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Cellular reproduction
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Recall that the cell theory states that
all cells come from preexisting cells.
Cell division is the process by which
new cells are produced from one cell.
The discovery of
chromosomes:
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Early biologist observed that just
before cell division, several short,
stringy structures suddenly appeared
in the nucleus.
Scientists soon noticed that these
structures seemed to vanish soon after
division of a cell.
The discovery of
chromosomes continued:
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These structures:
– Contained DNA
– Become darkly colored when stained
– Are called chromosomes.
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Scientists learned that chromosomes
are the carriers of the genetic material
that is copied and passed from
generation to generation of cells.
The structure of
eukaryotic chromosomes
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For most of a cell’s lifetime,
chromosomes exist as chromatin, long
strands of DNA wrapped around
proteins called histones.
Under a microscope chromatin looks
like beads on a string.
Chromatin: DNA wrapped
around histones:
Nucleosome
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The “beads” of the chromatin are a
group of 8 histones. We call this
group of histones the nucleosome
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Before a cell can divide, the long
strands of chromatin must be
reorganized. (just as you would coil a
long strand of rope before storing it.
As the nucleus begins to divide,
chromosomes take on a different
structure in which the chromatin
becomes tightly packed.
The Cell cycle
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Many events in nature follow a cyclical
pattern:
– Night follows day
– Fall follows summer
– Low tide follows high tide
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The cell also follows a cyclical pattern.
It is called the cell cycle.
Cell cycle
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The cell goes through 2 general
periods:
– A period of growth
– A period of division
An overview of
Interphase
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The majority of a cell’s life is spent in
the growth period
This period is known a interphase.
During interphase:
– A cell grows in size and carries on
metabolism
– Chromosomes are duplicated in
preparation for the period of division.
An overview of mitosis
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Mitosis
– is the period of nuclear division in a cell
– Follows interphase (the growth stage)
– This process produces two daughter cells.
Each daughter cell contains a complete
set of chromosomes.
cytokinesis
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Interphase and mitosis make up the
bulk of the cell cycle.
Following mitosis, the cytoplasm
divides, separating the two daughter
cells. This is called cytokinesis
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Figure 2: Different levels of DNA condensation. (1)
Single DNA strand. (2) Chromatin strand (DNA with
histones). (3) Condensed chromatin during interphase
with centromere. (4) Condensed chromatin during
prophase. (Two copies of the DNA molecule are now
present) (5) Chromosome during metaphase.