Cellular Reproduction
Beginning with Chromosomes
DNA double helix
DNA double
helix
Chromatin
Histones
TEM
“Beads on
a string”
Nucleosome
Tight helical fiber
Looped domains
TEM
Duplicated chromosomes
(sister chromatids)
Centromere
Figure 8.4
DNA
• DNA is found in chromosomes
• Chromatin consists of DNA and proteins
called histones and surround the DNA in
a spiral shape
• Organisms can reproduce sexually or
asexually. Amoeba reproduce asexually
Vocabulary Check
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1. What is a histone?
2. What is a nucleosome?
3. What is chromatin?
4. What is a centromere?
CELL DIVISION
MITOSIS & MEIOSIS
Mitosis = somatic cell division
http://youtu.be/1cVZBV9tD-A
Meiosis = egg and sperm division
http://youtu.be/rB_8dTuh73c
Cell Cycle
2 distinct phases
S
Chromosome duplication
G0
Non reproducing
cells
Interphase
G2
G1
Mitotic
What's the most important
event of interphase?
Cell Cycle results in the
production of two cells,
each with the same amount
of genetic material and the
same genetic information
What is significant about DNA in
the S and G2 phases?
All chromosomes are duplicated
That means: two copies of each chromosome
What are the copies called?
Sister chromatids
Do they contain identical genes?
Yes, but…
What about meiosis?
Some terminology
• Chromatin
• Homologous chromosomes
• Chromatid
• Double-chromatid chromosomes
• Single-chromatid chromosomes
When do each of these occur?
MITOSIS
THE STEPS OF MITOSIS
• Interphase – this is when DNA is duplicated
(actually, this is not part of mitosis itself)
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Prophase
Metaphase
Anaphase
Telophase
Stages of Mitosis – G2, prophase, metaphase, anaphase, telophase and cytokensis
(cytokenesis occurs during telophase)
Mitosis
Interphase
Onion root tip
Whitefish blastula
• Nuclear envelop intact
• Chromosomes duplicated but not apparent
• DNA loose, uncondensed, called chromatin
• 2 nucleoli visible
in onion root tip
• Nucleoli not visible
in whitefish
blastula
Prophase
Onion root tip
Mitosis
Whitefish blastula
• Double-chromatid chromosomes evident
• Chromatin becomes super-coiled & compact
• Nuclear envelop breaks down
• Centrioles migrate toward poles of cell
forming the spindle.
• Mitotic spindle begins to form
• Nucleoli disappear
in onion root tip
Double-chromatid chromosomes
Mitosis
Metaphase
Onion root tip
Whitefish blastula
• Double-chromatid chromosomes line up
on equatorial plate of cell
• The spindle fibers attach to the
chromosomes at the centromeres
Mitosis
Anaphase
Onion root tip
Whitefish blastula
• Spindle fibers pull chromatids apart
• Centromeres are pulled apart
• Groups of single-chromatid chromosomes
move opposite poles of the cell
Telophase
Onion root tip
Mitosis
Whitefish blastula
• Groups of single-chromatid
chromosomes reach poles of cell
• Nuclear envelop begins to reform
• 2 new daughter cells formed
• Cytokinesis begins with
appearance of cell plate
• Nucleoli reform
• Cytokinesis begins
with appearance of
cleavage furrow
Mitosis
Returning to Interphase
Onion root tip
Whitefish blastula
• Cytokinesis completes
• Chromosomes ‘disappear’ as
interphase chromatin reforms
• 2 new daughter cells enter G1 phase of
the cell cycle
A SUMMARY OF MITOSIS
Mitosis Questions
1. What does “diploid” mean?
2. We have __ total chromosomes.
3. In the term 2n, what does “n” stand for in
us? In a gypsy moth?
4. Why does mitosis occur? Major functions?
5. In what cells (general term) does mitosis
occur?
Mitosis Questions
6. What are chromatids?
7. What happens in Anaphase to result in
each new cell receiving duplicate
parental DNA?
8. In a species whose diploid number is
224, what would its sperm/eggs contain?
Why is this duplication of
parental DNA necessary?
Something to do with passing on genetic information?
Why undergo mitosis at all?
Something to do with cells getting damaged, old, lost?
Something to do with the organism growing, infant to adult?
What are typical body cells called?
SOMATIC CELLS
These cells divide continuously
The new cells receive an exact
copy of all the parent cell’s:
DNA
What is the process of somatic cell
duplication called?
Mitosis
What are these somatic cells?
Diploid or 2n
What does this mean?
They contain the full
number of chromosomes
in pairs
How many
in humans?
46
23 pairs
Know this!
Mitosis occurs only in
somatic cells
What about sex cells?
Called gametes
Eggs and sperm
Produced in ovaries or testes
• Gametes are not diploid (2n)
• Instead, they are haploid (n)
Our haploid (n) number is 23
So our eggs and sperm have how many
chromosomes?
Half the number
Why?
Homologous chromosomes
• Carry the same
genes
MEIOSIS
Mitoses results in identical cells and is used for replacement of dead or worn out
cells, wound repair and growth
Meiosis is strictly to produce eggs or sperm cells which are diverse from the
parent cell and are used in sexual reproduction
Fertilization is the union of an
egg and a sperm
If the egg and sperm were
both diploid, what would the
fertilized egg (zygote) be?
It would be a genetic mess!
Instead, gametes are haploid (n).
Egg and sperm both
have exactly half the
number of
chromosomes of
somatic cells
At fertilization, n + n = 23; 23 + 23 = 46!
Note how mitosis and meiosis
differ:
• Number of divisions?
• Number of chromosomes?
• Number of products?
Meiosis !
Somehow somatic cells (2n) in our
ovaries or testes must produce
gametes (n)
Meiosis is the cell division process that enables
the transformation from 2n to n
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
How is meiosis more complicated
than mitosis?
Gametes must contain precisely half the diploid
number of chromosomes
They must contain one of each homologous pair of
chromosomes
Remember karyotypes?
Each chromosome has a partner
They come in pairs
One from mom
One from dad
Human Karyotype
Which pair of chromosomes in us
in not homologous?
It’s the 23rd pair in males, the XY pair
Remember what homologous
means?
All our other pairs of
chromosomes are
homologous and are
called autosomes
How many divisions does meiosis have?
What are the divisions called?
MEIOSIS
MITOSIS
Prophase I
Prophase
Chromosome
duplication
Duplicated chromosome
(two sister chromatids)
MEIOSIS I
Chromosome
duplication
Parent cell
(before chromosome duplication)
2n  4
Homologous
chromosomes come
together in pairs.
Site of crossing over
between homologous
(nonsister) chromatids
Metaphase I
Metaphase
Homologous pairs
align at the middle
of the cell.
Chromosomes
align at the
middle of the
cell.
Anaphase I
Telophase I
Anaphase
Telophase
2n
Sister
chromatids
separate
during
anaphase.
Daughter cells
of mitosis
2n
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Chromosome with two
sister chromatids
Haploid
n2
Daughter
cells of meiosis I
MEIOSIS II
Sister chromatids
separate during
anaphase II.
n
n
n
Daughter cells of meiosis II
n
Figure 8.15
Prophase I has four times as
much genetic material
• Crossing over occurs in prophase I but
NOT in prophase II
1
2
3
4
1. One pair of homologues to start
2. DNA is replicated
3. Meiosis 1 = 2n ---> n
4. Meiosis 2 = chromatids separate into 4 products
In males, how many progeny
are produced?
Typically 4 viable sperm are produced
following each Meiosis 2
In females, how many progeny are
produced?
Just one viable ovum (egg) is produced,
plus 3 small polar bodies
Side by Side Comparision of mitosis
and meiosis
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
Genetic Variation
Independent Assortment
Crossing over
When does it occur?
Only during Prophase of Meiosis 1
Homologous chromosomes get
together in temporary tetrads
Overlap (cross over) and trade their DNA
Why is this a good thing to do,
generally?
Meiosis Summary
1. Meiosis 1
a. DNA replication takes place
b. A parent cell produces two daughter cells
each with one member of each original
pair of homologous chromosomes (to
create haploid daughter cells)
c. Crossing over may occur
Meiosis Summary
2. Meiosis 2
a. There is no more DNA replication
b. The chromatids of each chromosome separate
and each daughter cell divides
c. At the end of Meiosis 2, there are 4 daughter
cells from each parent cell. Each daughter cell
has half the number of chromosomes as the
parent cell
Meiosis Questions
1. The cells produced in meiosis are _____
(haploid or diploid?)
2. Sex cells are called ________.
_____ are produced by males, ____ by
females.
3. What’s crossing over and why is it
important?
Meiosis Questions
4. In what meiotic stage does crossing
over occur?
5. Why are sex cells n, not 2n?
6. If a species’ diploid number is 50, what
is n?
Cancer Cells: Growing Out of
Control
– Normal plant and animal cells have a cell cycle
control system that consists of specialized
proteins, which send “stop” and “go-ahead”
signals at certain key points during the cell
cycle.
– Cancer is a disease of the cell cycle.
– Cancer cells do not respond normally to the cell
cycle control system.
When Meiosis doesn’t work
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Nondisjunction
Down Syndrome an extra chromosome 21
Trisomy 21 and Down Syndrome
XXY – Klinefelter syndrome (males)
XYY – looks normal (male)
XXX – (female) Looks normal
XO – Turner syndrome (female)
NONDISJUNCTION IN MEIOSIS II
NONDISJUNCTION IN MEIOSIS I
Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Gametes
n1
n1
n–1
Abnormal gametes
Number of
n – 1 chromosomes
n1
n–1
Abnormal gametes
n
n
Normal gametes
Figure 8.20-3
Resources
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http://www.pbslearningmedia.org/asset/lsps07_int_celldivision/
•
http://www2.le.ac.uk/departments/genetics/vgec/schoolscolleges/topics/cellcyclemitosis-meiosis
Essentials of the Living World – Textbook
Education-Portal Short videos covering all aspects of cell division
Mitosis Quiz See how you do on this quiz!
Mitosis Exam OK, here are 20 more questions!
Meiosis Quiz
Meiosis Exam
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Vocabulary
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DNA
Chromosome
Chromatin
Histone
Nucleosome
Centromere
Sister chromatids
Mitosis
Meiosis
Homologous
chromsomes
Chromatid
Interphase
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
Spindle fiber
Diploid
Haploid
Apoptosis
Somatic
Sex cell
Gametes
Zygote
Karyotype
Tetrad
Crossing over
Independent assortment
Nondisjunction
Down syndrome
Trisomy
Cancer
Klinefelter syndrome
Turner syndrome