Cell Division

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Cell Division Knit of Identity, Source of
Distinction
Question How do cells conserve genetic
information?
Corollary Why care?
Question How do cells reduce and rearrange
genetic information?
Corollary Why care?
2
Mitosis and Meiosis
Mitosis retains genetic information because it
maintains chromosome number through nuclear
division.
Meiosis reduces and rearranges genetic
information because it rearranges chromosomes and
reduces their number through nuclear division.
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Mitosis and Meiosis
Meiosis is used to produce sex cells (gametes),
the thread of life that connects generations.
Mitosis is used for all other cell divisions.
And there is plenty of cell division about
4,000,000 per second in each of us.
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So, Whats a Chromosome? (and why care?)
A human chromosome at metaphase
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So, Whats a Chromosome (and why care)?
Chromosomes are structures that organize the
genetic material (DNA).
Each chromosome contains one (G1 phase) or two
(G2 phase) immensely long molecules of DNA
wrapped with proteins.
The DNA molecule is twisted and turned, but if it
were stretched out, one would find a linear array
of genes extending from one end to the other.
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So, Whats a Chromosome (and why care)?
If you take care of your chromosomes, youll take
care of your DNA (i.e. your genetic blueprint).
Not a bad idea.
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Packing Up DNA into Chromatin into Chromosomes
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The Link Between DNA and Chromosome Replication
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Chromosome Anatomy
A few key points -
The chromosome shown on the right is seen for a
fleeting period during the cell division cycle.
The rest of the time the DNA is much less tightly
packed (condensed).
The most confusing point - a chromosome can have
one chromatid or two, but in either case its
still one chromosome.
Chromosomes come in matched pairs (homologous
pairs).
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Cell division requires coordinated division of
chromosomes (mitosis) ..
and division of the cytoplasm (cytokinesis).
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Coordination of DNA Replication, Mitosis and
Cytokinesis in Cell Division
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Cell Division is Cyclic
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Mitosis a Knit of Identity
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Mitosis in Action
Blue shows DNA, green shows spindle fibers.
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Meiosis A Source of Distinction
Why do you share some but not all characters of
each parent?
What are the rules of this sharing game?
At one level, the answers lie in meiosis.
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Meiosis A Source of Distinction
Meiosis does two things -
1) Meiosis takes a cell with two copies of every
gene (a diploid 2n) and makes cells with a
single copy of every gene (a haploid 1n).
This is a good idea if youre going to combine
two cells to make a new organism. This trick is
accomplished by halving chromosome number (2n ?
1n).
2) Meiosis scrambles the specific forms of each
gene that the sex cells receive.
This makes for a lot of genetic diversity. This
trick is accomplished through independent
assortment and crossing-over.
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Meiosis With a Single Chromosome Pair
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Meiosis I the first and most significant cell
division of meiosis.
19
Meiosis II finishing business in the second
cell division of meiosis.
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One way meiosis makes lots of different sex cells
(gametes) Independent Assortment.
Independent assortment produces 2n different
possible gametes, where n the number of unique
chromosomes. In humans, n 23 and 223
6,000,0000.
Thats a lot of diversity by this mechanism alone.
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Another way meiosis makes lots of different sex
cells (gametes) Crossing-Over.
Crossing-over multiplies the already huge number
of different gamete types produced by independent
assortment.
Crossing over scrambles genes because each one of
the paired chromosomes often contain different
forms of the same gene.
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The Fundamental Distinction Between Mitosis and
Meiosis is in Chromosome Pairing and Alignment
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Boy or Girl? The Y Chromosome Decides.
The sex chromosomes are paired in females (XX)
and unpaired in males (XY).
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Spermatogenesis One path of gametogenesis
following meiosis
Men are busy meiosis produces roughly
250,000,000 sperm per day.
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Oogenesis Another path of gametogenesis
following meiosis
Woman are less busy in meiosis (but at least as
important) than men meiosis produces only a few
hundred mature eggs over a lifetime.
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What Meiosis is About
A way to create a new and unique individual
through sexual reproduction.