7
The Cell Cycle and Cell
Division
Chapter 7 The Cell Cycle and Cell Division
Key Concepts
• 7.4 Meiosis Halves the Nuclear
Chromosome Content and Generates
Diversity
• 7.5 Programmed Cell Death Is a
Necessary Process in Living Organisms
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiosis consists of two nuclear divisions but
DNA is replicated only once. The function of
meiosis is to:
• Reduce the chromosome number from diploid
to haploid (2n  1n) (46  23)
• Ensure that each haploid has a complete set of
chromosomes
• Generate diversity among the products
Meiosis I
• Occurs in Germ Cells
• Specialized reproductive cells of the
testis and ovaries
• G1, S, G2 happens in Interphase
• Consists of Prophase I, Metaphase I,
Anaphase I and Telophase/Cytokinesis I
Prophase I
• Homologous Chromosomes
pair up (tetrads) and do
crossing over
• Genes from Mom and Dad’s
chromosomes swap
• Called synapsis – pieces of the
chromsomes actually break off
and swap locations at chiasmata
• Creates genetic diversity
• Explains why you don’t look
exactly like your parents or
siblings
• Nuclear membrane and
nucleolus disappear and
centrosome/spindle fibers start
to organize
Figure 7.13 Crossing Over Forms Genetically Diverse Chromosomes
Metaphase I
•Homologous
Chromosome Pairs line
up at the equatorial
plate
Anaphase I
•Homologous Pairs of
chromosomes split into
regular chromosomes
•At this point, chromosome
number is reduced. 2n 
1n
Telophase and Cytokinesis I
• Chromosomes reach poles
• Nuclear membrane and
nucleolus reappear
• Cytokinesis separates the
cytoplasm
• Both cells have half the number
of chromosomes as the parent
cell
• DNA Replication will not occur
between divisions, just an
intermission occurs
Meiosis II
• Prophase II, Metaphase II, Anaphase II, Telophase II
• Follows same sequence of events as mitosis
• Prophase II – chromosomes become visible, nuclear
membrane and nucleolus disappear, centrosome and
spindles organize
• Metaphase II – chromosomes line up at equator
• Anaphase II – Chromosomes split into chromatids and
get pulled to opposite poles by the spindle fibers
• Telophase II – chromatids become chromatin and we
now have 4 CELLS!
Figure 7.12 Meiosis: Generating Haploid Cells (Part 4)
Meiosis Products
Males - Spermatogenesis
•4 mature sperm are made
•Each sperm have 23
chromosomes
•Each sperm also develop
flagella for movement
towards fertilization
Females - Oogenesis
•1 large egg or ova is
produced that survives
•3 small polar bodies are
produced which don’t
survive due to lack of
nutrients due to lack of
cytoplasm
Figure 7.11 Mitosis and Meiosis: A Comparison
Mitosis
Meiosis
•Asexual
reproduction
•1 division
•Done by all
somatic cells
•Identical
daughter cells
produced
•No crossing
over = no
genetic
diversity
•2n  2n
•How cells
reproduce,
repair and how
organisms
grow
•Sexual
reproduction
•2 divisions
•Done by germ
cells
•4 gametes are
produced
•Crossing over
– genetic
diversity
•2n  1n
•How sperm
and ova are
made
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Meiotic errors:
Nondisjunction—homologous pairs fail to
separate at anaphase I—sister chromatids fail
to separate, or homologous chromosomes may
not remain together
Either results in aneuploidy—chromosomes
lacking or present in excess
Example: Down syndrome
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
Organisms with triploid (3n), tetraploid (4n), and
even higher levels are called polyploid.
This can occur through an extra round of DNA
duplication before meiosis, or the lack of
spindle formation in meiosis II.
• Polyploidy occurs naturally in some species,
and can be desirable in plants.
• Produces larger flowers, fruits and seeds for
example
Concept 7.4 Meiosis Halves the Nuclear Chromosome Content
and Generates Diversity
If crossing over happens between nonhomologous chromosomes, the result is a
translocation.
A piece of chromosome may rejoin another
chromosome, and its location can have
profound effects on the expression of other
genes.
Example: Leukemia
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Cell death occurs in two ways:
1.In necrosis, the cell is damaged or starved for
oxygen or nutrients. The cell swells and bursts.
2.Apoptosis – genetically programmed cell death
 Reasons for this?

Cell is no longer needed

Genetic damage/cancerous
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Events of apoptosis:
• Cell detaches from its neighbors
• Cuts up its chromatin into nucleosome-sized
pieces
• Forms membranous lobes called “blebs” that
break into fragments
• Surrounding living cells ingest the remains of
the dead cell
Figure 7.14 Apoptosis: Programmed Cell Death (Part 1)
Concept 7.5 Programmed Cell Death Is a Necessary Process in
Living Organisms
Cell death cycle is controlled by signals:
• Lack of a mitotic signal (growth factor)
• Recognition of damaged DNA