Chapter 9-10
Mitosis & Meiosis
Chromosomes
• Rod shaped structures made of DNA and
proteins.
DNA wraps around proteins, then keeps coiling until it forms a super
They
are
coil.• The
super
coilthe
coilscell’s
furtherDNA
until ittightly
becomespackaged.
a chromosome.
Chromosome Structure
Chromosomes
are onlyThe
visible
in the cell right
Centromere:
condensed
before cell
division.
area
which holds the two
The rest chromatids
of the time the
cell’s DNA is unraveled
together.
and called chromatin.
Right beforeChromatids:
cell division,The
thetwo
cell’s
DNA of
makes
halves
a copy of itself.
an x-shaped chromosome.
The chromatids
are
two
copies
X-shaped chromosomes,
like
this
one
are really
SAME
THING
TWO copiesofofthe
theEXACT
DNA held
together.
Chromosome Numbers
Each species has a specific number of
chromosomes in their cells.
Two Main Types of Chromosomes
1. Sex Chromosomes – Those that determine
gender XX = Female
XY = Male
2. Autosomes – Those chromosomes that do
not determine gender.
Chromosome Number continued
The two copies of each chromosome are
called HOMOLOGOUS
IndividualsCHROMOSOMES.
formed
sexual are the same
Homologousthrough
chromosomes
reproduction have TWO
size, thecopies
same of
shape
and carry genes for
each autosome
the same traits.
I’m from
the Dad!
I’m from
the Mom!
One copy
from dad
One copy
from mom
Chromosome Number continued
• Karyotype:
– A picture of the chromosomes in a dividing cell.
– They pair up the homologous chromosomes.
What is the
– Thegender
only chromosomes
which don’t have a
of
homologous
pair
are
the
two
sex
chromosomes.
this
individual?
Chromosome Number continued
• Diploid and Haploid Cells
– Not all cells in your body will have two copies of
Ifeach
egg chromosome
and sperm weren’t haploid, when
– the
Diploid
– (diduring
= two) both
chromosomes
two(2n)
fused
fertilization,
the of
each homologous pair
resulting baby would have 46 + 46 = 96
– Haploid (n) – Having only one chromosome of
chromosomes!
each homologous
pair
This
would
be a bad thing.
– Humans
have
46 chromosomes
in 23 pairs.
way, the
resultingcells
baby
will
23
–This
Gametes
(reproductive
like
egghave
and sperm)
only have
23 =chromosomes.
+ 23
46 chromosomes!
Cell Division
• Prokaryotic (Bacterial) Cell Division
– Bacteria just have ONE chromosome.
– They copy their chromosome and then split their
cell in half.
Cell Division continued
•
But how do we
make sure each new
cell gets ONE copy
Eukaryotic Cell Division
of each
– Eukaryotes have LOTS of chromosomes
chromosome?
– The cell needs a way of dividing the chromosomes
between the new cells to make sure each new cell
has all the chromosomes it needs.
• Two Types of Eukaryotic Nucleus Division
– Mitosis – Keeps the number of chromosomes the
same. a PROCESS!
It requires
– Meiosis – Reduces the number of chromosomes
by ½
The first 3 phases are known
as Interphase.
The cell spends most of its
time in interphase.
The Cell Cycle
• When do cells reproduce?
• Cell Cycle – The repeating series of events which
make up the life of the cell.
• There are 5 phases to the cell cycle
–
G1 (First Growth) – The cell grows to mature size
Some cells enter G0 phase where they exit the cell cycle.
–
S (Synthesis)
The DNA is copied
Cells
in G0 do not– reproduce.
– G2 (Second Growth) – The cell prepares for cell
division
– M phase (Mitosis) – Division of the nucleus
The last 2 phases make up
– Cytokinesis – Division of the cytoplasm (rest of the
cell division.
cell)
Mitosis
We will use a cell with
4 chromosomes
because it’s easier
drawing(How
46.
thethan
nucleus.
we
These are organelles
called centrioles. They
•areMitosis
isduring
the division of
only active
cellget
division.
each cell a copy of every chromosome.)
• To look at mitosis, we will simplify our cell.
Prophase
Centrioles move toward opposite sides of
the cell and spindle fibers form.
Nuclear membrane breaks down and
disappears
Metaphase
Centrioles reach opposite sides of
the cell and spindle fibers attach
to the chromosomes and line
them up in the center of the cell.
Anaphase
Spindle fibers pull CHROMATIDS to
opposite sides of the cell
Telophase
Chromatids reach opposite sides of the
cell.
Spindle fibers disappear.
Nuclear membrane reforms.
Cytokinesis begins.
End result of mitosis
Two new cells which are identical to the original cell.
Original cell had 4 chromosomes, new cells have 4 chromosomes.
Meiosis
• Type of nuclear division which results in cells
which have one half the DNA of the original
cell.
• Produces gametes – haploid reproductive cells
(egg or sperm)
• Occurs in two parts: Meiosis 1 and Meiosis 2
– It’s ONE process split into two halves!
Prophase I
Centrioles begin to move to opposite sides of
the cell and spindle fibers form.
Nucleus breaks down and disappears.
Synapsis – Homologous chromosomes line
up next to each other. Each pair of
homologous chromosomes is called a
TETRAD.
Crossing Over occurs. Where maternal and
paternal homologous chromosomes swap
corresponding segments.
Metaphase I
Centrioles have reached opposite
sides of the cell.
Spindle fibers have attached to
chromosomes and lined the tetrads
up in the center of the cell.
How is this
different from
metaphase of
mitosis?
Anaphase I
How is this
different from
anaphase of
mitosis?
Spindle fibers pull homologous
chromosomes toward opposite
sides of the cell.
Telophase I
Chromosomes reach opposite sides
of the cell.
Spindle fibers break down.
Cytokinesis begins.
After cytokinesis of Meiosis I
We aren’t done yet! We have 2 copies of each chromosome!
But we do have 2 HAPLOID cells.
Prophase II
Centrioles duplicate and begin moving to
opposite sides of the cell.
Spindle fibers form.
Metaphase II
Spindle fibers attach to the chromosomes and line them up in the center
of the cell.
Anaphase II
Spindle fibers pull chromatids
to opposite sides of the cell
Telophase II
Chromatids reach opposite sides.
Spindle fibers disappear.
Nucleus re-forms.
Cytokinesis begins.
End Result of Meiosis
Meiosis forms 4 haploid
cells.
Meiosis in males vs. females
In males the process is called
In females the process is called spermatogenesis and the
oogenesis – and the result is one
result is 4 spermatids.
egg and 3 polar bodies.
This is because the cell divisions are
not equal, one cell ends up with the
bulk of the cytoplasm.
Types of Reproduction
• Asexual Reproduction – Production of
offspring from just one parent.
– Mitosis is a form of asexual reproduction.
• Sexual Reproduction – Production of offspring
through meiosis and the union of sperm and
egg.
– Requires two parents.
Let’s think about it!
How about a Venn?
Mitosis
Meiosis