Cell Growth and Division:
Mitosis vs. Meiosis and What’s
going on the rest of the time
Why does a cell need to divide?
• A little geometry: As an object
grows, the volume increases at a
faster rate than the surface area
The same happens with a cell
• What is the surface area?
• What is the volume?
• The more cytoplasm there is the
more materials are needed.
• How do the materials enter?
Oh no… diffusion is back!
• By limiting the ratio of membrane
to cytoplasm you limit the
“doorways” into the cell.
• Cell can’t get enough materials
to support its large size
• Cell dies, unless it divides in half!
What do our cells need to do
before they can divide?
• Get bigger
• Make another copy of DNA
• Make more organelles
Cell Cycle
Cell Cycle
G1 – growth and protein synthesis
S – DNA replication (copying the
DNA)
G2 – Make organelles
M – Mitosis (Nuclear division) and
Cytokinesis – division of
cytoplasm and membrane
What is DNA again?
•
•
•
•
A Chain of nucleotides
Twisted into a double helix (spiral)
VERY LONG
Contains ALL the recipes for every protein
our body needs
• Recipes are called genes
Understanding DNA structure
• Most of the time out DNA is in
the form of chromatin: strings
of DNA wrapped around
proteins called histones
Understanding DNA structure
• DNA is in chromatin form
through G1.
• In S phase, each strand of
chromatin is duplicated and the
duplicated copies remain
attached together at the
centromere.
Understanding DNA structure
• During M phase the chromatin
is folded into chromosomes
• DNA remains in chromosome
form until cell division is over
Understanding DNA structure
Remember DNA contains the
information needed to build an
organism
• Each chromosome contains
some of the information.
• Each organism has a specific
number of chromosomes.
Humans have 23 types of
chromosomes and 2 of each type
= total of 46 chromosomes
• One of each type came from your
mom the other from your dad.
• Every cell in your body has all 46
chromosomes with the exception
of egg/sperm cells
• Cells that contain two of each
chromosome are called Diploid
cells
• Cells that contain one of each
chromosome are called Haploid
cells
Chromosome Analogy
• Think of the Information in a cell
as an Encyclopedia
- Each chromosome is one book
– Haploid = one set of info / one
encyclopedia
A BCDE
Chromosome Analogy
Diploid = two different sets of
info/ two different
encyclopedias!
A BCDE
A BCDE
Chromosome Analogy
• Genome = total information in cell
– If a cell is haploid, the genome
consists of all the information in
one encyclopedia
–If Diploid, all the info in both
encyclopedias
A BCDE
A B CD E
Chromosome Analogy
• Chromosome = One volume
– Genes: segments of DNA; each
contains a specific message
–Genes are like Articles in
encyclopedia
F jkasdkfjh
Jadlfl he;
A
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
F jkasdkfjh
Jadlfl he;
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
Chromosome Analogy
• Genes can have different
variations. The variations are
called Alleles.
• Think of the alleles as two
different articles on the aardvark!
F jkasdkfjh
Jadlfl he;
A
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
F jkasdkfjh
Jadlfl he;
Kjadh fchw
Laksjdfh
Kasjdf;aj
Skdjfa;ie
Chromosome Analogy
• Homologous Chromosomes :
–Contain same genes, but may
contain different alleles
–Example: both might contain
Hair color gene but one might
have brown hair info the other
blonde
–One from Mom, one from Dad
Chromosome Analogy
• Homologous Chromosomes =
Volume “A” from each encycl.
Homologous
Chromosomes
A
A
Chromosome Analogy
When DNA duplicates itself before cell
division it makes two identical copies
of each of chromosome
Identical Copies are Sister Chromatids
Sister
Chromatids
Sister
Chromatids
A A
A A
Single, unduplicated
Chromosome
1 duplicated
chromosome, 2
sister chromatids
2 separated
chromatids
become individual
chromosomes
Centromere
Telomeres
Mitosis
• One Fluid Event; no stopping and
starting.
• BUT: for ease of study, we break it
into 4 stages
• REMEMBER: all phases are
continuous and may, in part,
overlap
1. Prophase
• Longest phase of Mitosis
• Chromatin folds up into
Chromosomes which can now be
seen
1. Prophase
• Centrosomes separate and move
toward opposite poles
• These are parts of the cell that
make microtubules
1. Prophase
• Centrosomes start to form the
Mitotic spindle
–Made of microtubules
1. Prophase
• Nuclear membrane breaks down
• Nucleolus disappears
2. Metaphase
• Chromosomes line up along
equator
• Spindle fibers attach to the
centromeres of chromosomes
3. Anaphase
• Centromeres connecting sister
chromatids separate and one
chromatid of each chromosome
moves toward each poles
4. Telophase
• Chromosomes begin to
unwind back into chromatin
• Nuclear Envelope reforms
around chromosomes
• Spindle breaks down
• Nucleolus reappears
Cell Division is not yet Done!!
• What have we made?
– One Cell with Two complete
Nuclei
What is left to do?
- Cytokinesis: Divide the
cytoplasm and separate the cells
Cytokinesis
• Animal cells:
Membrane pinches
inward forming a
Cleavage Furrow
until it divides the
cytoplasm into two
equal parts
Cytokinesis
• Plant cell: Cell
plate forms in the
middle of the
cytoplasm and
extends toward
the edges. Cell
wall forms from
this cell plate
Cell division is done!
• Now you have Two Identical
daughter cells
Controlling Cell Division
• Cells know when they need to
divide
–When?
•During growth
•Repair – injury
•Replacement (cells are not
immortal!)
Controlling Cell Division
• Cells know when they don’t need
to divide
– Cells stop when they reach
other cells
How do cells “know”?
• Cells communicate by releasing
molecules
• To control cell division cells release
proteins called “cyclins”
• Some cyclins are “Go” signals
–Trigger the cell division process
• Some cyclins are “Stop” signals
– shut down the cell division process
Loss of control
• What happens if the control
signals don’t work?!?
– cells divide uncontrollably
–Pile up on top of each other
–Form big balls of cells
called?????
»TUMORS!!!
Tumors
• Tumor cells do not respond to
(or do not have) the body’s
control signals
–missing a “stop” signal so
cell division doesn’t stop
–Hyperactive “go” signal so
cell is constantly dividing
Tumors Vs. Cancer
• Tumor = uncontrolled but isolated
growth of cells
• Tumor cells become cancer when
they start to invade healthy tissue
–What if 1 cancer cell breaks off
and enters the blood stream?
–Where ever it “lands” = new
tumor = metastasis
That’s how Somatic (or body) cells
divide!
• What would happen if we made
Egg and Sperm cells this way?
–Way too much DNA
• What do we have to do when
forming these cells?
–Reduce the amount of DNA
Meiosis
• Happens ONLY in sex cells
• Reduces information by ½
• Requires two different
divisions
–How many cells at the end??
Meiosis begins the same as Mitosis
• Cell in G1 enters S phase.
• ALL DNA is copied
• Chromatin folds up to form 46
duplicated chromosomes
Prophase I
Meiosis I
- homologous chromosomes
pair up forming tetrad; 4
chromatids together;
Meiosis I
Prophase I
- Centrosomes separate to poles
- Nuclear envelope breaks down
Meiosis I
Prophase I
- Crossing occurs between
homologous chromosomes
Meiosis I
Metaphase I
- Spindle fibers attach to
chromosomes at metaphase plate
Anaphase I
Meiosis I
- Fibers pull apart homologous
chromosomes toward opposite
poles
Anaphase I
Meiosis I
- Fibers pull apart homologous
chromosomes toward opposite
poles
Meiosis I
Telophase I
- Two cells are formed
- only half of the info of
the Original cell
- 2N (diploid at start)
 N (haploid at end of
meiosis I)
Meiosis II
Prophase II
- Spindle reforms in each haploid
daughter cell
Meiosis II
Metaphase II
- Chromosomes line up at the
equator
Meiosis II
Anaphase II
- Sister chromatids are pulled to opposite
poles
Meiosis II
Telophase II
- Four haploid (N) cells created
Compare and Contrast Mitosis and
Meiosis