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Cell Growth and Division:
Mitosis vs. Meiosis and What’s
going on the rest of the time
Cell Growth and Division:
Why does a cell need to divide?
• As an object grows, the volume increases at a faster
rate than the surface area
To Survive
• SA:V ratio must be “right” T
– More cytoplasm means more energy needed.
– By limiting the ratio of membrane to cytoplasm you
limit the “doorways” into the cell.
RESULT:
Cell can’t get enough materials to support its large
size
• Cell dies, unless it divides in half!
In order to divide, a cell must:
• Grow (includes protein synthesis)
• Replicate DNA
• Replicate organelles
Cell Cycle
Goal: To create two genetically identical cells
Interphase
Mitotic Phase
How Long is a Cell Cycle
• Dependent on the organism
– Bacteria
20 minutes for some
– Yeast
90 – 120 minutes
– Mammalian
24 hours or more
Cell Cycle
INTERPHASE
G1 – growth and protein synthesis
S – DNA replication (copying the DNA)
G2 – Make organelles
M PHASE
Mitosis (Nuclear division)
Cytokinesis – division of cytoplasm and
membrane
How Does DNA Duplicate?
• During S phase of Interphase
• Uncoiled string of DNA called Chromatin (uncoiled
DNA-protein complex) is duplicated
• During early prophase
– Chromatin (each piece) coils up to form a
chromosome with two chromatids (due to
duplication)
• During anaphase
– Chromatids will split forming chromosomes
with unduplicated DNA
How does DNA Duplicate?
• Each daughter cell gets 1 copy of each
chromosome
• At the end of mitosis, chromosomes
unravel (back to chromatin)
• THE STRUCTURE OF DNA
• Most of the time DNA is in
the form of chromatin:
strings of DNA wrapped
around proteins called
histones
• DNA is in chromatin form in
G1
• In S, each strand of
chromatin is copied
• Then, at the beginning of
mitosis, the chromatin is
folded into a duplicated
chromosome
Remember DNA contains the info needed to build an
organism
• Segments of DNA
which contain
information for a step
(a protein or enzyme)
are called a genes.
There are many
genes on each
chromosome.
• Each organism has a
specific number of
chromosomes.
Human 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 (only 1 of each type = 23)
Diploid cell
(human)
23 “types” of
chromosomes
2 of each type
1 from Mom
1 from Dad
Haploid cells
Cells that contain
one of each
chromosome are
called Haploid
cells.
Egg and sperm
cells are haploid.
This egg has an
extra chromosome
INTERPHASE: G1 cell:
- all DNA is UNDUPLICATED and in stringy chromatin form
- One piece of chromatin for every chromosome
Chromatin/
Chromosomes
from mom
Chromatin/
Chromosomes
from dad
Green, blue and red
represent the TYPES of
chromosomes.
- So a dotted red
chromosome is
homologous to solid
red chromosome
INTERPHASE: S phase cell:
- Every piece of chromatin is copied
- At the end each piece of chromatin is attached to its
identical copy
G2 phase cell:
- DNA does not change at all
- Cell prepares for Mitosis by duplicating organelles such as
centrosomes .
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
M Phase – Mitosis
Early Prophase:
- Chromatin coils/folds up
into dense chromosomes.
Chromosomes have
duplicated DNA - “sister
chromatids”
- Nuclear membrane and
nucleolus begin breaking
down
- Centrosomes start
moving toward opposite
sides of the cell. They are
parts of the cells that make
microtubule fibers
M Phase – Mitosis
Late Prophase:
- Centrosomes move closer to poles
- Microtubule fibers begin
extend from centrosomes
- Chromosomes are
fully formed
M Phase – Mitosis
Metaphase:
- Duplicated chromosomes line up on metaphase plate
-Microtubule “Cage”
(spindle) extends from
the centrosome to the
chromosomes and attach
to the centromeres
(middles) of each
chromosome –
- Each “bar” of the cage
is called a spindle fiber
M Phase – Mitosis
Anaphase:
- Sister chromatids
(identical copies) are
pulled apart by the
spindle fibers
- One copy of each
chromosome is pulled
towards each end of
the cell
M Phase – Mitosis
Telophase:
- A nuclear
membrane reforms
around each group of
chromosomes
- TWO nuclei at this
point!!
M Phase – Mitosis
Telophase:
Nuclear membrane is
formed
Unravels back into
chromatin
M Phase – Mitosis
Telophase:
- Chromosomes all
uncoil/unfold back into
chromatin.
- Notice that each nucleus
has UNDUPLICATED
chromatin like it did during
G1
Cytokinesis
- Sides of membrane start
pinching in forming cleavage
furrow
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
• Animal cells: Membrane
pinches inward forming a
Cleavage Furrow until it
divides the cytoplasm into
two equal parts
• Plant cell: Cell plate forms
in the middle of the
cytoplasm and extends
toward the edges. Cell wall
forms from this cell plate
M Phase – Mitosis
Cytokinesis:
- Cleavage furrow grows
until two cells are entirely
separate
M phase is complete and you
now have two cells that look
JUST like the cell you started
with!!!
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!)
– Reproduce
Controlling Cell Division
• Cells know when the don’t need to divide
(or when to stop dividing)
• Cell division “stops” when cells touch other
cells.
Identifying the Signal
EXPERIMENT
• A sample of the cytoplasm from a cell in
mitosis is taken and injected into an
interphase cell.
• Cell division will begin in the interphase
cell
• CONCLUSION: Suggests there is a signal
that triggers mitosis
Cyclins
• Mystery signal is one of a family of
proteins they call “cyclins”
• Some cyclins are “Go” signals
– Trigger the cell division process
• Some cyclins are “Stop” signals
– shut down the cell division process
Scarring – see handout
• Basically occurs due to the body’s
“emergency medicine response”
• Rather than being “neat and organized”, it
simply wants to “cover the hole”
• RESULT: unorganized collagen matrix 
scarring (will “soften” over time..but still be
present).
What if control is LOST????
• Cells begin to divide uncontrollably
• Pile up on top of each other
• Form big balls of cells called?????
TUMORS!!!
Tumor response
• 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 OR
• Hyperactive “go” signal so cell is
constantly dividing
Definition of a Tumor
• A tumor or tumour is commonly used as
a synonym for a neoplasm (a solid or
fluid-filled (cystic) lesion that may or may
not be formed by an abnormal growth of
neoplastic cells) that appears enlarged in
size.[1] Tumor is not synonymous with
cancer. While cancer is by definition
malignant, a tumor can be benign, premalignant, or malignant, or can represent
a lesion without any cancerous potential
whatsoever.
Tumors
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 (spreading of cancer from one
site to the next)
Cancer Terms
• Invasive:
•
Cancer has spread from the layer
(tissue) in which it developed and is
growing into (and replacing) other
“healthy” tissues that surround it.
• Metastatis - spread of cancer from one
organ or part to a non-adjoining area or
organ. We can look at cells to determine
whether or not the cancer is metastatic
RECAP
• Body cells are made through mitosis
• Most of the time they “know what to
do”,but sometimes we have an “out of
control situation  Cancer.
Asexual Reproduction
• Remember that mitosis is also used to
create new organisms (not just repair)
Mitosis is also utilized in
asexual reproduction
• Examples
– Budding
Hydra
Cell division – Sexual
Reproduction
• When cells participate in sexual
reproduction they are “joining”.
• Joining means that all of the DNA will
combine
• Too much DNA is usually an issue
– Unless, of course you are a seedless
watermelon! (3N)
Gametes
• What would happen if we made Gametes
(Egg and Sperm) cells this way?
– Way too much DNA!!!! (since two combine
together)
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
Now lets imagine that we need to make Gametes! Begins the same
G1 cell:
- all DNA is UNDUPLICATED and in stringy chromatin form
- One piece of chromatin for every chromosome
Chromatin/
Chromosomes
from mom
Chromatin/
Chromosomes
from dad
Green, blue and red
represent the TYPES
of chromosomes.
- So a dotted red
chromosome is
holomogous to solid
red chromosome
S phase cell:
- Every piece of chromatin is copied
- At the end each piece of chromatin is attached to its identical copy
G2 phase cell:
- DNA does not change at all
- Cell prepares for Mitosis
M Phase – Meiosis
Prophase I:
- Chromatin Coils/folds up
into dense chromosomes.
Each copy of a piece of
chromatin becomes a
“sister chromatid”
- Nuclear membrane and
nucleolus begin breaking
down
- Centrosomes start
moving toward opposite
sides of the cell. They are
parts of the cells that make
microtubule fibers
M Phase – Meiosis
Prophase:
-New event: Homologous
chromosomes pair up to
form Tetrads
M Phase – Meiosis
Prophase:
-Crossing over happens in each tetrad
- homologous chromosomes swap parts of their chromosomes
- Creates 4 DIFFERENT chromatids, each has a different combination of info
A
B
A
B
a
b
a
b
A
B
a
B
A
b
a
b
Meiosis I
• Crossing over in prophase
• Line up in homologous pairs (metaphase
• Homologous pairs separate BUT NOT
SISTERS!
• RESULT: 2N (with double DNA)  1 N
(double DNA)
M Phase – Meiosis
Prophase I:
- Centrosomes move closer to poles
- Microtubule fibers extend from centrosomes
M Phase – Meiosis
Metaphase I:
- Tetrads line up at the
metaphase plate
- Microtubules from the
centrosome extend to the
chromosomes and attach
to the centromeres
(middles) of each
chromosome - Microtubule
“cage” is called the
Spindle
- Each “bar” of the cage is
called a spindle fiber
M Phase – Meiosis
Anaphase I :
- Tetrads are separated
- any combination of
mom/dad chromosomes can
be pulled to either side
M Phase – Meiosis
Telophase I:
- A nuclear membrane
reforms around each group of
chromosomes
- cell will have TWO nuclei at
this point!!
Meiosis II
• Can occur without telophase or two
separate cells redivide
• Chromosomes line up “single file
• Sisters split (1N  1 N)
• RESULT: 4 cells, all 1N, all unique (due to
crossing over)
M Phase – Meiosis II
Prophase II:
- Same events as before
- Only this time it is
happening in each of the two
cells
M Phase – Meiosis II
Metaphase II:
- Remaining chromosomes
line up at metaphase plate of
each cell
- spindle fibers attach
M Phase – Meiosis II
Anaphase II:
- Attached chromatids are
pulled apart in each cell
M Phase – Meiosis II
Telophase II:
- Nuclear membranes reform
and spindle breaks down
M Phase – Meiosis II
Telophase II:
- chromosomes unwind back
to chromatin
Cytokinesis
- Four cells are formed
- each one has a completely
different combination of DNA
thanks to crossing over and
the division of homologous
chromosomes
A
a
c
b
C
e
d
F
E
d
b
F
Possible sets of
chromosomes in each
of the 4 cells made
during meiosis
A
B
C
D
e
f
a
c
B
D
E
f
Humans and Meiosis
• Germ Cell
– any biological cell that gives rise to the
gametes of an organism that reproduces
sexually.
– Found in the gonads (organs that make
gametes) Testis (male) Ovary (female)
Spermatogenesis
• Germ cell is called Spermatogonium – 2N
– Makes sperm (1N) through meiosis
– Makes additional spermatogonia (2N) through
mitosis
– Meiosis I
• Reduces chromosome number from 2N  1N
• BUT.. Still have duplicated chromosomes
– Meiosis II
• Splits sister chromatids  each chromatid
becomes a chromosome
Oogenesis
• Asymmetrical cell division leads to
formation of the ovum and 3 polar bodies
• Polar bodies eventually disintegrate.
Karyotype
• Photomicrograph (microscopic picture) of
chromosomes in a normal dividing cell
• Organized based on size of the chromosome
and position of the centromere. They are
grouped by homologous pairs. Sex
chromosomes are the 23rd pair
• Can use chromosomes in telophase (before they
uncoil) or prophase (duplicated)
Types of Chromosomes
• Sex chromosomes
– Determine the gender of an organism and
may also carry other genes
• X or Y
XX
XY
• Autosomes
– All other chromosomes (pairs 1-22)
Chromosomal Mutations
Mutation
a change in the nucleotide-base sequence
of a gene or DNA molecule.
Possible Effects:
Germ cell mutations - occur in gametes
Do not affect the organism, but affect
the offspring.
Somatic cell mutations – affect the body
cells (but not gametes) and do affect the
organism.
Classification of Mutations
Lethal mutations
– Cause death, often before birth
Beneficial mutations
– Mutations which provide higher survivability or
reproducibility
Silent mutations
– neither harmful nor beneficial
Chromosomal vs. Gene
Mutations
• Chromosomal Mutations
– Involve changes in the structure of a chromosome
or the loss or gain of a chromosome.
– Involve more than one gene
– Generally seen in a karyotype
• Gene mutations – WE WILL STUDY LATER
– The addition or removal of a single nucleotide within
a single gene
– AKA point mutation.
Karyotyping and Chromosomal
Mutations
• Karyotypes can identify chromosomal
mutations
• Can be seen as a change in banding
pattern, shape or size of a chromosome
• Can involve losing or gaining a “whole”
chromosome.
Chromosome Mutations
• Deletion
– Loss of a piece of chromosome due to
breakage (genetic information is missing)
• Inversion
– Chromsome segment breaks off, flips around
(upside down) and reattaches
• Translocation
– Broken off piece of a chromosome attaches to
a non-homologous chromosome
• Non-disjunction – non-separation of
homologues or sister chromatids
Chromosomal Diseases
• Learn about these at the following site
• http://learn.genetics.utah.edu/content/begi
n/traits/
• Know the karyotype of:
– Down syndrome, trisomy 18,, Turner
syndrome (XO, Klinefelter syndrome (XXY)
Questions to Answer
• What is the difference between monosomy
and trisomy? How can both occur “at the
same time”?
• What are two possible ways deletions can
occur?
Stem Cells
• have the ability to either divide again and
again to create more of itself (exact
copies), OR to differentiate into another
type of cell
• . Because of their ability to develop into
any cell type, they could potentially
provide an unlimited source of adult cells,
such as bone, muscle, liver, or blood cells.
Three types
• Totipotent
– Occur during cleavage (first few hours after
fertilization)
• Pleuripotent– One “step” differentiated
– Can form many but not all cell types (most
researched currently for medical use)
• Multipotent
– Bone marrow cells for all blood cells. Best
understood.
Medical Technology and
Controversy
• Can we “grow an organ”?
• Controversy:
– Source of stem cells
– Limitations of adult stem cells
– Cord blood – only blood diseases.
Embryology
• Know definitions
• Know difference between
Fertilization
Egg and
sperm join
Zygote
begins to
divide
(mitosis!)
Sperm cell
breaks down
but leaves the
chromosomes
Zygote:
A Fertilized
Egg
Cells continue to
divide and start to
rearrange
themselves
Morula: solid
ball of 10 to 30
cells (Day 4)
8-cell stage
4-cell stage
(Day 2)
(Day 3)
2-cell stage
No G1 or G2 phases just M and S during cleavage
GASTRULATION
More dividing, three
distinct layers of
cells are formed
Still dividing,
outer cells start
to fold inward
Blastula: Hollow,
fluid filled ball of
cells;
(Day 5)
Gastrula (W3)
Ectoderm
(outer)
Nervous
System and
skin
Mesoderm
(middle)
Muscle, skeleton
Dermis, Heart,
Kidneys
Endoderm
(inner)
Lining of
digestive
system
Different view of previous slide
1. Are the egg and sperm haploid or diploid? What about the zygote?
HAPLOID.
DIPLOID
2. What type of division is occurring to create a morula? A blastula?
What term is used to describe the first divisions of the zygote?
MITOSIS, Again MITOSIS, CLEAVAGE
3. In which stage do you see definite differentiation? What causes this?
4. Where would you find totipotent cells? Pleuripotent cells?
Other Important Terms:
•
Gamete – a haploid reproductive cell that unites with another haploid
reproductive cell to form a zygote.
•
Zygote - The cell that results from the union of gametes; also called a
fertilized egg
Fertilization – the union of a male and female gamete to form a zygote
•
Embryo – an organism in the early stages of development . A developing
human is called an embryo from two weeks after conception until the end of
the eighth week.
•
Differentiation – the structural and functional specialization of cells during
an organisms’s development
Other Important Terms:
•
Identical twins (monozygotic)- --
•
- single egg fertilized by one sperm.
•
Fertilized egg splits into two embryos
early in development.
•
Most IT share the same placenta. (get
oxygen and nutrients from the mother
and get rid of wastes through the
placenta.)
•
Fraternal twins (dizygotic)
- Fraternal
Usually in separate amniotic sacs.
twins develop when two eggs are fertilized by two
separate sperms. The fetuses have separate
placentas and amniotic sacs.