Chapter 8:
Cell Reproduction
Chromosome Structure
•Chromatin – made of DNA and protein
•Seen in a non-dividing cell
•Chromosomes – Chromatin that has coiled
tightly
•Seen in a dividing cell
•Histones – proteins that help to coil the
DNA and maintain the shape of
chromosomes
•After replication, there are 2 identical
sister chromatids, held together by a
centromere
•Each species has a set number of
chromosomes:
•Humans – 46
•Crayfish – 200
•Corn – 20
•Adder’s tongue fern – 1262
•Chimpanzee - 48
•Sand dollar – 52
•Dog – 78
•Cat - 32
Viewing the Chromosomes
• Karyotype - a photograph of the
chromosomes in a dividing cell
• Shows the gender of the child
• Shows abnormalities in chromosome
number or structure.
• Humans have 23 pairs of homologous
chromosomes (chromosomes with the same
genes), or 46 chromosomes total.
• You get 1 of each chromosome from
mom, and 1 from dad.
•Of the 23 pairs of chromosomes:
•1 pair are sex chromosomes –
determine your gender; may determine
other traits as well
•Females are XX
•Males are XY
•22 pairs are autosomes – all other
pairs of chromosomes; do NOT
determine gender
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How is karyotyping done?
Cells are taken from the fetus of a
pregnant woman
Chemicals added to the cells make them
divide
Another chemical stops division at the
midpoint
The stained cells are photographed and
can be paired based on cross-bands, size,
and shape.
Human karyotype preparation
Normal male karyotype
Down syndrome karyotype
•Body cells contain the diploid (2n)
number of chromosomes – 2
chromosomes of each kind (1 from each
parent)
•Sex cells (eggs and sperm) contain only
1 chromosome of each kind – haploid
(1n) number of chromosomes
Why do cells divide – why can’t they just grow
bigger?
• Surface area represents ability to take in/get
rid of materials.
• Volume represents needs of the cell – food,
oxygen, water, etc.
•Small cube 1 mm
tall
•Surface area = 6
mm2
•Volume = 1 mm3
•Surface area to
volume ratio = 6:1
•Larger cube 2 mm tall
•Surface area = 24
mm2
•Volume = 8 mm3
•Surface area to
volume ratio = 3:1
•As a cell grows, the volume of the inside
grows faster than the cell membrane
around the outside
•What happens when a cell gets too big?
•It could starve – can’t take in enough
food to support needs
•It could poison itself – can’t get rid of
waste fast enough
•So cells stay small for exchange of
materials
•Rather than grow bigger, cells divide and
increase in number
•Cell division occurs when:
•Body growth
•Maintenance and repair
•Fighting infection
•Replacing worn/dead cells
The Cell Cycle
• An orderly sequence of events that occurs
from the time a cell is first formed until it
divides into two new cells.
http://www.cellsalive.com/cell_cycle.htm
• Most of the cell cycle is spent in interphase
(time between cell divisions):
• G1 stage – cell growth and activity
• S stage – DNA replication occurs (S
stands for synthesis of DNA)
• G2 stage – cell prepares for cell division
•Amount of time spent in interphase
varies – average for adult mammals is 20
hours
•Nerve cells and muscle cells exit the cell
cycle  G0 phase
•Body never makes more of them
• Following interphase is the M stage,
including mitosis and cytokinesis
• Mitosis:
• 2 daughter cells are produced that are
identical to the
• Occurs in body cells
• Diploid cells produce other diploid cells
• Takes about 4 hours
http://www.cellsalive.com/mitosis.htm
http://www.copernicusproject.ucr.edu/ssi/HighSchoolBioResources/C
ell Division/Mitosis/Introduction_to_Mitosis.asf
Mitosis overview
1.
2.
3.
4.
5.
6.
7.
8.
Centriole
Chromatin
Nucleolus (in yellow)
Nuclear membrane
Spindle fibers
Chromosome (replicated)
Centromere
Sister Chromatids (each half of replicated
chromosomes) – become daughter chromosomes
9. Cell membrane
10. Cell membrane pinches in
Early Prophase
•Chromatin coils to
become chromosomes
•Centrioles move to
opposite ends of the
cell
•Nuclear membrane
and nucleolus
disappear
Late Prophase
•Spindle fibers
form
•Chromosomes
become attached
to spindle fibers
– centromere
attaches to
spindle fibers
Metaphase
•Chromosomes
line up at
equator
(middle)
Anaphase
•Sister chromatids
separate, becoming
daughter
chromosomes, and
move toward
opposite ends of cell
Telophase
•Spindle fibers
disappear
•Nuclear membrane
and nucleolus
reappear
•Chromosomes uncoil
and turn back into
chromatin
• Cytokinesis, or division of cytoplasm,
accompanies mitosis.
• Division of the cytoplasm begins in
anaphase, but is not completed until end
of telophase.
• In animal cell, cell membrane is flexible
and pinches in to divide cell.
• In plant cells, rigid cell wall does not
bend, so a cell plate forms to divide the
cell.
Mitosis in Plant Cells
•Same phases as in animal cells
•No centrioles
•Cell plate forms instead of cell membrane
pinching in
Cytokinesis in animal cells
http://www.copernicusproject.ucr.edu/ssi/HighSchoolBioResources
/Cell Division/Mitosis/Mitosis_Video_Quiz.asf
Cytokinesis in plant cells
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Cell Division in Prokaryotes
The process of asexual reproduction in
prokaryotes is called binary fission.
The two daughter cells are identical to the
original parent cell, each with a single
chromosome.
Following DNA replication, the two
resulting chromosomes separate as the
cell elongates.
Cell divides without cell structures seen
in plants & animals
http://www.microbelibrary.org/images/MondoMedia/2bhirez.mov
Meiosis
• Produces sex cells (gametes)
• Female gamete – egg
• Male gamete - sperm
• Reduces the chromosome number
• Egg or sperm cells each have only one
of each kind of chromosome
• Diploid cells (2n)  haploid cells (1n)
Overview of meiosis
http://www.cellsalive.com/meiosis.htm
• Meiosis involves two cell divisions
(meiosis I and II)
• Produces four haploid cells.
• Fertilization (fusion of egg and sperm) restores the diploid number of
chromosomes
Meiosis I
•Original cell produces 2 new cells
•Each new cell has 1 chromosome from
each homologous pair
•Each new cell is haploid (1n)
Meiosis II
•Each of the 2 cells made in meiosis I
splits
•Result is 4 haploid cells
•Chromatids of chromosomes are
separated
•Errors in meiosis can result in missing
or extra chromosomes
Meiosis in males vs. females:
•Males:
•Occurs in testes
•Spermatogenesis
•Makes 4 haploid sperm
•Females:
•Occurs in ovaries
•Oogenesis
•Makes 1 large egg & 3 polar bodies
(that die) – cells divide unevenly
Spermatogenesis
Oogenesis
Meiosis vs. Mitosis
Mitosis
•Body cells
•Diploid cells make
diploid cells
•End result 2 cells
•No variation in
cells produced
Meiosis
•Sex cells
•Diploid cells make
haploid cells
•End result up to 4
cells
•Cells made are
different from parent
Meiosis compared to mitosis
Sexual vs. Asexual Reproduction
Sexual:
•2 parents
•Offspring are
genetically different
from parent – mix of
traits
•Union of egg &
sperm
Asexual:
•1 parent
•Offspring are
identical to parent
•Binary fission in
bacteria
•Regeneration in
worms
Cloning: Produces an organism that is
genetically identical to the parent
• Steps:
1. DNA is removed from a fertilized
egg
2. Fertilized egg cell is fused with cell
of individual being cloned
3. Egg with new DNA is implanted
into a woman
http://gslc.genetics.utah.edu/units/cloning/clickandclone/