Download Cell Reproduction - Ursuline High School

Cell Reproduction
or
How cells make copies of themselves
Also called
Cell Division
Cell Division
• Cell division consists of two phases:
• 1. the division of the nucleus, and
• 2. the division of the cytoplasm (cytokinesis)
2 Kinds of Nuclear Division
• 1. Mitosis – mitosis divides the nucleus so
that both resulting new cells (daughter cells)
are genetically identical…Same amount of
DNA
• 2. Meiosis – meiosis produces daughter
cells that contain half the genetic
information… half the amount of DNA.
Before Division Begins
• Before a cell can successfully divide, the
DNA must be packaged so it does not get
damaged. The “stringy” form of DNA
(chromatin) is coiled into structures called
“chromosomes”.
Each Chromosome
is made up of two
identical halves
called “sister
chromatids” joined
at the centromere.
Each Chromatid is a
single, coiled DNA
molecule.
The point where two
sister chromatids are
connected.
Sister Chromatids
Organizing Chromatin into a
Chromosome.
Diploid Cells
(symbolized as 2n)
• Diploid cells (2n) have 2 copies of every
chromosome, forming what is called a
“Homologous” chromosome pair.
In Diploid cells, one pair of chromosomes comes
from the mother and the other pair comes from
the father.
The diploid number for humans is 46, or
2n = 46,…. Or you could say…..Humans have 23
homologous pairs, or…. Humans have 92
chromatids.
Chromosomes
Chromosomes
The Life Cycle of a cell is called
The “Cell Cycle”
The Cell Cycle consists of 5 Phases
1.Interphase (part of the cell cycle, but not part of mitosis)
2.Prophase
3.Metaphase These 4 phases are known
collectively as “Mitosis”
4.Anaphase
5.Telophase
Interphase
90% of cell’s life is spent in interphase
During Interphase the cell grows, duplicates its
chromosomes and performs its normal job.
Interphase has 3 stages
The Events of Interphase
G1 stage - first gap - Cell grows and carries out regular biochemical functions.
S stage – synthesis - DNA is replicated or synthesized.
G2 stage - second gap - Cell completes preparations for division…..a cell can
complete S, but fail to enter G2.
nuclear envelope
Plant Cells
Animal Cells
Mitosis
(the splitting of the nucleus)
prophase
metaphase
anaphase
telophase
Mitosis
• A cell that has grown
in size and is about to
divide is called a
“Mother Cell”.
• As a result of Mitosis
and cytokinesis the
Mother cell splits into
two genetically
identical “Daughter
Cells”.
Mother
Cell
The Events of Prophase
• Nucleoli disappear.
• Chromatin condenses into
the chromosomes.
• Nuclear Envelope
dissolves… the nucleus
comes apart
• Centrioles (MTOC’s)
separate and move to
opposite ends of the cell.
• Microtubules from each
MTOC connect to a
specialized region of the
centromere called the
kinetochore. This moves the
chromosomes back and
forth.
• Mitotic spindle begins to
form.
Plant Cells
Animal Cells
Events of Metaphase
• Chromosomes line up at the
equator of the cell…. called
the “metaphase plane”.
• Centrioles arrive at opposite
ends of the cell.
• Spindle apparatus fully
developed.
• Metaphase ends when the
microtubules pull each
chromosome apart into two
chromatids. Once separated
from its sister chromatid,
each chromatid is now called
a chromosome. To count the
number of chromosomes, at
any time, count the number
of centromeres.
centriole
centriole
metaphase plane
Plant Cells
Animal Cells
Events of Anaphase
Chromosomes
• Anaphase begins when the
chromosomes separate.
• Microtubules shorten as
tubulin units uncouple, the
chromosomes are pulled
away from each other
toward opposite ends of
the cell.
• Cell elongates; poles move
slightly further apart.
• Anaphase ends when the
chromosomes reach their
respective ends of the cell.
Plant Cells
Animal Cells
Events of Telophase
• Chromosomes uncoil back to chromatin.
• Nuclear envelope reforms. The nucleus
reforms in each newly formed cell.
• Nucleoli reappear.
• Spindle fibers disappear.
• Simultaneously Cytokinesis usually starts.
Plant Cells
Animal Cells
Animal Cell Cytokinesis
• “Cleavage furrow” forms.
• Microfilaments contracts and divides the
cytoplasm into two parts.
Plant Cell Cytokinesis
• Cell plate develops from Golgi vesicles.
• New cell wall developed around the cell plate.
p
Cell Plate
Cytokinesis
Animal Cell - Mitosis
Plant Cell - Mitosis
Regulation of Cell Division
• Must be controlled.
• Rate of cell division depends on the cell type.
• Example: skin cells divide frequently
liver cells divide as needed
brain cells rarely or never divide
Cells will stop dividing when the surrounding cell
density reaches a specific level….... this is called
“Density-Dependent Inhibition”
•
When density is high - no cell division.
• When density is low - cells divide.
Cancer Cells
• A tumor is a
large mass of
cells. Tumors
form because
cells do not stop
dividing. The
control
mechanisms for
cell division have
failed.
Metastasis
Why Cells Divide
• As cells grow in size they become less healthy....
If they don’t divide (split in two) they will die.
2 Reasons Why Cells Become
Less Healthy
• 1. Surface area-to-Volume ratio
becomes too small.
• 2. The nucleus is limited in regulating
cell activities…. its genome-to-volume
ratio becomes too small.
Surface area-to-Volume Ratio
• When a cell grows, the volume of the cell increases faster than the
surface area surrounding it.
• When we say “the surface area-to-volume ratio is large”, that means
there is a large surface area relative to volume.
• When we say “the surface area-to-volume ratio is small”, that means
the surface area is small relative to volume.
• When the surface-to-volume ratio is large, the cell can effectively
react with the outside environment…..for example…. adequate
amounts of water and oxygen can move into the cell, and wastes
can rapidly be eliminated.
• When the surface-to-volume ratio is small the cell is unable to
exchange enough substances to service the cell. The cell dies.
Genome-to-Volume Ratio
• The DNA in a cell is referred to as its “genome” (all of its genes).
• The genome controls all of the cell’s activities by producing
enzymes…. which trigger and control the cell’s necessary functions.
• Because there is a finite amount of DNA, the amount of enzymes is
limited.
• As the cell grows the volume increases, but the amount of DNA
remains the same…. In other words “the genome-to-volume ratio”
decreases….. Eventually there is not enough DNA to regulate the
cell….. cell functions decrease and the cell dies.