Download Cell Division

Cell Division
Mitosis & Meiosis
Learning Target
• I can describe the basic process
of mitosis.
(Quality Core Standard B.1.j)
Cell Division
What is it?
Why do
Cells do it?
Why is it
important to
me?
Cell Division
Also known as
Mitosis
Takes place in
Regular Body Cells
Keeps Cells Living
and Growing
Why Do Cells Divide?
Two main reasons:
1. The larger a cell
becomes, the more
demands the cell
places on it's DNA.
Food goes in
Waste goes out
2. The bigger the
cell gets the harder
it becomes to
move food and
waste across the
membrane
Protein Pumps
This happens
because the
surface area
and volume
ratio does not
stay the same
as the cell size
increases.
The cell's ability
to either get
substances
from the
outside or
eliminate waste
from the inside
is related to the
surface area of
the cell
membrane.
(outside)
How much food
and other
material is
required, and
how much
waste the cell
produces and
has to get rid of,
is related to the
volume of the
cell. (inside)
As a cell gets
bigger there
comes a time
when its surface
area is not large
enough to meet
the demands of
the cell's volume
and the cell stops
growing.
So, once cells
reach a certain
size they must
divide in order to
continue to
function – or they
will no longer be
able to take in
nutrients and
eliminate waste.
Why Is Cell Division Important?
1. All Living Things are made of Cells
2. The Cell is the basic unit of Structure and
Function in Living Things.
3. All Cells come from pre existing Cells
These three statements are part of what?
You are a living
organism, made of
cells.
In order to keep
living, your cells
must stay alive.
In order for cells to
keep living, they
must divide and
multiply
Phases of Mitosis
The Basic Phases of a
Cell’s Life Cycle:
•Interphase
•G1
•S
•G2
•Mitosis
•Prophase
•Metaphase
•Anaphase
•Telophase
•Cytokinesis
Interphase
The longest stage
of a Cell’s life
The time spent
between divisions
Produces all
materials required
for growth
Preparation for
division
Interphase: G1
Growth Phase
Cells do most of
their growing
and synthesize
new proteins
and organelles.
Interphase: S Phase
(Synthesis Phase)
Chromosomes are
duplicated and DNA is
synthesized
If the cell gets to this
phase, division will be
completed.
Interphase: G2
Shortest phase
Many organelles and
other molecules are
made
Next step –
Mitosis
Mitosis:
Cell Division
The last part of
the Cell Cycle
is called
Mitosis and
has 4 phases
during which
the cell divides
into 2 cells
Prophase
The Cell begins the
division process
1.The nucleolus
disappears
2.The nuclear membrane
breaks apart
3. The chromosomes
become visible
4. The spindle
apparatus forms
and attaches to
the centromeres
of the
chromosomes
Metaphase
The Second Phase
of Mitosis
1. The Nuclear
Membrane is
completely gone
2. The duplicated
chromosomes line
up along the cell's
equator.
Anaphase
The third phase of
Mitosis
Daughter
chromosomes separate
They are pushed and
pulled toward opposite
poles of the cell by the
spindle fibers
Spindle Fibers
Telophase
The nuclear
membrane and
nucleoli and
nucleus reform.
Cytokinesis is
nearly complete,
The Cell Plate
begins to form
The Cell
prepares for final
division
Cytokinesis – The final stage of the cell
cycle
The cytoplasm, organelles, and nuclear
material are evenly split and two new
cells are formed.
Cell Plate
The two new cells – each exactly like the
other – are called Daughter Cells
Learning Target
• I can describe the basic process
of mitosis
(Quality Core Standard B.1.j)
Meiosis
Why We Are Who We Are
Learning Target
• I can describe the basic process
of meiosis.
• (Quality Core Standard C.1.f)
Meiosis
• Process of reduction division. Number of
chromosomes per cell is cut in half.
• Occurs when two organisms join gametes
to form the first cell of a new individual
• Gametes are sperm and egg; or pollen
and ovule
• All of our cells are diploid meaning we
have two copies of each chromosome
Meiosis
• One chromosome comes from the male
parent; the other from the female:
homologous
• Our sex cells: sperm and egg are haploid
meaning these special cells only have half
the chromosomal complement
• Involves two divisions of the nucleus
called Meiosis I and Meiosis II
Prophase I
• Chromosomes condense
• Nuclear envelope breaks down
• Homologous chromosomes
pair all along their length
forming tetrads (4 chromatids)
• Crossing over occurs when
parts of one chromosome are
broken and exchanged with
the same parts on its
corresponding chromosome
Metaphase I
• Pairs of homologous
chromosomes
are moved by spindle fibers to the
equator of the cell
• Homologous chromosomes,
each made of two chromatids
stay together
Anaphase I
• Homologous chromosomes
separate
• Chromosomes of each pair are
pulled to opposite poles by
spindle fibers
• Chromatids do not separate at
the centromeres—each is still
made of two chromatids
Telophase I & Cytokinesis
• Individual chromosomes gather
at each pole
• Cytokinesis occurs forming
two new cells
• Both cells contain one chromosome
from each pair of homologous
chromosomes
• The chromosomes do not replicate
between Meiosis I and II
Prophase II
 New spindle forms around chromosomes
Metaphase II
• Chromosomes line up at equator
• Attached at centromeres to spindle fibers
Anaphase II
• Centromeres divide
• Chromatids (chromosomes) move to opposite poles
Telophase II
• Nuclear envelope forms around each set of
chromosomes
• Spindle breaks down
• Cytokinesis occurs
• Four haploid cells are created, each carrying one set
of chromosomes
Meiosis
• Contributes to genetic variation by way of
three mechanisms:
• Independent assortment
• Crossing over
• Random fertilization
Independent Assortment
• Each gamete receives one chromosome from
each of the 23 pairs of homologous
chromosomes
• The random distribution of homologous
chromosomes is independent assortment
• Each of the 23 pairs independently separates
so that 223 (about 8 million gametes are
possible from just one cell)
Crossing over
• An exchange of DNA segments between two
homologous chromosomes
• In Prophase I, the chromosomes have made a
tetrad with each pair of chromatids joined at
centromeres
• Crossing over occurs at chiasma
• When recombined offspring may have one trait
from each parent: dad’s eyes and mom’s hair
Crossing over
Random Fertilization
• The zygote that forms a new individual is
the result of random joining of gametes
• Each new sperm or new egg is vastly
different from one another due to the
different combination
Meiosis in Males
Meiosis in Males
• Process is called spermatogenesis
• Occurs in testes
• Can occur throughout males life once hits
puberty
• At end of meiosis, four complete sperm
cells are made
Meiosis in Females
Meiosis in Females
• Called oogenesis
• Occurs in ovaries
• Cytokinesis causes uneven splitting of cytoplasm
in Meiosis I
– One cell gets most of cytoplasm and is much larger;
small one is called polar body
• Meiosis II; same process again. One large cell is
made; three small polar bodies die
• Large ovum is made that has enough nutrients for
zygote during its initial formation
Learning Target
• I can describe the basic process
of meiosis.
(Quality Core Standard C.1.f)