1. Chapter 9 Lecture Outline
Cell Division

2. The Need for Cell Division
Renewal and repair of tissues
Stem cells
capable of self-renewal
give rise to descendant cells
Asexual and sexual reproduction
Prokaryotes divide through binary fission
Eukaryote cell division is more complicated
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3. Cell cycle: Two Main Stages
Series of events in life cycle of a cell
Time to complete cell cycle
Dependent on organism, type of cell, life stage
Two main stages
Interphase
Most cells spend 90% of lifespan in this stage
Cell division
Mitosis and cytokinesis
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4. Interphase: The Longest Stage
The period between divisions
Longest phase of the cell cycle
The cell prepares to divide
Divided into 3 stages:
G1: growth after mitosis
S: synthesis of DNA
G2: growth before mitosis
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The Cell Cycle
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G1 and G2 phases
G stands for “gap”
Early biologists saw a gap
Between S phase and cell division
Important for two reasons
periods of growth
size of cell and protein content increase
preparation for next phase
checkpoint that ensures conditions are suitable
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G0 Phase
Most cells are not actively dividing
These cells are in G0 phase
Can last days to years
Some cells will divide again; e.g. liver cells
Some cells stay in G0; e.g. nerve cells
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8. DNA Packaged as Chromosome
DNA molecules are enormously long
Double helix nearly 2 meters in length
DNA is tightly packaged with proteins
Chromatin
DNA and proteins
Chromosome
Tightly packed
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Karyotype
Chromosomes are visible during mitosis
Their number and shape can be studied
Humans have 46 chromosomes
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10. Homologous Chromosomes
46 chromosomes are arranged in 23 pairs
One came from each parent
22 pairs are autosomes
Both chromosomes are homologues
1 pair are sex chromosomes
Can be homologous; XX for females
Can be different; XY for males
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Mitosis
Consists of five phases
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis: division of the cytoplasm
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Prophase
Cell enters mitosis
Chromosomes condense
Centrosomes move apart
Go to the poles of the cell
Mitotic spindle begins to form
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Prometaphase
Mitosis proceeds
Chromosome condensation completed
Nuclear envelope breaks down
Mitotic spindle extends from centrosomes
Attaches to centromeres of chromosomes
Kinetochore: site of attachment
Chromatids linked to opposite poles
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Metaphase
Chromosomes line up
Metaphase plate
Align sister chromatids
Equal and balanced
segregation
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Anaphase
Chromatids separate
Break free and dragged to opposite sides
Microtubules shorten
Result:
Equal segregation of chromosomes in two daughter cells
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16. Telophase and Cytokinesis
Chromosomes reach the poles
Mitotic spindle falls apart
Chromosomes unfold
Nuclear membrane reforms
Cytokinesis
Cytoplasm is divided
Two cells are formed
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Mitosis
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Meiosis
Used to make gametes
Eggs and sperm
Chromosome number is halved (haploid)
Zygote is diploid after fertilization
Meiosis
Fertilization
Mitosis
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Meiosis I vs. Mitosis
Sister Chromatids
separate
Sister Chromatids
remain attached
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Meiosis II
Metaphase
Anaphase
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Concept Quiz
Which of the following is not true about interphase?
The cell grows larger during interphase.
Chromosomes are duplicated during interphase.
Interphase is divided into three phases.
Chromosomes segregate to daughter cells during interphase.
The correct answer is D. This does not occur during interphase, but during mitosis.
It is important to stress that interphase is composed of 3 phases in which the cell grows and the chromosomes are duplicated. Students often believe that G1, S, and G2 are in addition to interphase.
Answers A – C: These are all true. Growth occurs during G1 and G2 phases and the chromosomes are duplicated during S phase.
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Concept Quiz
Which of the following is true about homologous chromosomes?
Both were received from the same parent.
One of each segregates to each daughter cell during mitosis.
Both stay together in meiosis I.
All 23 pairs are always homologous.
The correct answer is B. It is important that students understand what homologous chromosomes are. These are not identical, but homologous; one came from each parent and they contain slightly different genes. One of each segregates into each daughter cell during mitosis so that each daughter cell is identical.
Answer A: As stated above, one came from each parent.
Answer C: They are separated during meiosis I, resulting in haploid daughter cells.
Answer D: Sex cells are not homologous in males, but are in females.
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Concept Quiz
Meiosis insures that
Each gamete receives the same genes
Chromosome number is doubled in the gametes
Zygotes produced by fertilization have the normal number of chromosomes
All paternal chromosomes end up in the same gamete
The correct answer is C. This is the key to meiosis – reduction division so that the fertilized egg restores the normal number of chromosomes.
Answer A: Each gamete doesn’t receive the same genes because the homologous chromosomes are randomly assorted during meiosis I.
Answer B: Chromosome number is halved in the gametes.
Answer D: As in the answer for A, the homologous chromosomes are randomly assorted during meiosis I.
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Biology In the News
“New Type of Stem Cells May Help Regenerate Heart Tissues”
Induced pluripotent stem cells
A genetically reprogrammed adult body cell
Behaves like pluripotent embryonic stem cells
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Terms
Chromatin
Chromosome
Crossing-over
Cytokinesis
Diploid
DNA segregation
Fertilization
G0 phase
G1 phase
G2 phase Gametes Genetic recombination Haploid Homologous pair Homologues Independent assortment Interphase Karyotype
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26. © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e
Terms
Prometaphase
Prophase
S phase
Sex chromosomes
Sexual reproduction
Sister chromatids
Somatic cells
Stem cells
Telophase
Kinetochores Maternal homologue Meiosis Meiosis I Meiosis II Metaphase Mitosis Mitotic spindle Paternal homologue
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27. © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e
Process Animation 9.1 The Cell Division Cycle
Macintosh
Windows
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28. © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e
Process Animation 9.2 Mitosis and Cell Division
Macintosh
Windows
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29. © 2009 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 4/e
Process Animation 9.3 Meiosis
Macintosh
Windows
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30. Discover Biology StudySpace 30

31. Additional Art from Chapter 9
All art files from the book are available in JPEG and PPT formats on the Instructor Resource Disc 31

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