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Cell Division
Chapter 8
http://www.youtube.com/watch?v=Q6ucKWIIFmg
Cell Division
 Doubling organelles and proteins
 DNA replication
 Nuclear division
 Cytoplasmic division

Cell Increase and Decrease
Maintain homeostasis
Cell numbers kept in check by this
mechanism
Through cell division of somatic cells and cell
death
Cell division – interphase, mitosis and cytokinesis
Cell death – apoptosis
Cell Increase and Decrease
 Somatic cells
 Asexual reproduction
 increase in number of
somatic cells
 Increase in number
unicellular organisms
 Germ cells
 Sexual reproduction
 requires the production
of eggs and sperm
Important terms:
DNA
Chromosomes
Chromatin
Chromatid
Mitosis verse Meiosis??
 Mitosis
Cell division mechanism
that occurs in
nonreproductive cells
somatic cell nuclei
 Meiosis
Cell division mechanism
that occurs in cells that
participate in sexual
reproduction
gamete nuclei
Cell Cycle – Somatic cells
 Set of stages that
involves cell growth
and nuclear division
 Consists of:
Interphase
 G1
S
 G2
Meiotic stage
 Mitosis and
Cytokinesis
http://www.cellsalive.com/cell_cycle.htm
Interphase
 When the cell carries
on its usual functions
 Main stages:
G1
 Gap before DNA
synthesis begins
S
 Time when DNA
duplicated***************
G2
 Gap between time
DNA duplication ends
and mitosis begins
Interphase in Meiosis?
 Meiosis is a "oneway" process
Cannot be said to
engage in a cell cycle
as mitosis does
 Preparatory steps that
lead up to meiosis are
identical in pattern
and name to the
interphase of the
mitotic cell cycle
Cytokinesis in Animal Cells
 Cytoplasmic cleavage
 Accompanies mitosis
Separate process
 Cleavage furrow forms
between daughter
nuclei
 Contractile ring
contracts deepening
the furrow
 Continues until
separation is complete
Human DNA in somatic cells
 22-23 pairs of
homologous
chromosomes
Difference?
Autosomes (1-22)
Sex chromosomes (23)
 Somatic cells have
46 chromosomes
Diploid
2n
Human DNA in gametes
 Due to reductional
division
 Halves the diploid
number (2n) to a
haploid number (n)
 23 total chromosomes
Division of the Nucleus
 Nucleus must be
divided
Parent cell’s DNA into
2 nuclei
 2 ways nucleus can
divide:
Mitosis
Meiosis
Mitosis
Maintaining the Chromosome Number
DNA Replication…Somatic cells
 Duplicated chromosome
 Composed of 2 sister
chromatids
 held together by a
centromere
 Sister chromatids
 Genetically identical
 When separate, each
daughter nucleus gets a
chromosome
DNA
copied
DNA
divided
Chromosomes
 Chromosomes are paired in
somatic cells
 homologous chromosomes,
homologues
 contain information about the same
traits but the information may vary
 Cells that have two of each type of
chromosome are called diploid
cells
 one chromosome of each pair is
inherited from the mother and the
other is inherited from the father
The difference between homologous
chromosomes and sister chromatids
Mitosis
Mitosis
4 main stages:
Prophase
Metaphase
Anaphase
Telophase
How the cell cycle works
Mitosis
 1. Prophase –
Mitosis begins!
Threadlike form
Spindle fibers appear
DNA start to condense
Aster formed
Nuclear envelope starts
to break apart
Centrioles move to
opposite sides of the
cell
Mitosis
 2. Metaphase
Duplicated chromosomes
aligned midway between the
poles
Associated with spindle fibers
Mitosis
 3. Anaphase
Sister chromatids
separate from each other
and move to opposite
poles
Become daughter
chromosomes
Mitosis
 4. Telophase
Return to threadlike form
as in prophase
New nuclear envelope
separates each
chromosome cluster
2 new nuclei!!!!!
http://www.cellsalive.com/mitosis.htm
Meiosis
Reducing the Chromosome Number
Meiosis
Mechanism for dividing the nucleus of
germ cells
Oogonia and spermatogonia
2n
Meiosis must take place prior to formation
of gametes
Sperm and eggs
n
First stage in sexual reproduction
Meiosis Overview
 Occurs in the life cycle of sexually reproducing
organisms
 Reduces the chromosome number
 2 divisions, 4 daughter cells
 Cells are diploid at beginning of meiosis
 TWO consecutive divisions
Result is 4 haploid nuclei
 Divided into:
Meiosis I
Meiosis II
Reducing the Chromosome Number
 Genetic Recombination
Promotes genetic
variability
Happens by:
 Crossing Over
 Independent Assortment of
paired chromosomes
 Random Fertilization
Meiosis comparison
Meiosis I
 Crossing over
 Homologous
chromosomes line up
 Homologous
chromosomes split
 Two haploid cells formed
 Cytokinesis occurs
Meiosis II
 Essentially the same as
Meiosis I
 Starts with a haploid cell
that has NOT undergone
chromosome duplication
Comparisons between males and females
 Spermatogenesis
Begins at puberty and
continues throughout
life
 Oogenesis
 Begins in the fetus
 Primary oocytes are
arrested in prophase I
 At puberty, one primary
oocyte continues the
process of meiosis during
each menstrual cycle
Comparisons
 Mitosis
DNA replication occurs
only once
Requires only one
division
Produces two daughter
cells
Diploid daughter cells
 2n
Genetically identical
cells produced
Occurs all the time
 Meiosis
DNA replication occurs
only once
Requires two divisions
Produces four daughter
cells
Haploid daughter cells
n
Genetically variable
cells produced
Occurs only at certain
times
MITOSIS
MEIOSIS
Parent cell
(before chromosome duplication)
Site of
crossing over
MEIOSIS I
Prophase I
Prophase
Duplicated
chromosome
(two sister
chromatids)
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
duplication
Chromosome
duplication
2n = 4
Chromosomes
align at the
metaphase plate
Metaphase
Anaphase
Telophase
Sister chromatids
separate during
anaphase
2n
2n
Daughter cells
of mitosis
Tetrads
align at the
metaphase plate
Homologous
chromosomes
separate
(anaphase I);
sister chromatids remain
together
No further
chromosomal
duplication;
sister
chromatids
separate
(anaphase II)
Metaphase I
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
Comparisons
Overview of the Life Cycle of Humans
Cell division in other
organisms
How Plant Cells Divide
 Occurs in meristematic tissues
 Same phases as animal cells
 Plant cells do not have centrioles or asters
Plant Cells
Cytokinesis in Plant Cells
 Flattened, small disk appears between daughter
cells
 Golgi apparatus produces vesicles which move
to disk
 Release molecules which build new cell walls
 Vesicle membranes complete plasma
membranes
Prokaryotes Have a Simple Cell
Cycle
 Cell division in prokaryotes takes place in
two stages (simple cell cycle)
1. copy the DNA

this process is called replication
2. split the cell in two to form daughter cells

this process is called binary fission
Cell Division in Prokaryotes
 Binary Fission
 Prokaryotes have a single
chromosome
 Chromosomal replication
occurs before division
 Cell elongates to twice its
length
 Cell membrane grows
inward until division is
complete
Alterations of chromosome
number and structure
Extra copy of chromosome 21 causes Down
syndrome
Trisomy 21 involves the inheritance of three
copies of chromosome 21
– Trisomy 21 is the most common human chromosome
abnormality
– Imbalance in chromosome number causes Down
syndrome, which is characterized by
– Characteristic facial features
– Susceptibility to disease
– Shortened life span
– Mental retardation
– Variation in characteristics
– The incidence increases with the age of the mother
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
40
30
35
Age of mother
45
50
Accidents during meiosis can alter
chromosome number
Nondisjunction is the failure of chromosomes or
chromatids to separate during meiosis
Fertilization after nondisjunction yields zygotes
with altered numbers of chromosomes
Abnormal numbers of sex
chromosomes
Sex chromosome abnormalities tend to be less
severe as a result of
– Small size of the Y chromosome
– X-chromosome inactivation
– In each cell of a human female, one of the two X
chromosomes becomes tightly coiled and inactive
– random process that inactivates either the maternal or
paternal chromosome
– Inactivation promotes a balance between the number of X
chromosomes and autosomes
New species can arise from errors in cell
division
Polyploid species have more than two
chromosome sets
– Observed in many plant species
– Seen less frequently in animals
Example
– Diploid gametes are produced by failures in meiosis
– Diploid gamete + Diploid gamete  Tetraploid
offspring
– The tetraploid offspring have four chromosome sets
Alterations of chromosome structure can cause
birth defects and cancer
Structure changes result from breakage and
rejoining of chromosome segments
– Deletion is the loss of a chromosome segment
– Duplication is the repeat of a chromosome
segment
– Inversion is the reversal of a chromosome segment
– Translocation is the attachment of a segment to a
nonhomologous chromosome; can be reciprocal
Altered chromosomes carried by gametes cause
birth defects
Chromosomal alterations in somatic cells can
cause cancer
Deletion
Duplication
Homologous
chromosomes
Inversion
Chromosome 9
Reciprocal
translocation
Chromosome 22
“Philadelphia chromosome”
Activated cancer-causing gene