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Chapter 8 – Cellular Reproduction
In order for organisms to grow
and reproduce, cells must divide.
Cell Division (8.1)
Cell division is part of asexual
reproduction
• Single-celled organisms reproduce by simple
cell division
• There is no fertilization of an egg by a sperm
Sexual reproduction
• Requires fertilization of an egg by a sperm
using a special type of cell division called
meiosis.
• Thus, sexually reproducing organisms use:
– Meiosis for reproduction
– Mitosis for growth and maintenance
Chromosomes Contain Our Genes
• Long stretches of DNA and proteins
• Humans have 23 different chromosomes
• Diploid – 2 of each chromosome (our body
cells)
• Haploid – 1 of each chromosome (our sex
cells)
DNA double helix
Histones
TEM
“Beads on
a string”
Nucleosome
Tight helical fiber
Looped domains
TEM
Duplicated chromosomes
(sister chromatids)
Centromere
Figure 8.4
Chromosomes 8.2
Cell Cycle
1.
2.
3.
4.
Cells grow and do “cell” things
Cells copy DNA
Each copy moves to opposite ends of cell.
Cells divide; each new cell has a copy of DNA
Chromosomes are duplicated
before cell division
Cell Cycle (8.6)
Eukaryote cells divide by mitosis or
meiosis
• Mitosis
– One cell divides producing two identical nuclei
followed by cell division
– Almost all cell reproduction
• Meiosis (only in testes and ovaries)
– One cell divides twice to produce four genetically
different cells
– Produces egg and sperm cells (gametes)
A Cell’s life cycle is divided into phases.
• Most of the time a cell is in Interphase
• During interphase, a cell:
– Performs its normal functions
– Doubles everything in its cytoplasm
– Grows in size
Interphase and Prophase (8.7)
Prophase
• Chromosomes condense and are visible
under light microscope
• Nuclear membrane breaks down
• Spindle microtubules grow from two
centrosomes, clouds of cytoplasmic
material that in animal cells contain
centrioles.
Metaphase and Anaphase
• Metaphase
• Mitotic spindle guides the separation of two sets of
daughter chromosomes.
• Chromosomes line up along middle axis of cell
(metaphase plate)
• Anaphase
• Spindle fibers pull the chromosomes away from center
• Centromeres break and each sister chromosome goes to
opposite poles of the cell.
Telophase and Cytokinesis
• Chromosomes unwind at the poles
• New nuclear membrane forms
• Cytokinesis typically:
–Occurs during telophase
–Divides the cytoplasm
–Is different in plant and animal cells
Cytokinesis (8.8)
Cytokinesis (8.8)
Meiosis
• Two cell divisions (Meiosis I and II)
• Four daughter cells result each containing half
of the chromosomes as the parent cell
• In males meiosis results in the production of 4
sperm cells
• In females meiosis results in the production of
1 large egg cell and 3 small polar bodies
Haploid gametes (n  23)
Egg cell
n
n
Sperm cell
FERTILIZATION
MEIOSIS
Multicellular
diploid adults
(2n  46)
2n
MITOSIS
and development
Diploid
zygote
(2n  46)
Key
Haploid (n)
Diploid (2n)
Figure 8.12
Meiosis has two stages
• Meiosis I
• Meiosis II
Meiosis I
• Similar to mitosis except during first
metaphase chromosome pair up side by side
• During first anaphase one set of chromosomes
move to either pole of the cell.
• Sister chromotids stay joined in the new
daughter cells
Meiosis I (8.14)
Meiosis II
• Anaphase II sister chromotids separate
• Daughter cells have only one of each
chromosome (Haploid)
Meiosis II (8.14)
Crossing Over Increases Genetic Variation
The number of times a cell can
divide is limited.
Cloning (NIB)
Dolly was euthanized
• When chromosomes replicate they lose part
of their ends (telomeres)
• Linked to aging
• Dolly’s chromosomes came from an adult
sheep with already shortened telomeres
• Dolly developed age-related illnesses
Dolly taught us a lot about
cellular aging.
Mitosis and Meiosis