(that can be seen with special staining techniques)
• It turns out that in a typical eukaryotic cell, there
are two of each type of chromosome.
– A cell with two of each type of chromosome is said to
be diploid.
Different animal and
plant species have
different numbers of
chromosomes— mice, for
example, have 40
chromosomes, making up
20 pairs per cell. But
although closely related
organisms usually have
similar chromosome
numbers, there's no
simple relationship
between how "complex"
an organism is and how
many chromosomes per
cell it has.
Gametes
• Animals, plants, and many other eukaryotes form
specialized cells for sexual reproduction called
gametes
– Usually these come in two forms: large ones (eggs) and
small ones (sperm)
– In many protists and fungi, all the gametes are the same
size, and can't be divided into eggs and sperm (they might
be designated + and –, or something like that)
• Gametes only have one of each pair of
chromosomes.
– This is called being haploid.
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Meiosis
• Meiosis is the specialized process of cell division
that produces haploid cells from diploid cells.
So here's a human egg cell being swarmed by a
number of human sperm cells. Both the egg cell and
each sperm cell have one of each pair of human
chromosomes: 23 chromosomes each.
– In animals, this is how eggs and sperm are produced.
(With plants, things get a little strange from our point of
view; maybe I can talk about that later.)
– The basics were worked out by the Belgian scientist
Edouard van Beneden in 1883.
• Another name is reduction division— because
unlike mitosis, meiosis has to reduce the number
of chromosomes per cell by half
When an egg and one sperm fuse together, they form
a cell known as a zygote, which now has two of each
chromosome—in this case 46 chromosomes.
And that zygote will divide by mitosis into 2, 4, 8, 16,
32, . . . , 100 trillion cells—each with 46 chromosomes.
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The result is that you
received one of each
pair of chromosomes
from your mother, and
the other one of each
pair from your father. . .
and you'll pass on one of
each pair of your
chromosomes to each of
your children.
An error in meiosis can
cause an egg or sperm to
have two copies of one
chromosome, or to be
missing a chromosome.
One result of this is
shown here: the
chromosomes of
someone with three
copies of chromosome
21 (two from one parent
and one from the other).
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