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Chapter 11
The Continuity of Life:
Cellular Reproduction
Chapter 11 Outline
• 1 Cellular Reproduction in the Lives of
Individual Cells and Entire Organisms
• 2 DNA Organization in Eukaryotes
• 3 Mitosis
• 4 Control of Cell Cycle
• 5 Sexual Reproduction
• 6 Meiosis
• 7 Eukaryotic Life Cycle
• 8 Genetic Variability
1 What Is the Role of Cellular Reproduction in the
Lives of Individual Cells and Entire Organisms?
– The Cell Cycle and Cellular Reproduction
– The Prokaryotic Cell Cycle Consists of Growth
and Binary Fission
– The Eukaryotic Cell Cycle Consists of Interphase
and Cell Division
– Eukaryotic Cells Grow and Replicate DNA in
Interphase
– The Process of Mitosis: Asexual Reproduction
– The Process of Meiosis: Prerequisite for Sexual
Reproduction
Cellular Reproduction
•
Intracellular activity between
one cell division to the next
is the cell cycle
– Some activities involve growth
(enlargement) of the cell
– Some activities involve
duplication of genetic material
and cellular division
(reproduction)
cell
division
cell growth and
DNA replication
Cellular Reproduction
•
Reproduction from a single parent is
asexual reproduction
– Some organisms reproduce asexually
Cellular Reproduction
•
Multicellular organisms grow by asexual
reproduction; some reproduce
The Prokaryotic Cell Cycle
Cell cycle in prokaryotes
1. Long growth phase
•
•
Replication of circular
DNA chromosome
Duplicate
chromosomes
anchored to membrane
2. Cell increases in size,
pulling duplicated
chromosomes apart…
The Prokaryotic Cell Cycle
Cell cycle in prokaryotes
3. Plasma membrane grows
inward between
chromosome copies
4. Fusion of membrane along
cell equator completes
separation (binary fission
or “splitting in two”)…
The Prokaryotic Cell Cycle
Cell cycle in prokaryotes
5. Daughter cells are genetically identical
– Under ideal conditions Escherichia coli
bacteria complete a cell cycle every 20
minutes
The Eukaryotic Cell Cycle
• Progression through cell cycle in
multicellular organisms is variable
– Cells may exit the cell cycle and never divide
again
– Cells may enter or continue through the cell
cycle and divide in response to growth
hormones
The Eukaryotic Cell Cycle
Eukaryotic cell cycle
divided into two phases
•
Interphase. Eukaryotic cells
spend most time in
interphase
-
•
Acquisition of nutrients,
growth, chromosome
duplication
Cell division
- One copy of every
chromosome and half of
cytoplasm and organelles
parceled out into two
daughter cells
The Eukaryotic Cell Cycle
Interphase is divided
into three phases
– G1 (growth phase 1)
– Acquisition of nutrients
and growth to proper
size
– S (synthesis) phase
– DNA synthesis occurs,
replicating every
chromosome
– G2 (growth phase 2)
– Completion of growth
and readying for division
The Eukaryotic Cell Cycle
Decision to proceed or exit
the cell cycle in G1
–
–
Internal and external signals
in G1 stimulate cells to
proceed through cycle and
divide
Cells may exit cycle to nondividing G0 phase
– Cells remain alive and
metabolically active in G0
– Specialization
(differentiation) occurs
» Unique features of cell
type develop
Mitosis and Meiosis
•
There are two types of cell division in
eukaryotes
– Mitotic cell division (mitosis)
– Meiotic cell division (meiosis)
Mitosis and Meiosis
•
Mitosis is the mechanism of asexual
reproduction in eukaryotic cells
– Used in the reproduction of unicellular organisms
– Used in growth of fertilized egg into adult
– Used in cloning and stem cell research
•
Mitotic cell division involves two steps
– Nuclear division
– Cytokinesis (cytoplasmic separation)
mitosis,
differentiation,
and growth
embryo
mitosis,
baby differentiation, and growth
adu
Mitosis and Meiosis
• Meiotic division occurs in animal
ovaries and testes
– Two divisional steps produce four
daughter cells that can become
gametes
– Daughter cells are genetically
different from parent cell and each
other
– Daughter cells have half the genetic
material of the parent cell
meiosis in
ovaries
egg
fertilized
egg
sperm
fertilization
meiosis in
testes
The Eukaryotic Chromosome
•
•
DNA must be condensed
(coiled) to fit into nucleus
for easy manipulation in
cell division
Each chromosome consists
of a DNA double helix
wound around spool
proteins
The Eukaryotic Chromosome
•
A chromosome contains
hundreds of DNA
sequences called genes
found at specific locations
(loci)
• Each chromosome contains
– A central centromere
– Telomeres
The Eukaryotic Chromosome
•
Centromere (“middle body”) is region where
chromosome can attach to a sister
chromatid
– Two sister chromatids bound at their
centromeres comprise a duplicated
chromosome
– Sister chromatids separate at their centromeres
during mitosis
The Eukaryotic Chromosome
•
Telomeres (“end bodies”) are the two ends
of a chromosome
– Essential in maintaining chromosome stability
Homologous Pairs of Chromosomes
•
Duplicated chromosomes are tightly coiled
“X” shapes
Homologous Pairs of Chromosomes
Chart showing entire set
of stained chromosomes
(karyotype) shows pairs
–
–
Every chromosome in a
non-reproductive cell has a
“partner” or homologous
chromosome
Homologues contain the
same kinds of genes and
have the same size, shape,
and banding pattern
Homologous Pairs of Chromosomes
Human cells have 23
homologous pairs of
chromosomes
–
Chromosome pairs 1-22 are
autosomes with similar
appearance between homologues
Chromosome pair 23 are sex
chromosomes which may have
similar or different appearances
–
–
Females have two X chromosomes
of similar appearance
Males have an X and a Y
chromosome (the Y is much
smaller)
Homologous Pairs of Chromosomes
•
•
A karyotype showing two chromosomes for
each pair comes from a diploid (meaning
“double”) cell
Cells with only one chromosome “per pair”
are haploid (containing half the diploid
number)
– Meiosis (in sexual reproduction) produces
haploid cells from one diploid cell
Homologous Pairs of Chromosomes
•
Diploid and haploid numbers
– Number of haploid chromosomes in a cell
designated “n”
– Number of diploid chromosomes in a cell
designated “2n”
Mitosis Consists of Four Phases
•
Cells prepare for
mitotic division
during interphase
– Chromosomes are
replicated in S
phase
– Necessary proteins
are synthesized in
G1 and G2
Mitosis Consists of Four Phases
•
Four phases of mitosis
– Prophase
– Metaphase
– Anaphase
– Telophase
Mitosis
• Prophase
– Nuclear Envelope
Breaks.
– Chromosomes
condense
– Nucleolus dissappears
– Spindle apparatus
assembles
– Microtubules connect
kinetochores on each
pair of sister
chromatids to the
spindle poles.
pole
kinetoc
pole
Mitosis
• Metaphase
– Chromosomes align in
cell’s center (equator).
• Metaphase plate.
Mitosis
• Anaphase
"free" spindle
fibers
– Spindle microtubules
shorten and pull daughter
chromosomes (formerly
sister chromatids) towards
each spindle pole
– Pole-pole microtubules
push cell poles apart.
ANAPHASE
Mitosis
• Telophase
chromosomes
extending
nuclear envelop
re-forming
– Spindle disassembles.
– Nuclear envelope forms
around each set of
daughter chromosomes.
– Nucleoli reappear
TELOPHASE
One set of chromosomes
reaches each pole and relaxes
into extended state; nuclear
envelopes start to form
around each set; spindle
microtubules begin to
disappear.
Cytokinesis
• Cleavage of cell into
two halves.
• Animal Cells:
– Actin filaments form a
“belt” around the cell’s
equator.
– The belt contracts,
pinching in the cell’s
“waist”, forming two
daughter cells.
INTERPHASE OF
DAUGHTER CELLS
CYTOKINESIS
Cell divides in two; each
daughter cell receives one
nucleus and about half of
the cytoplasm.
Spindles disappear, intact
nuclear envelopes form,
chromosomes extend
completely, and the
nucleolus reappears.
INTERPHASE
nuclear
envelope
MITOSIS
chromatin
nucleolus
centriole
pairs
LATE INTERPHASE
Duplicated chromosomes in
relaxed state; duplicated
centrioles remain clustered.
condensing
chromosomes
pole
beginning of
spindle formation pole
EARLY PROPHASE
Chromosomes condense
and shorten; spindle
microtubules begin to form
between separating
centriole pairs.
spindle
microtubules
kinetochore
LATE PROPHASE
Nucleolus disappears;
nuclear envelope breaks
down; spindle microtubules
attach to the kinetochore
of each sister chromatid.
METAPHASE
Kinetochores interact;
spindle microtubules line up
chromosomes at cell's
equator.
INTERPHASE
"free" spindle
fibers
chromosomes
extending
ANAPHASE
Sister chromatids separate
and move to opposite poles
of the cell; spindle
microtubules push poles
apart.
nuclear envelope
re-forming
TELOPHASE
One set of chromosomes
reaches each pole and relaxes
into extended state; nuclear
envelopes start to form
around each set; spindle
microtubules begin to
disappear.
CYTOKINESIS
Cell divides in two; each
daughter cell receives one
nucleus and about half of
the cytoplasm.
INTERPHASE OF
DAUGHTER CELLS
Spindles disappear, intact
nuclear envelopes form,
chromosomes extend
completely, and the
nucleolus reappears.
Cytokinesis
• Plant Cells: Cell plate
Golgi complex
cell wall
plasma
membrane
carbohydratefilled vesicles
Carbohydratefilled vesicles
bud off the Golgi
and move to the
equator of the
cell.
Vesicles fuse
to form a new cell
wall and plasma
membrane
between daughter
cells.
Complete
separation of
daughter cells.
Mitosis in Onion Roots
• Interphase
early Prophase
late Prophase
• Metaphase
Anaphase
late Anaphase
• Telophase
Daughter cells
Control of Cell Cycle
•
The cells of some tissues divide frequently
throughout lifespan
• e.g. skin, intestine
•
Cell division occurs rarely or not at all in
other tissues
• e.g. brain, heart, skeletal muscles
•
Cell division in eukaryotes is driven by
enzymes and controlled at specific
checkpoints
Enzymes Drive the Cell Cycle
– The cell cycle is driven by proteins called Cyclindependent kinases, or Cdk’s
– Kinases are enzymes that phosphorylate (add a
phosphate group to) other proteins, stimulating or
inhibiting their activity
– Cdk’s are active only when they bind to other
proteins called cyclins
Enzymes Drive the Cell Cycle
• Cell division occurs when growth
factors bind to cell surface receptors,
which leads to cyclin synthesis
• Cyclins then bind to and activate specific
Cdk’s
Enzymes Drive the Cell Cycle
• Activated Cdk’s promote a variety of
cell cycle events
– Synthesis and activation of proteins
required for DNA synthesis
– Chromosome condensation
– Nuclear membrane breakdown
– Spindle formation
– Attachment of chromosomes to spindle
– Sister chromatid separation and
movement
Checkpoints Control Cell Cycle
• Although Cdk’s drive the cell cycle,
multiple checkpoints ensure that
– The cell successfully completes DNA
synthesis during interphase
– Proper chromosome movements occur
during mitotic cell division
Checkpoints Control Cell Cycle
•
There are three
major
checkpoints in
the eukaryotic cell
cycle, each
regulated by
protein
complexes
– G1 to S:
– G2 to mitosis
– Metaphase to
anaphase
Checkpoints Control Cell Cycle
•
G1 to S: Ensures that
the cell’s DNA is
suitable for replication
– p53 protein expressed
when DNA is damaged
• Inhibits replication
• Stimulates synthesis
of DNA repair
enzymes
• Triggers cell death
(apoptosis) if damage
can’t be repaired
Checkpoints Control Cell Cycle
•
G2 to mitosis: Ensures that
DNA has been completely
and accurately replicated
– p53 protein expression leads
to decrease in synthesis and
activity of an enzyme that
facilitates chromosome
condensation
– chromosomes remain
extended and accessible to
DNA repair enzymes, which fix
DNA before cell enters mitosis
Checkpoints Control Cell Cycle
• Metaphase to
anaphase: Ensures
that the chromosomes
are aligned properly at
the metaphase plate
– a variety of proteins
prevent separation of
the sister chromatids if
there are defects in
chromosome alignment
or spindle function
Why do So Many Organisms
Reproduce Sexually?
• Meiosis
– Sexual reproduction involves production of haploid
gametes through meiosis.
• Fertilization
– A gene might have alternate forms (alleles). Sexual
reproduction allows new gene combinations.
meiosis in
ovaries
meiosis in
testes
egg
fertilized
egg
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sperm
adults
gene 1
same alleles
gene 2
different alleles
How Does Meiotic Cell Division
Produce Haploid Cells?
• There are two cell divisions during Meiosis; Meiosis I
and II.
• Meiosis I Separates Homologous Chromosomes into
Two Haploid Daughter Nuclei
• The total number of cells produced is 4
Meiosis I

Meiosis II

n
2n
meiotic
cell division
2n
2n
n
fertilization
Meiotic Cell Division Followed by Fusion of Gametes
Keeps the Chromosome Number Constant
from Generation to Generation
Unique Features of Meiosis (3)
• Synapsis
– Homologues pair along their length.
• Homologous Recombination
– Crossing over occurs between homologous
chromosomes.
• Reduction Division
– 2 Cell divisions: Chromosomes do not
replicate between Meiosis I and II.
MEIOSIS I
Prophase I
• Duplicated chromosomes
condense.
• Homologous chromosomes pair
up and chiasmata occur as
chromatids of homologues
exchange parts.
• The nuclear envelope
disintegrates, and spindle
chiasma
microtubules form
paired homologous
chromosomes
spindle
microtubule
Crossing Over
• Presence of chiasmata [chiasma] indicates
crossing over has occurred.
Metaphase I
• Terminal chiasmata holds
homologous pair together.
• Spindle microtubules attach to
kinetochore proteins only on the
outside of each centromere.
• Metaphase plate: Each joined
pair of homologues lines up.
– Orientation of each pair is random
• Mendel’s “independent assortment” is
explained
Completing Meiosis
• Anaphase I
– Spindle fibers begin to shorten
and pull whole centromeres
towards poles.
• Each pole receives a member of
each homologous pair.
Completing Meiosis
• Telophase I
– Chromosomes segregated into
two clusters at opposite ends
of cell
• Nuclear membrane re-forms.
– Sister chromatids are no longer
identical.
MEIOSIS I
paired homologous
chromosomes
recombined
chromosomes
spindle
microtubule
chiasma
Prophase I. Duplicated
chromosomes condense.
Homologous chromosomes
pair up and chiasmata occur
as chromatids of homologues
exchange parts. The nuclear
envelope disintegrates, and
spindle microtubules form.
Metaphase I. Paired
homologous chromosomes
line up along the equator of
the cell. One homologue of
each pair faces each pole
of the cell and attaches to
spindle microtubules via its
kinetochore (blue).
Anaphase I. Homologues
separate, one member of
each pair going to each pole
of the cell. Sister chromatids
do not separate.
Telophase I. Spindle
microtubules disappear. Two
clusters of chromosomes have
formed, each containing one
member of each pair of
homologues. The daughter nuclei
are therefore haploid. Cytokinesis
commonly occurs at this stage.
There is little or no interphase
between meiosis I and meiosis II.
Second Meiotic Division
• Meiosis II resembles normal
mitotic division but with few
chromosomes
– Prophase II - Nuclear envelope
breaks down.
– Metaphase II - Spindle fibers bind to
both sides of centromere
– Anaphase II - Spindle fibers contract
and sister chromatids move to
opposite poles
– Telophase II - Nuclear envelope reforms; chromosomes relax.
Second Meiotic Division
Second Meiotic Division
– Cytokinesis results in:
• Four non-identical haploid
cells
MEIOSIS II
Prophase II.
If chromosomes
have relaxed after
telophase I, they
recondense. Spindle
microtubules re-form
and attach to the
sister chromatids.
Metaphase II.
Chromosomes line
up along the equator,
with sister chromatids
of each chromosome
attached to spindle
microtubules that lead
to opposite poles.
Anaphase II.
Chromatids separate
into independent
daughter chromosomes,
one former chromatid
moving toward each
pole.
Telophase II.
Chromosomes finish
moving to opposite
poles. Nuclear
envelopes re-form,
and the chromosomes
become extended
again (not shown here).
Four haploid
cells.
Cytokinesis results in
four haploid cells,
each containing one
member of each pair
of homologous
chromosomes (shown
here in condensed
state).
The mechanism of crossing over
sister
chromatids of
one duplicated
homologue
protein strands
joining duplicated
chromosomes
direction of
“zipper”
formation
pair of homologous,
duplicated chromosomes
Duplicated homologous
chromosomes pair up side
by side.
Protein strands “zip” the
homologous chromosomes
together.
recombinatio
n
enzymes
Recombination
enzymes bind to the
joined chromosomes.
chiasm
a
Recombination
enzymes snip
chromatids apart
and reattach the free
ends.
Chiasmata (the sites
of crossing over)
form when
one end of the
paternal chromatid
(yellow) attaches to
the other end
of a maternal
chromatid
(purple).
chiasma
Recombination enzymes
and protein zippers leave.
Chiasmata remain, helping
to hold homologous
chromosomes together.
duplicated
chromosomes
spindle
microtubules
Sexual Reproduction Produces Genetic
Variability in Three Ways :
• Shuffling of
Homologues Creates
Novel Combinations
of Chromosomes
• Crossing Over
Creates
Chromosomes with
Novel Combinations
of Genes
• Fusion of Gametes
Adds Further Genetic
Variability to the
Offspring
meiosi
ovaries
egg
fertilized
egg
fertilization
sper
m
Evolutionary Consequences of Sex
• Evolutionary process is revolutionary and
conservative.
– Pace of evolutionary change is accelerated by
genetic recombination
– Evolutionary change not always favored by
selection
• May act to preserve existing gene combinations
•
Life Cycles
In Haploid Life
Cycles, the
Majority of the
Cycle Consists
of Haploid
Cells
•
In Diploid Life •
Cycles, the
Majority of the
Cycle Consists
of Diploid Cells
In Alternation-ofGeneration Life
Cycles, There Are
Both Diploid and
Haploid Multicellular
Stages
Haploid Life Cycles
•
•
•
Fungi and unicellular algae
Most of life cycle is haploid
Asexual reproduction by mitotic cell
division produces a population of identical,
haploid cells
Haploid Life Cycles
•
•
Under certain
environmental
conditions, “sexual”
haploid cells are
produced
Two sexual haploid
cells fuse, forming a
diploid cell that
immediately
undergoes meiosis,
producing haploid
cells again
Diploid Life Cycles
•
•
•
•
•
Most animals
Most of the cycle is
in diploid state
Haploid gametes
are formed by
meiosis
Gametes fuse to
form a diploid
zygote
Zygote develops
into adult through
mitotic cell divisions
Alternation-of-Generation Cycles
•
•
•
•
Plants
Includes both
multicellular diploid and
multicellular haploid body
forms
Multicellular diploid body
gives rise to haploid
spores, through meiosis
Spores undergo mitosis
to produce a multicellular
haploid generation
Alternation-of-Generation Cycles
•
•
•
Eventually, certain
haploid cells
differentiate into
haploid gametes
Two gametes fuse to
form a diploid zygote
The zygote grows by
mitotic cell division into
a diploid multicellular
diploid generation
The making of Dolly
Finn Dorset ewe
donor cell
from udder
electric pulse fused cells
Cells from the udder of a Finn Dorset ewe
are grown in culture with low nutrient levels.
The starved cells stop dividing and enter the
non-dividing G0 phase of the cell cycle.
Blackface ewe
egg
cell
nucleus is
removed
DNA
Meanwhile, the nucleus is
sucked out of an unfertilized egg
cell taken from a Scottish Blackface
ewe. This egg will provide cytoplasm
and organelles but no chromosomes.
The egg cell without a
nucleus and the quiescent
udder cell are placed side by
side in a culture dish. An
electric pulse stimulates the
cells to fuse and initiates mitotic
cell division.
The cell divides, forming an
The ball of cells is implanted
embryo that consists of a hollow ball into the uterus of another
of cells.
Blackface ewe.
The Blackface ewe gives
birth to Dolly, a female Finn
Dorset lamb, a genetic twin
of the Finn Dorset ewe.