7-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Chapter 7: Cell division

7-2Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

7-3Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

7-4Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Cell cycle in eukaryotes

• Cell division in eukaryotes involves two processes:– nuclear division

division of nuclear DNA

– cytokinesis physical division of cell

7-5Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

• G1 phase– gap between mitosis and synthesis

• S phase– replication of DNA generates sister chromatids

• G2 phase– gap between synthesis and mitosis

7-8Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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

7-9Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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.)

7-10Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 7.6 (a) + (b): Cell dividing (interphase, prophase)

7-11Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

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.)

7-12Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 7.6 (c) + (d): Cell dividing (prometaphase, metaphase)

7-13Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Stages of mitosis (cont.)

• 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

7-14Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 7.6 (e) + (f): Cell dividing (anaphase,

telophase)

7-15Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Cytokinesis in animal cells

• 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

7-16Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Fig. 7.6 (g) + (h): Cytokinesis

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Mitosis in plants • Plant cells differ from animal cells• Lack centrosomes and astral spindles

– microtubules form barrel-shaped spindle

• 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

7-18Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Control of cell cycle progression

• Progression regulated by cyclins and cyclin-dependent (Cdks) kinases

• S-phase-promoting factor (SPF)– phosphorylates and activates proteins required for DNA

replication

• 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.)

7-19Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Control of cell cycle progression (cont.)• Protease degrades cyclin component of MPF

– proteins dephosphorylated– chromosomes decondense– nuclear envelope reforms

7-20Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Checkpoint control

• 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

• 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

• 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.)

7-24Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Meiosis I (cont.)

• Anaphase I– homologous chromosomes move to opposite poles– sister chromatids do not separate

• At end of meiosis I, each daughter cell contains one set of chromosomes

7-25Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Meiosis II

• Resembles mitosis• Anaphase II

– sister chromatids separate to form chromosomes– chromosomes migrate to opposite poles– results in four haploid cells

(cont.)

7-26Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Meiosis II (cont.)

• Males– two rounds of division results in four haploid sperm

• 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

7-27Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Genetic consequence of meiosis

• Generates genetic diversity in sexually reproducing organisms through recombination

– crossing over during prophase of meiosis I– mixing of maternal and paternal genomes in zygote