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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?
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.
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.
So, What’s a Chromosome?
(and why care?)
A human
chromosome at
metaphase
So, What’s 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.
So, What’s a Chromosome
(and why care)?
If you take care of your chromosomes,
you’ll take care of your DNA (i.e. your
genetic blueprint). Not a bad idea.
Packing Up – DNA into Chromatin into Chromosomes
The Link
Between DNA
and
Chromosome
Replication
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).
telomere
Cell division requires coordinated division of
chromosomes (mitosis) …..
…… and division of the
cytoplasm (cytokinesis).
Cytokinesis in animals
Coordination of
DNA
Replication,
Mitosis and
Cytokinesis in
Cell Division
Cell
Division is
Cyclic
The cell cycle
Mitosis – a Knit of Identity
Mitosis in Action
Blue
shows
DNA,
green
shows
spindle
fibers.
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.
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 you’re 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.
Meiosis With a Single Chromosome Pair
Diploid
Reduction of
Chromosome Number
Haploid
Meiosis I – the first and most significant cell
division of meiosis.
Reduction of Chromosome Number
Meiosis II – finishing business in the second cell division of
meiosis.
2 chromatids/chromosome
1 chromatid/chromosome
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.
That’s a lot of diversity by this
mechanism alone.
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.
The Fundamental Distinction Between Mitosis and
Meiosis is in Chromosome Pairing and Alignment
Mitosis
Meiosis
Boy or Girl? The Y Chromosome Decides.
The sex
chromosomes
are paired in
females (XX)
and unpaired in
males (XY).
Spermatogenesis – One path
of gametogenesis following
meiosis
Men are busy – meiosis
produces roughly
250,000,000 sperm per day.
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.
What Meiosis is About
A way to create a new and unique individual through sexual reproduction.