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Warm Up
• Quote:
–“
• Team Competition
The Cell Cycle
Why the cell must divide:
• Reason 1:
– The cell only contains one copy of DNA to serve
as the instructions for everything that occurs in
the cell. If the cell were too large it would not
be capable of providing instruction for all
necessary functions.
• Reason 2:
– The volume of an object will increase faster
than the surface area as the object increases in
size. The cell’s needs are based on how large
the cell is (volume) but everything the cell
needs, such as food, oxygen, water, and getting
rid of waste, must enter or leave through the
cell membrane (surface area). Therefore the
cell’s needs will increase faster than the cell’s
ability to provide for the needs.
– The diagram shows what would happen to the volume to
surface area ratio if a cube were to double in size
– A cube that is 1 mm by 1 mm by 1 mm has a volume of 1 mm3
and a surface are of 6 mm2. The ratio is _______
– A cube that is 2 mm by 2 mm by 2 mm has a volume of 8 mm3
and a surface are of 24 mm2. The ratio is _______
– This demonstrates that a cell’s needs increase faster than the
cell can provide for its needs.
4 mm
1 mm
1 mm
2 mm
4 mm
2 mm
1 mm
2 mm
Surface area = 6 mm2
Volume = 1 mm3
Surface area = 24 mm2
Volume = 8 mm3
4 mm
Steps of the Cell Cycle
• The cell cycle is the events that occur in the cell from creation to
division into new cells. This process has several steps.
• Interphase: the period in between cell divisions. This will take up
the majority of the cell’s life. This phase is divided into three
parts:
– G1 phase (growth phase one): the cell will increase in size and
carry out its specific function. Some cells will remain in this
stage for their entire life.
– S phase (synthesis phase): creates a copy of the cell’s DNA
through a processes called replication. This allows the cells
that are going to be created in division to receive identical
copies of the DNA creating identical cells.
– G2 phase: the creation of other materials needed for division
occurs in this phase.
• M phase (mitosis): series of steps used to organize and divide the
DNA in the nucleus to ensure each cell gets an identical and
complete copy. Mitosis has four steps:
Steps of the Cell Cycle
– Prophase:
• Normally the longest step of mitosis.
• DNA will coil into individual
chromosomes in order to be sorted
and divided easily. The chromosomes
are the two copies of the same
section of DNA held together by a
protein which gives them and X
shape. Before this the DNA was
uncoiled in a mass of strings.
• The nucleus disappears.
• In animal cells centrioles will move to
the poles.
• Spindle fibers are created from the
poles that will be used to move and
divide the DNA.
Steps of the Cell Cycle
– Metaphase:
• Normally the shortest phase of mitosis.
• The chromosomes line up single file in the
center of the cell.
– Anaphase:
• The spindle fibers separate the
centromeres that hold the two copies of
each piece of DNA together and pull one
half of each chromosome (one copy of the
DNA) to each side of the cell.
– Telophase:
• The DNA uncoils.
• The spindle fibers break down and the
cetrioles move back to their original
location.
• Two nuclei form.
Steps of the Cell Cycle
• C Phase (cytokinesis):
– This phase will divide the cell into two
identical cells.
– Animal cells: the cell membrane pinches in
until it completely divides the cell.
– Plant cells: the cell wall is not flexible so
the cell will construct a new cell wall down
the center by using a cell plate that has
appeared as a starting point.
Interphase
S: DNA synthesis and replication
G1: Rapid growth and
metabolic activity
G2: Centrioles replicate;
cell prepares for division
M: Divides the
DNA
C: Divides the
cell
Purpose of Mitosis
• This process is used to create two new
identical cells. This can be done to replace
dead or damaged cells or increase the size
of an organism (growth).
• Mitosis can also be used as a form of
reproduction. Since the new cells only had
one parent this is called asexual
reproduction.
– Benefits: asexual reproduction can
occur very quickly and is relatively
simple to accomplish.
– Negatives: all of the organisms are
genetically identical so this does not
allow for diversity or evolution.
Alternative to Asexual Reproduction
• Sexual reproduction uses cells that
contain only half the DNA founds in
normal body cells from two different
individuals to create the next
generation. This way the next
generation does not have double the
DNA which would be too much for the
cell to go through.
– Benefits: This produces genetic
variations among organisms of the
same species. Genetic variation
allows a species to have options so
that they can select the best version
of a trait for the environment to
pass on to the next generation.
– Negatives: This process takes much
longer than mitosis and requires two
parents.
The Cells of Sexual Reproduction
• The cells used in sexual reproduction
only contain half of the DNA from the
parent. However, this is still one copy of
every trait that the species creates.
• Each parent has two chromosomes to
code for the production of each trait.
The two copies may be the same or they
may be different. The two
chromosomes that code for the same
traits are called homologous
chromosomes.
• Typically only one copy of the DNA is
expressed. This explains why a parent
with brown eyes can have a baby with
blue eyes. The parent has both DNA
sequences, but only expresses the
brown eye sequence.
• When the cells from both parents fuse
during fertilization the offspring will have
two DNA sequences for every trait.
The Cells of Sexual Reproduction
• Human traits are carried on 23
chromosomes. The typical
human body cell contains 46
chromosomes (23 that were
inherited from each parent).
These cells are called diploid
because they have two of each
chromosome.
• The cells used in sexual
reproduction contain 23 of the
46 chromosomes which means
they are haploid. These cells are
called gametes and can be
either eggs or sperm.
• Since the cells used in sexual
reproduction contain ½ the
amount of DNA as the parent
they cannot be created using
mitosis. The process that makes
these cells is called meiosis.
Steps of Meiosis
• Interphase: this occurs the same as the
interphase before mitosis which means the cell
will grow, make a copy of each piece of DNA so
that each chromosome will have two copies of
the same DNA, and prepare to divide.
• Prophase I:
– DNA coils into chromosomes where the two
copies of each piece of DNA are held
together by a protein giving them the X
shape.
– Nucleus disappears.
– Spindle fibers form and centrioles move to
the poles.
– The two chromosomes that code for the same
trait (homologous chromosomes) will pair
together. Pieces of the chromosomes can
become tangled, break and switch places in a
process called crossing over. This increases
the number of different cells the parent can
create so that all offspring are different.
Steps of Meiosis
• Metaphase I:
– Chromosomes are moved to the center of
the cell and lined up.
– Chromosomes are lined up in homologous
pairs.
• Anaphase I:
– One whole chromosome from each pair is
pulled to each pole.
• Telophase I:
– Chromosomes are uncoiled, nuclei form,
spindles break down, and centrioles move
back to their original location.
• Cytokinises I:
– The cell divides into two cells that have
two copies of DNA for each trait (diploid).
Steps of Meiosis
• Interphase:
– DOES NOT OCCUR AGAIN
SO THIS HALF OF
MEIOSIS STARTS WITH 46
PIECES OF DNA NOT
WITH 2 COPIES OF 46
PIECES OF DNA.
• Prophase II:
– DNA coils, the nucleus
disappears, centrioles
move to poles and
spindle fibers form.
• Metaphase II:
– Chromosomes line up at
the center of the cell
single file.
Steps of Meiosis
• Anaphase II:
– Half of each chromosome
is pulled to each pole.
• Telophase II:
– DNA uncoils, nuclei form,
the centrioles move back
to their original location,
and spindle fibers break
down.
• Cytokinises II:
– Both cells divide creating
four genetically different
cells that each contain half
the amount of DNA
(haploid).