Mitosis and Meiosis
Specialized diploid cell
splits in half twice and
produces four separate
cells
Mitosis
Cells are able to
reproduce themselves Reproduction allows
organisms to develop,
grow, heal, and stay
alive
DNA is organized into chromosomes
Mitosis allows one cell with 46 chromosomes to split into two
genetically identical cells
Cells with 46 chromosomes are
called diploid cells (two sets)
Cells with 23 chromosomes
are called haploid cells
Somatic cells have 46
chromosomes grouped into 23
pairs
Mitosis stages:
Metaphase
Anaphase
Prophase
Telophase
Interphase During interphase, the "strings" of DNA are messy and are called
chromatin
Cells in between episodes of mitosis (mostly growing)
During mitosis, a set of
protein cylinders next to the
nucleus (called centrosome)
duplicates itself Centrosomes regulate everything moved around during mitosis
As mitosis occurs, the DNA begins to multiply itself and the cell
ends up with two copies of every strand of DNA
After all that, the cell enters the prophase as the
"mess" of chromatin coils upon itself to produce
thick strands of DNA wrapped around proteins
These are called chromosomes
The duplicates stay attached to the original in
a shape that resembles an "X"
These are called the chromatids and
they are made up of a left and arm
and a right leg and arm
The meetup in the middle
is called the centromere
As the chromosomes form, the nuclear envelope disintegrates and
the centrosomes go to the opposite ends of the cell and leave behind
a trail of protein ropes called microtubules
Microtubules help provide a structure to the cell
Longest phase of mitosis (up to 20 minutes)
Chromosomes attach
themselves to the ropey
microtubules at their
centromeres
Chromosomes are moved around by molecules called motor proteins
(two of these proteins on each side of the centromere → centromere-
associated protein E)
At the same time, a protein called dynein is pulling the other ends of
the ropes near the cell membranes
After being moved around, the chromosomes line up in the middle of
the cell (down)
Motor proteins start pulling
really hard on the ropes so
the X-shaped chromosomes
split into their individual
chromosomes
As they are detached
from one another, they
are dragged toward
each end of the cell
Each of the new cell's
structures is reconstructed
Nuclear membrane reforms and
nucleoli form within them +
chromosomes go back into chromatin
Crease forms between the
new cells and signals the
final split
The division between the cells is called cleavage
Cytokinesis
Two new nuclei move apart and the cells separate
Daughter cells
Clones/genetic copies of the original cell
These cells are genetically different from each other
Meiosis goes through the
four stages of mitosis but
does it twice
Second round includes diploid cells called primary oocytes or
primary spermatocytes (depending on the kind of gamete they
make)
Alleles
Variations of the same genes
Homologous
chromosome pairs
Pairs of chromosomes
Sperm and
egg cells
Only have 23 chromosomes
Need each other to combine and complete the 46
chromosomes
Haploid cells
Prophase I
Same as mitosis
Each double
chromosome has two
chromatids
Prophase I
includes two
additional steps
Crossover Each double chromosome
lines up next to its
homolog (mother's version
lined up next to father's)
One chromatid from each X
gets tangled up with the other X
in a process called crossover
Homologous recombination
During crossover, the
chromosomes trade
sections of DNA
Genetic information being traded is equivalent
(both hair color or both eye color - but never
eye color WITH hair color, for example)
All four chromatids are now different
Each chromatid will end up in a different sex cell later on
There is one pair of chromosomes that
doesn’t always go through the crossover or
recombination
That's the 23rd pair and the sex chromosomes
Female: two X chromosomes (which can do the
crossover and recombination since they are the
same)
X and Y chromosomes are not homologous
(don't match up!)
Metaphase I
In meiosis, each chromosome lines up
next to its homologous pair-partner (that
it's already swapped genes with)
Anaphase I
Homologous pairs get
pulled apart and migrate to
either end of the cell
Telophase I
Pretty much the same as mitosis
End of telophase I marks the end of round one
Leaves us with two haploid cells, each with 23 double
chromosomes with new, unique combinations
Centromeres still look like X's
The next round begins and the process is the exact same as
mitosis except the aim is different
Not to duplicate the double chromosomes
but to pull them apart and separate them
into single-strand chromosomes
No DNA replication involved in Prophase II
Instead, the DNA just clumps up
again into chromosomes and the
infrastructure for moving them, the
microtubules are put back into place
In metaphase II, the chromosomes are
moved into alignment into the middle of
the cell
In anaphase II, the chromatids are pulled apart into single
chromosomes
Chromosomes uncoil into chromatin and the crease-
forming cleavage in the final separation of cytokinesis
mark the end of Telophase II
