Cell Division
Cell division consists of two phases:
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nuclear division
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cytokinesis = division of the cytoplasm
There are two kinds of nuclear division:
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MITOSIS = creates two identical daughter cells
MEIOSIS = a reduction division that produces
daughter cells containing half the genetic info. of
the parent cell
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During periods of non-cell division the genetic
material exists in an uncoiled form called
Chromatin
Before cell division can take place the
chromatin must begin to condense into tightly
coiled bodies called Chromosomes
Each chromosome is made of two identical
halves called Sister Chromatids that are joined
by a Centromere
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Diploid Cells (2n): two copies of every
chromosome; form a Homologous Pair
Homologous Pair: one homologue is
from the mother, one from the father
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HUMANS = 46 chromosomes, 23 homologous
pairs, 92 chromatids
When a cell is not dividing = chromatin is
enclosed in the nuclear envelope
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Microtubule Organizing Centers (MTOC) =
Centrosomes contain a pair of centrioles used in
cell division; located in cytoplasm
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The non-dividing period of the cell cycle =
INTERPHASE
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MITOSIS: there are four phases in mitosis =
prophase, metaphase, anaphase, and
telophase
PROPHASE:
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Nucleoli disappear
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Chromatin condenses into chromosomes
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Nuclear envelope breaks down
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Mitotic Spindleis assembled as Centrosomes
(MTOC) move toward the poles of the cell
Microtubules from the centrosomes connect to a
region of the centromere called the Kinetochore
Metaphase
Chromosomes line up along the Metaphase
Plate
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Plate exists between the poles of the cell
Identical chromatids are attached to kinetochore
fibers radiating from opposite ends of the cell
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Anaphase
Paired centromeres of each chromosome move
apart
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Move toward opposite ends of cell
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Cell elongates
Result = 2 poles have identical collections of
chromosomes
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Telophase
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Concludes nuclear division
Nuclear envelope develops around each pole,
forming 2 nuclei
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Chromosomes uncoil into chromatin
Cytokinesis occurs – divides the cytoplasm into
2 cells
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Once Mitosis is completed, Interphase begins.
Interphase is characterized as a period of
growth.
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Growth is divided into 3 periods: G1, S, G2 (G =
Gap)
● G1 = growth and synthesis of non-DNA
compounds like lipids, proteins, and carbs.
Begins as soon as mitosis ends.
● S = DNA is synthesized as chromosomes are
duplicated
● G2 = second growth phase, metabolic
activities, preparation for mitosis
Fast Facts:
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90% of the cell cycle is spent in Interphase
A cell that does not enter the S phase will not
divide
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Mitosis is unique to Eukaryotes
Mitosis is reliable, only one error per 100,000 cell
divisions
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MEIOSIS
Cell division of the sex cells (gametes), sperm
cell and egg cell
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Consists of two groups of divisions (Meiosis I &
Meiosis II)
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Interphase I :
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Chromosomes replicate as in mitosis
Duplicated chromosomes consist of two
identical sister chromatids
MEIOSIS I
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Prophase I:
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Nucleolus disappears
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Chromatin condenses into chromosomes
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Nuclear envelope breaks down
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Spindle apparatus develops
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Homologous chromosomes pair, a process
called synapsis
Pairs of homologues = Tetrads
Synapsis
During synapsis, corresponding regions along
non-sister chromatids form close associations
called Chiasmata = sites where genetic material
is exchanged
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Crossing Over= the exchange of genetic
material between non-sister chromatids
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Metaphase I:
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Homologous pairs are lined up at the
metaphase plate
Microtubules extend from one pole and attach
to centromere (kinetochore) of one member
from each homologous pair
Anaphase I:
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Homologues within tetrads uncouple
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Move towards opposite poles
Telophase I:
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Chromosomes have reached poles
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Nuclear envelope redevelops
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Each pole has half the number of chromosomes
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Each chromosome contains 2 chromatids
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Cytokinesis follows
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Interphase II may follow (no duplication)
Prophase II:
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Nuclear envelope disappears
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Spindle fibers form
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NO CROSSING OVER!
Metaphase II:
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Chromosomes align singly at the Metaphase
plate
Anaphase II:
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Each chromosome is pulled apart into 2
chromatids
Chromatids migrate to poles
Telophase II:
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Nuclear envelope reappears
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Cytokinesis occurs
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END RESULT = 4 Haploid daughter cells
Genetic Variation
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Occurs as a result of meiosis = Sexual Reproduction
Reassortment = Genetic Recombination; 3 events are
responsible:
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Crossing over: Occurs during Prophase I; non-sister
chromatids of homologous chromosomes exchange
pieces of genetic material
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Independent Assortment of Homologues: random
separation during Metaphase I
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Random Joining of Gametes: which sperm cell
fertilizes the egg is random
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Why Do Cells Divide?
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Maintain a low surface area to volume ratio
Many microscopic cells are more efficient than a
few large ones
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Maintain a low genome to volume ratio (genome
controls the cell)
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Density Dependent Inhibition= cell division
stops when cell density reaches a certain
maximum
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