Cell Reproduction or How cells make copies of themselves Also called Cell Division.
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Transcript of Cell Reproduction or How cells make copies of themselves Also called Cell Division.
Cell Reproductionor
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”.
Sister Chromatids
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.
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 Phases1.Interphase (part of the cell cycle, but not part of mitosis)
2.Prophase
3.Metaphase
4.Anaphase
5.Telophase
These 4 phases are known collectively as “Mitosis”
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.
Plant Cells
Animal Cells
nuclear envelope
Mitosis(the splitting of the nucleus)
prophasemetaphaseanaphase 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.metaphase plane
centriole
centriole
Plant Cells
Animal Cells
Events of Anaphase
• 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.
Chromosomes
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 divideCells 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.