Download Mitosis PPT

Mitosis
The Cell Cycle & Cell
Division
DNA and Cell Division:
• First, a look at Chromosomes:
• Chromatin: DNA in long, thin,
loose strands
• Visible and seen when the
cell is NOT Dividing
• Just before cell division, the
DNA replicates, then coils
and condenses into thicker
rod like structures called
chromosomes
DNA, or deoxyribose nucleic acid, contains all of
the genetic instructions to run an organism. DNA
has a unique double helix structure, which
resembles a twisted ladder. In eukaryotes, DNA is
stored in the nucleus. In prokaryotes it is twisted
into a nucleoid.
Most of the time your DNA is in a loose pile
called chromatin (Think of a plate of spaghetti).
During certain processes your DNA condenses
into tight structures called chromosomes
(when you wrap a noodle around your fork).
Every species has a unique number of
chromosomes.
• Chromatin

Chromosome
Chromosomes
• Chromosomes: DNA coils
and condenses around small
proteins called histones
• Visible and seen when the
cell is actively dividing.
A closer look at chromosomes:
Replicated
Chromosome
Un-replicated
Chromosome
Centromere
This basically shows
one copy of DNA
This basically
shows the two
copies of DNA
side by side
Sister Chromatids
The Importance of Chromosome Number
• Every organism has a “normal” total
number of Chromosomes in each cell.
• In all sexually reproducing organisms,
the chromosomes occur in
Homologous Pairs:
– Each pair contains similar genetic
information, and you get one from
each of your parents.
Organism
Typical
Chromosome
Number
Human
46
Chimp
48
Bat
44
Goldfish
94
Amoeba
50
Cat
38
Dog
78
Fruit Fly
8
Mold
4
Humans: 46 chromosomes, 23 pairs
Fruit flies: 8 chromosomes, 4 pairs
Adder’s Tongue Fern: 1262 chromosomes, 631
pairs
*** The number of chromosomes has NO
relationship to complexity! ***
The number of chromosomes is ALWAYS an
even number because “Normal” cells have
two copies of each chromosome. You
received one copy from mom and the other
from dad. For example, Mom gave you a
copy of her 14th chromosome, and so did
Dad. Now you walk around with two
Chromosome #14s in every cell. Since the
pairs code for the same information they are
called homologous. We call a cell with two
copies of each chromosome diploid, and
represent them with a 2n notation.
Diploid and Haploid Cells
• Any cell that contains all
paired chromosomes is
called a diploid cell
• Any cell that contains only
half or unpaired
chromosomes is called a
Haploid cell
Chromosome Number Symbolism
and Terminology:
• N = Symbol indicating number of
different types of chromosome in
a species
 Humans have 23 types of
chromosomes;
Therefore N in Humans = 23
 In Chimps N = 24
Diploid
• 2N = A diploid cell; A
cell with pairs of
chromosomes
• In humans 2N = 46;
In Chimps 2N = 48
• Body Cells or Somatic
Cells are diploid
Haploid
• 1N = A haploid cell; A
cell with unpaired
chromosomes
• In humans 1N = 23; In
Chimps 1N = 24
• Sex Cells or gametes
are haploid
Cell Growth
There are two reasons why cells divide, instead
of continuously grow:
A. Demands on the genetic material
a. Similar to a library serving more and
more people, but not buying
more books.
B. Difficult for osmosis and diffusion to
occur across the cell membrane
a. Depends on surface area to volume
ratio.
b. As size increases the volume
increases faster than the surface
area.
The Cell Cycle
When a cell becomes too large, it needs to
divide. Cells follow a predictable life cycle,
called the cell cycle where the cell grows,
prepares to divide, and actually divides, over
and over and over, etc. This cell division
process is happening in your body right now, in
all of your somatic, or body cells (all of your
cells except those inside of your testes or
ovaries).
Cell division has two predictable steps, first
mitosis copies and divides the nucleus in half,
and second cytokinesis divides the cytoplasm
in half. The two newly formed cells are called
daughter cells.
The Cell Cycle:
• Cells go through phases or a
cell cycle during their life
before they divide to form new
cells
• The cell cycle includes 2 main
parts --- interphase, and cell
division
• Interphase is the longest part of
a cell's life cycle and is called
the "resting stage" because the
cell isn't dividing
• Cell division includes mitosis
(nuclear division) and
cytokinesis (division of the
cytoplasm)
We use the following
diagram to represent the
Cell Cycle.
The Cell Cycle
Cell Cycle Phases in Detail
Interphase is the period of time between cell
divisions. Somatic cells spend 94% of their life
in this phase.
**This is NOT a phase where NOTHING
happens.**
This is a phase is when normal life occurs (protein
building)!
Interphase:
• G1 (Gap 1) = Cell grows
and doubles in size,
organelles doubled
– Any cell that will never
divide again such as nerve
cells stay “arrested” here
(G0)
• S (Synthesis) = DNA is
replicated or synthesized
here.
• G2 (Gap 2) = Cell
makes structures like
organelles in preparation
for division
Other features of Interphase:
• DNA is in chromatin
form (loose threadlike)
• Nucleus and
nucleolus are
visible
• Centrioles are nonactive
Mitosis Phases
Mitosis has four phases: prophase, metaphase,
anaphase, and telophase (Often called PMAT.)
Prophase:
• DNA begins to condense to
form chromosomes
Spindle
• Nucleolus and nuclear
membrane begin to
disappear
• Centrioles begin migration
to opposite ends of the cell
• Spindle fibers form from
the centrioles
Centrioles
Chromosomes
Metaphase:
• Chromosomes attach to
the spindle
• Microtubules connect
the centromere to the
spindle fibers.
• Chromosomes align in
the middle (equator) of
the cell
Equator
Anaphase:
Events:
1. Chromosomes separate into
individual sister chromatids
and move to opposite poles
2. Spindle fibers begin to
break down
Sister
Chromatids
Telophase
Events:
1. Chromosomes loosen into Chromatin.
2. Nuclear envelope reappears.
3. Spindle fibers disappear completely.
4. Cleavage furrow forms in animal cells, or Cell plate
forms in plant cells to BEGIN cytokinesis.
Daughter Cells Forming
Cytokinesis
• Spindle begins to
disappear
• Chromosomes uncondense and revert to
chromatin
• Animal cells simply
pinch off at the
cleavage furrow.
• In plant cells the
cell plate builds a
new cell wall.
Two Identical Daughter Cells – Back in
Interphase
Cytokinesis is not a step of mitosis or interphase. It overlaps
both mitosis and the G1 phase.
Mitosis Animation
Mitosis Video
Cell Division Controls
All cells grow and divide at different rates. For example,
skin cells rapidly and continuously divide throughout
life, while nerve cells almost never divide after
development. Cyclins are proteins which regulate
division and growth in eukaryotic cells. They respond to
internal or external cell conditions, such as the amount
of DNA, or the loss of contact with neighboring cells.
Cancer, an uncontrolled growth of cells, is caused by
the failure of cells to respond to cyclins.
Breast Cancer Cell
Brain Cancer Cell