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Chapter 8
The Cellular Basis of
Reproduction and Inheritance
Rain Forest Rescue
• Scientists in Hawaii are attempting to "rescue"
endangered species from extinction by
promoting reproduction
• Reproduction is one phase of an organism's
life cycle
– Sexual reproduction
• Fertilization of sperm and egg produces
offspring
– Asexual reproduction
• Offspring are produced by a single parent,
without the participation of sperm and egg
• Cell division is at the heart of organismal
reproduction
CONNECTIONS BETWEEN CELL DIVISION
AND REPRODUCTION
8.1 Like begets like, more or less
• Asexual reproduction
– Chromosomes are duplicated and cell
divides
– Each daughter cell is genetically identical to
the parent and the other daughter
• Sexual reproduction
– Each offspring inherits a unique
combination of genes from both parents
– Offspring can show great variation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
8.2 Cells arise only from preexisting cells
• "Every cell from a cell" is at the heart of the
perpetuation of life
– Can reproduce an entire unicellular
organism
– Is the basis of sperm and egg formation
– Allows for development from a single
fertilized egg to an adult organism
– Functions in an organism's renewal and
repair
8.3 Prokaryotes reproduce by binary fission
• Prokaryotic cells reproduce asexually by a type
of cell division called binary fission
– Genes are on one circular DNA molecule
– The cell replicates its single chromosome
– The chromosome copies move apart
– The cell elongates
– The plasma membrane grows inward,
dividing the parent into two daughter cells
LE 8-3a
Plasma
membrane
Prokaryotic
chromosome
Cell wall
Duplication of chromosome
and separation of copies
Continued elongation of the
cell and movement of copies
Division into
two daughter cells
LE 8-3b
Colorized TEM 32,500
Prokaryotic chromosomes
THE EUKARYOTIC CELL CYCLE AND MITOSIS
8.4 The large, complex chromosomes of
eukaryotes duplicate with each cell division
• Eukaryotic genes
– Many more than in prokaryotes
– Grouped into multiple chromosomes in the
nucleus
• Eukaryotic chromosomes
– Contain a very long DNA molecule
associated with proteins
– Most of the time occur in the form of thin,
loosely packed chromatin fibers
– Condense into visible chromosomes just
before cell division
• Eukaryotic cell division
– Chromosomes replicate
• Sister chromatids joined together at the
centromere
– Sister chromatids separate
• Now called chromosomes
– Cell divides into two daughter cells
• Each with a complete and identical set of
chromosomes
LE 8-4b
Sister chromatids
Centromere
LE 8-4c
Chromosome
duplication
Centromere
Sister
chromatids
Chromosome
distribution
to
daughter
cells
8.5 The cell cycle multiplies cells
• The cell cycle is an ordered series of events
extending from the time a cell is formed until it
divides into two
• Most of the cell cycle is in interphase
– G1: cell grows in size
– S: DNA synthesis (replication) occurs
– G2: Cell continues to grow and prepare for
division
• The cell actually divides in mitotic (M) phase
– Mitosis: nuclear division
– Cytokinesis: cytoplasmic division
– Duplicated chromosomes evenly distributed
into two daughter nuclei
LE 8-5
INTERPHASE
S
(DNA synthesis)
G1
G2
8.6 Cell division is a continuum of dynamic
changes
• Interphase: Duplication of the genetic material
ends when chromosomes begin to become
visible
• Prophase (the first stage of mitosis): The
mitotic spindle is forming. Centrosomes
migrate to opposite ends of the cell
• Prometaphase: Chromatins completely coil
into chromosomes; nucleoli and nuclear
membrane disperse
• Metaphase: The spindle is fully formed;
chromosomes are aligned single file with
centromeres on the metaphase plate
• Anaphase: Chromosomes separate from the
centromere, dividing to arrive at poles
• Telophase: Cell elongation continues, a
nuclear envelope forms around chromosomes,
chromosomes uncoil, and nucleoli reappear
• Cytokinesis: The cytoplasm divides
LE 8-6a
INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Nuclear
envelope
PROPHASE
Chromatin
Plasma
membrane
Early mitotic
spindle
PROMETAPHASE
Centrosome
Chromosome, consisting
of two sister chromatids
Centromere
Fragments
of nuclear
envelope
Kinetochore
Spindle
microtubules
LE 8-6b
METAPHASE
ANAPHASE
Cleavage
furrow
Metaphase
plate
Spindle
TELOPHASE AND CYTOKINESIS
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
8.7 Cytokinesis differs for plant and animal cells
• Animals
– Ring of microfilaments contracts into
cleavage furrow
– Cleavage occurs
• Plants
– Vesicles fuse into a membranous cell plate
– Cell plate develops into a new wall between
two daughter cells
Animation: Cytokinesis
LE 8-7a
Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
LE 8-7b
Cell plate
forming
Wall of
parent cell
Cell wall
Vesicles containing
cell wall material
Daughter
nucleus
New cell wall
Cell plate
Daughter cells
8.8 Anchorage, cell density, and chemical growth
factors affect cell division
• An organism must be able to control the timing
of cell division
• Anchorage dependence
– Most animal cells must be in contact with a
solid surface to divide
• Density-dependent inhibition
– Cells form a single layer
– Cells stop dividing when they touch one
another
– Inadequate supply of growth factor causes
division to stop
LE 8-8a
Cells anchor to
dish surface
and divide.
When cells have
formed a complete
single layer, they
stop dividing
(density-dependent
Inhibition).
If some cells are
scraped away, the
remaining cells
divide to fill the dish
with a single layer
and then stop
(density-dependent
inhibition).
LE 8-8b
After forming a
single layer,
cells have
stopped dividing.
Providing an
additional supply of
growth factors
stimulates
further cell division.
8.9 Growth factors signal the cell cycle control
system
• The cell cycle control system regulates the
events of the cell cycle
• If a growth factor is not released at three major
checkpoints, the cell cycle will stop
– G1 of interphase
– G2 of interphase
– M phase
LE 8-9a
G1 checkpoint
G0
Control
system
G1
M
M checkpoint
G2 checkpoint
G2
S
• How a growth factor might affect the cell cycle
control system
– Cell has receptor protein in plasma
membrane
– Binding of growth factor to receptor triggers
a signal transduction pathway
• Molecules induce changes in other
molecules
– Signal finally overrides brakes on the cell
cycle control system
LE 8-9b
Growth factor
Plasma membrane
Relay
proteins
Receptor
protein
Signal
transduction
pathway
G1 checkpoint
Control
system
G1
M
S
G2
CONNECTION
8.10 Growing out of control, cancer cells produce
malignant tumors
• Cancer cells do not respond normally to the
cell cycle control system
– Divide excessively
– Can invade other tissues
– May kill the organism
• If an abnormal cell avoids destruction by the
immune system, it may form a tumor
– Benign: abnormal cells remain at original
site
– Malignant: abnormal cells can spread to
other tissues and parts of the body
– Metastasis: spread of cancer cells through
the circulatory system
LE 8-10
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows from a
single cancer cell.
Cancer cells invade
Neighboring tissue.
Cancer cells spread through
lymph and blood vessels to
other parts of the body.
• Cancers are named according to location of
origin
– Carcinoma: external or internal body
coverings
– Sarcoma: tissues that support the body
– Leukemia and lymphoma: blood-forming
tissues
• Radiation and chemotherapy are effective as
cancer treatments because they interfere with
cell division
8.11 Review of the functions of mitosis: growth,
cell replacement, and asexual reproduction
• When the cell cycle operates normally, mitotic
cell division functions in
– Growth
– Replacement of damaged or lost cells
– Asexual reproduction
Video: Hydra Budding
MEIOSIS AND CROSSING OVER
8.12 Chromosomes are matched in homologous
pairs
• The somatic (body) cells of each species
contain a specific number of chromosomes
• Humans and most other organisms have pairs
of homologous chromosomes
– Carry genes for the same characteristics at
the same place, or locus
• Except sex chromosomes
– One chromosome is inherited from the
female parent, one from the male
LE 8-12
Chromosomes
Centromere
Sister chromatids
8.13 Gametes have a single set of chromosomes
• Diploid cells have two sets of chromosomes
(2n)
– Somatic cells
• Haploid cells have one set of chromosomes (n)
– Gametes (egg and sperm cells)
• Sexual life cycles involve the alternation of
haploid and diploid stages
– Fusion of haploid gametes in fertilization
forms a diploid zygote
LE 8-13
Haploid gametes (n = 23)
n
Egg cell
n
Sperm cell
Meiosis
Fertilization
Diploid
zygote
(2n = 46)
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
2n
8.14 Meiosis reduces the chromosome number
from diploid to haploid
• Meiosis
– Like mitosis, is preceded by chromosome
duplication
– Unlike mitosis, cell divides twice to form four
haploid daughter cells
• The process of meiosis includes two
consecutive divisions
– Meiosis I
• In synapsis, homologous chromosomes are
paired
• In crossing over, homologous chromosomes
exchange corresponding segments
• Each homologous pair divides into two
daughter cells, each with one set of
chromosomes consisting of two chromatids
• Meiosis II
– Essentially the same as mitosis
– Sister chromatids of each chromosome
separate
– Result is four cells, each with half as many
chromosomes as the parent
LE 8-14a
MEIOSIS I : Homologous chromosome separate
INTERPHASE
PROPHASE I
METAPHASE I
ANAPHASE I
Centrosomes
(with centriole
pairs)
Microtubules
Sister chromatids
Sites of crossing over attached to Metaphase remain attached
plate
kinetochore
Spindle
Nuclear
envelope
Sister
chromatids
Chromatin
Tetrad Centromere
(with kinetochore)
Homologous
chromosomes separate
LE 8-14b
MEIOSIS II : Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
Haploid daughter
cells forming
8.15 Review: A comparison of mitosis and
meiosis
• Mitosis
– Provides for growth, tissue repair, and
asexual reproduction
– Produces daughter cells genetically
identical to the parent
• Meiosis
– Needed for sexual reproduction
– Produces daughter cells with one member
of each homologous chromosome pair
LE 8-15
MITOSIS
MEIOSIS
Site of
crossing over
Parent cell
(before chromosome replication)
MEIOSIS I
Prophase I
Prophase
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
replication
Chromosome
replication
Duplicated
chromosome (two
sister chromatids)
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
Metaphase I
Anaphase I
Telophase I
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
No further
chromosomal
replication;
sister
chromatids
separate during
anaphase II
Haploid
n=2
Daughter
cells of
meiosis I
MEIOSIS II
n
n
n
Daughter cells of meiosis II
n
8.16 Independent orientation of chromosomes in
meiosis and random fertilization lead to varied
offspring
• Reshuffling of the different versions of genes
during sexual reproduction produces genetic
variation
– Random arrangements of chromosome pairs at
metaphase I of meiosis lead to many different
combinations of chromosomes
– Random fertilization of eggs by sperm greatly
increases this variation
Animation: Genetic Variation
LE 8-16
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
8.17 Homologous chromosomes carry different
versions of genes
• Each chromosome of a homologous pair can
bear different versions of genes at
corresponding loci
– Makes gametes (and thus offspring)
different from one another
– Examples: coat color and eye color in mice
LE 8-17a
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
Meiosis
c
e
White
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Chromosomes of
the four gametes
LE 8-17b
Brown coat (C); black eyes (E)
White coat (c); pink eyes (e)
8.18 Crossing over further increases genetic
variability
• Crossing over is a genetic rearrangement
between two homologous chromosomes
– Homologues pair up into a tetrad during
prophase I of meiosis
– Maternal and paternal chromatids break at
the same place
– The two broken chromatids join together in
a new way at the chiasma
Animation: Crossing Over
TEM 2,200
LE 8-18a
Tetrad
Chiasma
Centromere
– When homologous chromosomes separate
at anaphase I, each contains a new
segment
– In meiosis II, each sister chromatid goes to
a different gamete
– Gametes of four genetic types result
LE 8-18b
Coat-color
genes
Eye-color
genes
C
E
Tetrad
(homologous pair of
chromosomes in synapsis)
c
e
Breakage of homologous chromatids
C
E
c
e
Joining of homologous chromatids
C
E
Chiasma
c
e
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
e
Separation of chromatids at
anaphase II and
completion of meiosis
C
E
Parental type of chromosome
C
e
Recombinant chromosome
c
E
c
e
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
ALTERATIONS OF CHROMOSOME NUMBER
AND STRUCTURE
8.19 A karyotype is a photographic inventory of an
individual's chromosomes
•
A blood sample is treated with a chemical that
stimulates mitosis
•
After several days, another chemical arrests mitosis at
anaphase, when chromosomes are most highly
condensed
•
Chromosomes are photographed and electronically
arranged by size and shape into the karyotype
•
Normal humans have 22 pairs of autosomes and two
sex chromosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 8-19
Packed red and
white blood cells
Blood
culture
Hypotonic
solution
Fixative
White
blood
cells
Centrifuge
Stain
Fluid
Centromere
Siste
c
r hromatids
2,600
Pair of homologous
chromosomes
CONNECTION
8.20 An extra copy of chromosome 21 causes
Down syndrome
• A person may have an abnormal number of
chromosomes
• Down syndrome is caused by trisomy 21, an
extra copy of chromosome 21
– The most common human chromosome
number abnormality
– Many physical and mental problems
– Increased incidence in older mothers
LE 8-20c
8.21 Accidents during meiosis can alter
chromosome number
• Abnormal chromosome count is a result of
nondisjunction
– The failure of homologous pairs to separate
during meiosis I
– The failure of sister chromatids to separate
during meiosis II
LE 8-21a
LE 8-21b
• Fertilization of an egg resulting from
nondisjunction with a normal sperm results in a
zygote with an abnormal chromosome number
– May be involved in trisomy 21
LE 8-21c
CONNECTION
8.22 Abnormal numbers of sex chromosomes do
not usually affect survival
• Nondisjunction can produce gametes with
extra or missing sex chromosomes
– Upset the genetic balance less than
unusual numbers of autosomes
– Lead to varying degrees of malfunction in
humans
– Usually do not affect survival
8.23 Alterations of chromosome structure can
cause birth defects and cancer
• Breakage can lead to rearrangements affecting
genes on one chromosome
– Deletion: loss of a fragment of chromosome
– Duplication: addition of a fragment to sister
chromatid
– Inversion: reattachment of a fragment in
reverse order
– Inversions least harmful because all genes
are present in normal number
LE 8-23a
• Translocation is the attachment of a
chromosomal fragment to a nonhomologous
chromosome
– Can be reciprocal
– May or may not be harmful
LE 8-23b
• Chromosomal changes in sperm or egg cells
can cause congenital disorders
• Chromosomal changes in a somatic cell may
contribute to the development of cancer
LE 8-23c