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CHAPTER 8
The Cellular Basis of
REPRODUCTION
CONNECTIONS BETWEEN CELL
DIVISION AND REPRODUCTION
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Like begets like, more or less
– Living organisms reproduce by two methods
–
Asexual reproduction
– Offspring are identical to the original cell or organism
– Involves inheritance of all genes from one parent
– Involves MITOSIS (Eukaryotic Organisms); BINARY FISSION
(Prokaryotic Organisms)
– CLONING is an asexual process
–
Sexual reproduction
– Offspring are similar to parents, but show variations in traits
– Involves inheritance of unique sets of genes from two parents
– Involves MEIOSIS
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Cells arise only from preexisting cells
– Cell division perpetuates life
–
–
Cell division is the reproduction of cells
Cells are composed of : Carbohydrates, Lipids, Proteins,
Nucleic Acids, so cell division requires the building of
these molecules from their monomers
– Is cell division an ENDERGONIC or EXERGONIC
process? What does it require?
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Cells arise only from preexisting cells
–
Roles of cell division
– Asexual reproduction
– Reproduction of an entire single-celled organism
(MITOSIS or BINARY FISSION)
– Growth of a multicellular organism (MITOSIS)
– Growth from a fertilized egg into an adult (MITOSIS)
– Repair and replacement of cells in an adult (MITOSIS)
– Sexual reproduction
– Sperm and egg production (MEIOSIS)
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Prokaryotes reproduce by binary fission
– Binary fission means “dividing in half”
– Occurs in prokaryotic cells
– Two identical cells arise from one cell
– Steps in the process
– A single circular chromosome duplicates, and the copies begin
to separate from each other
– The cell elongates, and the chromosomal copies separate
further
– The plasma membrane grows inward at the midpoint to divide
the cells
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Plasma
membrane
Prokaryotic
chromosome
Cell wall
1
Duplication of chromosome
and separation of copies
Plasma
membrane
Prokaryotic
chromosome
Cell wall
1
Duplication of chromosome
and separation of copies
2
Continued elongation of the
cell and movement of copies
Plasma
membrane
Prokaryotic
chromosome
Cell wall
3
1
Duplication of chromosome
and separation of copies
2
Continued elongation of the
cell and movement of copies
Division into
two daughter cells
Prokaryotic chromosomes
THE EUKARYOTIC CELL CYCLE AND
MITOSIS
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The large, complex chromosomes of
eukaryotes duplicate with each cell division
– Eukaryotic chromosomes are composed of chromatin
– Chromatin = DNA + proteins
– To prepare for division, the chromatin becomes highly
compact, and the chromosomes are visible with a
microscope
– Early in the division process, chromosomes duplicate
– Each chromosome appears as two sister chromatids,
containing identical DNA molecules
– Sister chromatids are joined at the centromere, a narrow
region
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Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to
daughter
cells
Sister chromatids
Centromere
The Cell Cycle
– The cell cycle is an ordered sequence of events for cell
division
– It consists of two stages
– Interphase: duplication of cell contents
– G1—growth, increase in cytoplasm
– S—duplication of chromosomes (DNA REPLICATION)
– G2—growth, preparation for division
– Mitotic phase: division
– Mitosis—division of the nucleus
– Cytokinesis—division of cytoplasm
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INTERPHASE
S
(DNA synthesis)
G1
G2
Cell division is a continuum of dynamic changes
– Mitosis progresses through a series of stages
– Prophase (Prometaphase)
– Metaphase
– Anaphase
– Telophase
– Cytokinesis overlaps telophase
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Cell division is a continuum of dynamic
changes
– Before Mitosis begins:
– Interphase
– In the cytoplasm
– Cytoplasmic contents double
– New organelles are formed
– In the nucleus
– Chromosomes (DNA) duplicate during
the S phase
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Cell division is a continuum of dynamic
changes
– Prophase and Metaprophase
– In the cytoplasm
– Microtubules begin to emerge forming
the spindle
– In the nucleus
– Chromosomes coil and become
compact
– Nuclear Membrane disappears
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INTERPHASE
Chromatin
Centrosomes
(with centriole pairs)
PROPHASE
Early mitotic Centrosome
spindle
PROMETAPHASE
Fragments
of nuclear
envelope
Centromere
Plasma
Nuclear
envelope membrane Chromosome, consisting
of two sister chromatids
Nucleolus
Kinetochore
Spindle
microtubules
Cell division is a continuum of dynamic
changes
– Metaphase
– Chromosomes align at the cell
equator (middle of the cell)
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Cell division is a continuum of dynamic
changes
– Anaphase
– Sister chromatids separate at the
centromeres and move to
opposite poles of the cell
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Cell division is a continuum of dynamic
changes
– Telophase
– Opposite of PROPHASE
– The nuclear membrane forms
– Chromatin uncoils
– The spindle disappears
– CYTOKINESIS occurs
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METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Daughter
chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming
Cytokinesis differs for plant and animal cells
– Cytokinesis
– Cleavage in animal cells
– A cleavage furrow forms from a contracting ring of
microfilaments, interacting with myosin
– The cleavage furrow deepens to separate the contents into
two cells
– Cytokinesis in plant cells
– A cell plate forms in the middle from vesicles containing cell
wall material
– The cell plate grows outward to reach the edges, dividing the
contents into two cells
– Each cell has a plasma membrane and cell wall
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Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Wall of
Cell plate Daughter
parent cell forming nucleus
Cell wall
New cell wall
Vesicles containing Cell plate Daughter cells
cell wall material
Cell division is a continuum of dynamic
changes
– Applying Your Knowledge
Human cells have 46 chromosomes (the
DIPLOID number or 2 sets)
– At the end of Mitosis, how many
chromosomes are in each cell?
– Is the genetic material identical in each
cell?
Copyright © 2009 Pearson Education, Inc.
CLONING
• A somatic cell from one parent is used
• The nucleus of a somatic cell in put into an
egg cell (ovum) that has had its nucleus
removed
• The ovum with the somatic cell nucleus
behaves like a Zygote
• A new eukaryotic organism is produced with
the DNA of only one parent (it is a clone of the
parent)
Donor
cell
Nucleus from
donor cell
Reproductive
cloning
Implant blastocyst in
surrogate mother
Remove
nucleus
from egg
cell
Add somatic cell
from adult donor
Grow in culture
to produce an Therapeutic
early embryo cloning
(blastocyst)
Remove embryonic
stem cells from
blastocyst and
grow in culture
Clone of
donor is born
Induce stem
cells to form
specialized cells
CONNECTION: Growing out of control, cancer cells
produce malignant tumors
– Cancer cells escape controls on the cell cycle
– Cancer cells divide rapidly
– They spread to other tissues through the
circulatory system
– Growth is not inhibited by other cells, and tumors
form
– Benign tumors remain at the original site
– Malignant tumors spread to other locations
by metastasis
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Review: Mitosis provides for growth, cell
replacement, and asexual reproduction
• Mitosis produces genetically
identical cells for
– Growth
– Replacement
– Asexual reproduction
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MEIOSIS AND
CROSSING OVER
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Gametes have a single set of chromosomes
– Meiosis is a process that converts diploid cells into
haploid cells
– Diploid cells have two homologous sets (2n) of
chromosomes
– Haploid cells have one set (1n) of chromosomes
– Meiosis occurs in the sex organs, producing gametes—
sperm and eggs
– Fertilization is the union of sperm and egg
– The zygote formed by fertilization has a diploid
chromosome number (2n), one set from each parent
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Chromosomes are matched in homologous pairs
– Somatic cells have pairs of homologous chromosomes,
receiving one member of each pair from each parent
– Homologous chromosomes are matched in
– Length
– Gene locations
– A locus (plural, loci) is the position of a gene
– Different versions of a gene may be found at the same locus on
maternal and paternal chromosomes
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Homologous pair of
Chromosomes: One from Mother; One from Father
Centromere
Sister chromatids
One duplicated
chromosome
Mitosis and meiosis have important
similarities and differences
– Which characteristics are similar for mitosis and
meiosis?
– One duplication of chromosomes
– Which characteristics are unique to meiosis?
– Two divisions of the cells (stages I and II): 4 new
cells formed instead of 2
– Pairing of homologous chromosomes during
PROPHASE I and exchange of genetic material by
CROSSING OVER
– Homologous pairs of chromosomes line up a the
cell equator during METAPHASE I
– Cells formed are NOT GENETICALLY IDENTICAL
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MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
ANAPHASE I
Microtubules Metaphase Sister chromatids
remain attached
plate
attached to
Spindle kinetochore
Sites of crossing over
Sister
Chromatin chromatids
Tetrad
Centromere
(with kinetochore)
Homologous
chromosomes separate
C
E
Chiasma
e
c
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
4
C
e
Separation of chromatids at
anaphase II and
completion of meiosis
E
Parental type of chromosome
C
e
c
E
c
e
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
MEIOSIS II: Sister chromatids separate
TELOPHASE II
AND CYTOKINESIS
PROPHASE I
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
Haploid daughter
cells forming
Mitosis and meiosis have important
similarities and differences
– What is the outcome of each process?
– Mitosis: two genetically identical cells,
with the same chromosome number
as the original cell
– Meiosis: four genetically different
cells, with half the chromosome
number of the original cell
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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
Independent orientation of chromosomes and
crossing over in meiosis and random fertilization lead
to varied offspring
 Independent orientation at Metaphase I
– Each pair of chromosomes independently aligns at the
cell equator; there is an equal probability of the
maternal or paternal chromosome facing a given pole
– The number of combinations for chromosomes
packaged into gametes is 2n where n = haploid number
of chromosomes (How many combinations for human?)
 Crossing over in Prophase I (How many
combinations in humans?)
 Random Fertilization
– The combination of each unique sperm with each
unique egg increases genetic variability
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1 Combination 2
Combination 3 Combination 4
Accidents during meiosis can alter
chromosome number
– Nondisjunction is the failure of chromosomes or
chromatids to separate during meiosis
– During Meiosis I
– Both members of a homologous pair go to one pole
– During Meiosis II
– Both sister chromatids go to one pole
– Fertilization after nondisjunction yields zygotes with
altered numbers of chromosomes
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Centromere
Sister
chromatids
Pair of homologous
chromosomes
5
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
Number of chromosomes
n–1
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n+1
n–1
n
Number of chromosomes
n
An 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
– An 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
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