Transcript 7-5 Cell cycle

Chapter 7: Cell division
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint
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Cell cycle
•
•
Period of time from origin of cell to division of cell
into daughter cells
Types of division
– mitosis produces daughter cells with genetic complement
identical to parent cell

somatic cells
– meiosis produces daughter cells with half the genetic
complement of parent cell

reproductive cells
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Cell division in prokaryotes
•
Binary fission
– division into two daughter cells, each with one copy of the
genetic material
•
Single, circular DNA molecule attached to plasma
membrane
– replicates
– new molecule attached to separate point of plasma
membrane
– membrane between two molecules lengthens
– plasma membrane and cell wall grow inward
– cell divides
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Cell cycle in eukaryotes
•
Cell division in eukaryotes involves two processes:
– nuclear division

division of nuclear DNA
– cytokinesis

physical division of cell
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Cell cycle
•
Synthesis of DNA and other molecules during
interphase
– G1 (first gap) phase
– S (synthesis) phase
– G2 (second gap) phase
•
Chromosomes become visible and divide during M
phase (mitosis)
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Fig. 7.3a: Cell cycle in actively growing
cells
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Interphase
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G1 phase
– gap between mitosis and synthesis
•
S phase
– replication of DNA generates sister chromatids
•
G2 phase
– gap between synthesis and mitosis
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Mitosis in animal cells
•
•
•
•
•
Chromosomes condense
Nuclear membrane breaks down
Chromosomes attach to mitotic spindle
(microtubules)
Identical copies of chromosomes migrate to
opposite poles of mitotic spindle
Nuclear membrane reforms around chromosomes
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Stages of mitosis
•
End of G2 phase of interphase leads into mitosis
• Prophase
– chromatin in nucleus condenses into chromosomes
– chromosomes composed of identical sister chromatids
joined by centromeres
– centrosome at each end produces microtubules that form
asters (radial arrays) in the spindle
(cont.)
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Fig. 7.6 (a) + (b): Cell dividing (interphase,
prophase)
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Stages of mitosis (cont.)
•
Prometaphase
– asters enclose nuclear envelope
– nuclear envelope disaggregates
– kinetochore fibres (microtubules) bind to kinetochores in
chromosomes
•
Metaphase
– chromosomes line up along middle of spindle in
metaphase plate
(cont.)
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Fig. 7.6 (c) + (d): Cell dividing (prometaphase,
metaphase)
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Stages of mitosis (cont.)
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Anaphase
– sister chromatids separate to form chromosomes
– chromosomes migrate to opposite poles (anaphase A)
– poles move apart, microtubules slide over one another,
elongating spindle (anaphase B)
– mechanism of migration remains unclear
•
Telophase
– chromosomes decondense
– new nuclear envelope forms, surrounding each group of
chromosomes
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Fig. 7.6 (e) + (f): Cell dividing (anaphase,
telophase)
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Cytokinesis in animal cells
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Cytokinesis occurs in late mitosis
– starts during anaphase B
– completed in telophase
•
Actin filaments form contractile ring that pulls
plasma membrane and constricts cell
– results in cleavage of cell to produce daughter cells
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Fig. 7.6 (g) + (h): Cytokinesis
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Mitosis in plants
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Plant cells differ from animal cells
Lack centrosomes and astral spindles
– microtubules form barrel-shaped spindle
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Enclosed in rigid walls
– during anaphase, fibres thicken between chromosomes
– phragmoplast forms
– inside phragmoplast, membrane vesicles form new
cytoplasmic membrane and cell plate (new wall)
– preprophase band close to cell wall marks site where
growing cell plate will fuse with existing cell wall
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Control of cell cycle progression
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Progression regulated by cyclins and cyclindependent (Cdks) kinases
S-phase-promoting factor (SPF)
– phosphorylates and activates proteins required for DNA
replication
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M-phase-promoting factor (MPF)
– phosphorylates and activates proteins that induce
chromosome condensation (histones) and nuclear
envelope breakdown (nuclear envelope scaffold proteins)
– remains active until chromosomes are aligned at
metaphase
(cont.)
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Control of cell cycle progression
(cont.)
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Protease degrades cyclin component of MPF
– proteins dephosphorylated
– chromosomes decondense
– nuclear envelope reforms
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Checkpoint control
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Mechanisms to determine integrity of processes
during cell division
Detect defects in DNA integrity or in attachment of
chromosomes to mitotic spindle
Detected errors result in inhibition of SPF or MPF
activity, blocking progression and interrupting cell
cycle
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Meiosis
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Reduction division producing haploid reproductive
cells (gametes)
During meiosis
– DNA replication → cell division (meiosis I) → cell division
(meiosis II)
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Fig. 7.13: Meiosis in animal cell
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Meiosis I
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Prophase I
– homologous chromosomes pair up (synapsis)
– chromatids of homologous chromosomes may cross over,
exchanging portions of genetic material
– crossing over occurs at chiasmata
– generates novel combinations of genetic material
•
Metaphase I
– homologous chromosomes aligned on central plane of
spindle
– kinetochores on each chromosome in a homologous pair
attach to opposite poles
(cont.)
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Meiosis I (cont.)
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Anaphase I
– homologous chromosomes move to opposite poles
– sister chromatids do not separate
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At end of meiosis I, each daughter cell contains
one set of chromosomes
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Meiosis II
•
Resembles mitosis
• Anaphase II
– sister chromatids separate to form chromosomes
– chromosomes migrate to opposite poles
– results in four haploid cells
(cont.)
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Meiosis II (cont.)
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Males
– two rounds of division results in four haploid sperm
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Females
– two rounds of division not accompanied by cytokinesis
– one cell with haploid nucleus
– remaining nuclei form polar bodies that degenerate or are
expelled
•
Gamete formation in some groups may involve a
subsequent mitotic division
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Genetic consequence of meiosis
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Generates genetic diversity in sexually
reproducing organisms through recombination
– crossing over during prophase of meiosis I
– mixing of maternal and paternal genomes in zygote
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