The Continuity of Life:
Cellular Reproduction
Chapter 11
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)
Cellular Reproduction
•
Reproduction from a single parent is
asexual reproduction
– Some organisms reproduce asexually
•
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
occurs
• Duplicate chromosomes anchored to
membrane
1. Cell increases in size, pulling duplicated
chromosomes apart…
The Prokaryotic Cell Cycle
Cell cycle in prokaryotes
1. Plasma membrane grows inward
between chromosome copies
2. Fusion of membrane along cell equator
completes separation (binary fission or
“splitting in two”)
3. 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
•
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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
Eukaryotic cells spend most time in
interphase
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 non-dividing 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
– Karyokinesis (nuclear division)
– Cytokinesis (cytoplasmic separation)
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
• 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
Homologous Pairs of
Chromosomes
– 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)
Haploid/Diploid
• Cells with two chromosomes “per pair” are
diploid.
• Cells with only one chromosome “per pair” are
haploid (containing half the diploid number)
• Polyploidy – more than 2 sets 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
Events of Mitotic Prophase
Three major events occur in prophase
1. Duplicated chromosomes condense
•
Sister chromatids in each duplicated
chromosome coil up, forming small compact
bodies
1. Spindle microtubules form
–
–
Centrioles serve as points from which spindle
microtubules form (points called spindle poles)
Plants, fungi, and some algae lack centrioles but still
form spindle poles
Events of Mitotic Prophase
Three major events occur in prophase
1. Chromosomes are captured by the
spindle
– Kinetochore structure at chromosome
centromeres allows spindle fibers to attach
– Each sister chromatid attached to
microtubules emanating from one of two
spindle poles
Events of Mitotic Metaphase
•
•
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Spindle microtubules attached to
chromosome kinetochores lengthen and
shorten
Chromosomes pulled into a line
perpendicular to the spindle
Each kinetochore of every duplicated
chromosome faces one of the spindle poles
Events of Mitotic Anaphase
•
•
•
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Sister chromatids separate in anaphase
Motor proteins in kinetochores pull
chromatids apart
Daughter chromosomes (formerly
chromatids) move towards each spindle
pole
Unattached spindle microtubules interact
to push cell poles apart
Events of Mitotic Telophase
Four events of telophase
1. Spindle microtubules disintegrate
2. Nuclear membrane forms around each group
of chromosomes at the poles
3. Chromosomes unwind and revert to
their “extended” state
1. Nucleoli (disappeared in prophase) reappear
Cytokinesis
•
Cytokinesis differs in animal and plant cells
– Animal cells
•
•
Microfilaments attach to plasma membrane at equator
of cell
Contraction of ring pinches off membrane, forming two
cells
• Plant cells
– Stiff cell walls prevent “pinching off”
– Cell plate with cell wall and membrane material
forms in the middle of the elongated cell
Methods of Asexual Reproduction
• Binary fission – equal division of
cytoplasm (ameoba, paramecium)
• Budding – unequal division of cytoplasm
(yeast, hydra)
• Regeneration – new organism from part of
original (starfish, planarian)
• Sporulation – production of spores (molds)
Methods of Asexual Reproduction
• Vegetative propagation – new plant from part of
original plant. Several different methods.
• Bulbs – onions, tulips
• Tubers – potato
• Runners – strawberries
• Rhizomes – Iris
• Layering – Blackberries
• Cuttings & Grafting – Fruit trees, Roses
Control of Cell Cycle
•
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 Cyclin-dependent 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
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
Meiosis Separates
Homologues
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•
•
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Meiosis is a specialized cell division
process that produces haploid gametes
Each gamete receives one member of
each pair of homologous chromosomes
Meiosis consists of one round of DNA
replication, followed by two rounds of
nuclear divisions
These events occur in two stages
– Meiosis I
– Meiosis II
Meiosis Separates
Homologues
•
•
Both members of each homologous
chromosome pair are replicated prior to
meiosis
After replication, each chromosome
consists of 2 sister chromatids
Meiosis Separates
Homologues
•
During meiosis I, each daughter cell
receives one member of each pair of
homologous chromosomes
Meiosis Separates
Homologues
•
During meiosis II, sister chromatids
separate into independent chromosomes.
Each daughter cell receives one of these
independent chromosomes
Fusion of Haploid Gametes
•
•
Meiosis reduces chromosome number by
half, producing 1n gametes (eggs and
sperm)
Fusion of gametes (fertilization)
combines two chromosome sets to
produce diploid (2N) zygote
Overview of Meiosis I & II
• The phases of meiosis have the same
names as the equivalent phases in
mitosis, followed by I or II to distinguish
the two nuclear divisions that occur in
meiosis
Overview of Meiosis I & II
• Meiosis I separates homologous
chromosomes into two haploid daughter
nuclei
Overview of Meiosis I & II
• Meiosis II separates sister chromatids into
four gametes
Crossing Over
• Crossing over (genetic recombination)
occurs between homologues
• Enzymes facilitate exchange of DNA
between arms of adjacent chromatids,
producing chiasmata