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Chapter 8
The Continuity of
Life: How Cells
Reproduce
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc.
8.1 Why Do Cells Divide?
 Cells reproduce by cell division.
• One cell gives rise to two or more cells, called
_______________ cells.
• Each daughter cell receives a complete set of heredity
information—from the parent cell—and about half of the
cytoplasm.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Cell division transmits hereditary information to each
daughter cell.
• The hereditary information in each cell is deoxyribonucleic
acid (DNA).
• DNA is contained in ________________.
• A molecule of DNA consists of smaller subunits called
_____________.
 Segments of different lengths along a DNA molecule
are the units of inheritance called genes.
 Each gene spells out the instructions for making
the proteins of the cell.
Copyright © 2009 Pearson Education Inc.
 The four bases are
adenine (A), thymine
(T), guanine (G), and
cytosine (C).
phosphate
nucleotide
base
T
A
sugar
C
G
G
 The nucleotides
are held together
by __________
bonding between
the bases.
C
C
C
G
A
A
A
T
C
G
A
T
T
A
T
(a) A single strand of DNA
(b) The double helix
Fig. 8-1
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 When a cell divides, it first ____________ its DNA, and
each copy is transferred into each daughter cell.
 Cell division is required for growth and development.
• Cell division in which the daughter cells are genetically
identical to the parent cell is called __________ cell division.
• After cell division, the daughter cells may grow and divide
again, or may differentiate, becoming specialized for specific
functions.
• The repeating pattern of division, growth, and
differentiation followed again by division is called the
cell cycle.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Most multicellular organisms have three categories of
cells.
• Stem cells: retain the ability to divide and can
differentiate into a variety of cell types
• Other cells capable of dividing: typically differentiate
only into one or two different cell types
• Permanently differentiated cells: differentiated cells
that can never divide again
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Cell division is required for sexual and asexual
reproduction.
• Sexual reproduction in eukaryotic organisms occurs
when offspring are produced by the fusion of gametes
(sperm and eggs) from two adults.
• Gametes are produced by __________ cell division, which
results in daughter cells with exactly half of the genetic
information of their parent cells.
• Fertilization of an egg by a sperm results in the
restoration of the full complement of hereditary
information in the offspring.
Copyright © 2009 Pearson Education Inc.
8.1 Why Do Cells Divide?
 Reproduction in which offspring are formed from a
single parent, without having a sperm fertilize an egg,
is called __________ reproduction.
• Asexual reproduction produces offspring that are
genetically identical to the parent.
• Examples of asexual reproduction occur in bacteria,
single-celled eukaryotic organisms, multicellular
organisms such as Hydra, and many trees, plants, and
fungi.
Copyright © 2009 Pearson Education Inc.
The trees in this
grove have already
lost their leaves
(a) Dividing bacteria
The trees in this
grove have begun
to change color
(b) Cell division in Paramecium
bud
(c) Hydra reproduces asexually by budding
The trees in this
grove are still green
(d) A grove of aspens often consists of genetically
identical trees produced by asexual reproduction
Copyright © 2009 Pearson Education Inc.
Fig. 8-2
8.2 What Occurs During The Prokaryotic
Cell Cycle?
 The prokaryotic cell cycle consists of a long
period of growth, during which the cell
duplicates its DNA.
cell division
by binary
fission
cell growth and
DNA replication
(a) The prokaryotic cell cycle
Fig. 8-3a
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8.2 What Occurs During The Prokaryotic
Cell Cycle?
 Cell division in prokaryotes occurs by binary fission, which
means “splitting in two.”
 The prokaryotic chromosome is attached at one point to the
plasma membrane of the cell.
attachment
site
cell
wall
plasma
membrane
circular
DNA
The circular DNA double helix is
attached to the plasma membrane at
one point.
Fig. 8-3b(1)
Copyright © 2009 Pearson Education Inc.
8.2 What Occurs During The Prokaryotic
Cell Cycle?
 During the growth phase of the cell cycle, the DNA is
replicated, producing two identical chromosomes that become
attached to the plasma membrane at two separate points.
 As the cell grows, new plasma membrane is added between
the attachment points of the chromosomes, pushing them
apart.
The DNA replicates and the two DNA
double helices attach to the plasma
membrane at nearby points.
New plasma membrane is added
between the attachment points, pushing
them further apart.
Fig. 8-3b(2)(3)
Copyright © 2009 Pearson Education Inc.
8.2 What Occurs During The Prokaryotic
Cell Cycle?
 Once the cell has doubled in size, the plasma membrane
in the middle of the cell grows inward between the two
DNA attachment sites.
The plasma membrane grows inward
at the middle of the cell.
Fig. 8-3b(4)
Copyright © 2009 Pearson Education Inc.
8.2 What Occurs During The Prokaryotic Cell
Cycle?
 Fusion of the plasma membrane along the equator of the cell
completes binary fission, producing two daughter cells, each
with its own chromosomes
 The two daughter cells are genetically identical to each other
and to the parent cell
The parent cell divides into two daughter
cells.
Fig. 8-3b(5)
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Unlike prokaryotic chromosomes, eukaryotic
chromosomes are separated from the cytoplasm by a
membrane-bound nucleus.
 Eukaryotic cells always have multiple chromosomes.
 Eukaryotic chromosomes contain more DNA than
prokaryotic chromosomes.
 The eukaryotic chromosome consists of DNA bound to
__________.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Prior to cell division, the DNA within each chromosome is
replicated.
• The duplicated chromosomes then consist of two DNA double
helixes and associated proteins that are attached to each other
at the centromere.
 During cell division, proteins fold up the DNA into compact
structures that are 10 times shorter than during the rest of
the cell cycle.
Fig. 8-4
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
• Each of the duplicated chromosomes attached at
the centromere is called a sister ___________.
• During mitotic cell division, the sister chromatids
separate and each becomes a separate
chromosome that is delivered to one of the two
resulting daughter cells.
Copyright © 2009 Pearson Education Inc.
8.3 How Is the DNA In Eukaryotic Cells
Organized?
centromere
genes
duplicated
sister
chromosome
chromatids
(2 DNA double
helices)
(a) A replicated chromosome consists of two sister chromatids
independent
daughter
chromosomes,
each with one
identical DNA
double helix
(b) Sister chromatids separate during cell division
Fig. 8-5
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Eukaryotic
chromosomes
usually occur in
pairs.
• An entire set of
stained
chromosomes
from a single cell
is called a
_____________.
sex
chromosomes
Fig. 8-6
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8.3 How Is The DNA In Eukaryotic Cells
Organized?
 The nonreproductive cells of many organisms have
chromosomes in pairs, with both members of the pair
being the same length.
• The chromosomes are the same length and have the same
staining properties because they have the same genes
arranged in the same order.
 Chromosomes with the same genes are called
homologous chromosomes, or _____________.
 Cells with pairs of homologous chromosomes are called
_________.
 Homologous chromosomes are usually not identical.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 A typical human cell has ____ pairs of chromosomes.
 22 of these pairs have a similar appearance and are called
_________.
 Human cells also have a pair of ____ chromosomes,
which differ from each other in appearance and in genetic
composition.
• Females have two ___ chromosomes.
• Males have one ___ and one ___ chromosome.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 Not all cells have paired chromosomes.
 The ovaries and testes undergo a special kind of cell
division, called meiotic cell division, to produce
gametes (________ and _________).
• Gametes contain only one member of each pair of
autosomes, plus one of the two sex chromosomes.
• Cells with half the number of each type of chromosome
are called __________ cells.
• Fusion of two haploid cells at ______________ produces
a diploid cell with the full complement of
chromosomes.
Copyright © 2009 Pearson Education Inc.
8.3 How Is The DNA In Eukaryotic Cells
Organized?
 The number of different types of chromosomes
in a species is called the haploid number and is
designated n.
• In humans, n = 23.
 Diploid cells contain 2n chromosomes.
• Humans body cells contain 2n = 46 (2 x 23)
chromosomes.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 The eukaryotic cell cycle is divided into two
major phases: interphase and cell division.
• During interphase, the cell acquires nutrients
from its environment, grows, and duplicates its
chromosomes.
• During cell division, one copy of each
chromosome and half of the cytoplasm are
parceled out into each of two daughter cells.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic Cell
Cycle?
 The eukaryotic cell cycle
cell growth and
differentiation
cell
growth
synthesis
of DNA;
chromosomes
are duplicated
Fig. 8-7
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 There are two types of division in eukaryotic cells:
mitotic cell division and meiotic cell division.
• ___________ cell division may be thought of as
ordinary cell division, such as occurs during
development from a fertilized egg, during asexual
reproduction, and in skin, liver, and the digestive tract
every day.
• ___________ cell division is a specialized type of cell
division required for sexual reproduction.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic Cell
Cycle?
 Mitotic cell division
• Mitotic cell division consists of nuclear division (called
mitosis) followed by cytoplasmic division (called
cytokinesis) and the formation of two daughter
cells.
 Meiotic cell division:
• Is a prerequisite for sexual reproduction in all eukaryotic
organisms.
• Meiotic cell division involves a specialized nuclear
division called meiosis.
• It involves two rounds of cytokinesis, producing four
daughter cells that can become gametes.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 The life cycle of eukaryotic organisms include both
mitotic and meiotic cell division.
• A new generation begins with the fusion of two gametes.
• Through mitosis and differentiation, the fertilized egg grows
and develops a multicellular body.
• Meiotic cell division generates new gametes that may unite
with other gametes to produce the next generation.
Copyright © 2009 Pearson Education Inc.
8.4 What Occurs During The Eukaryotic
Cell Cycle?
 Mitotic and meiotic cell division in the
human life cycle
mitotic cell division,
differentiation, and growth
mitotic cell division,
differentiation,
and growth
baby
adults
embryo
meiotic cell
division in
meiotic cell
ovaries
division in
testes
mitotic
cell division,
differentiation,
and growth
haploid
diploid
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egg
fertilized
egg
sperm
fusion of gametes
Fig. 8-8