Cellular Reproduction

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Cellular Reproduction

• Beginning with Chromosomes

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DNA double helix
DNA double helix
Histones
Chromatin
Beads on a string
TEM
Nucleosome
Tight helical fiber
Looped domains
Duplicated chromosomes (sister chromatids)
TEM
Centromere
Figure 8.4
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Vocabulary Check

• 1. What is a histone?
• 2. What is a nucleosome?
• 3. What is chromatin?
• 4. What is a centromere?

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CELL DIVISIONMITOSIS MEIOSIS
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Cell Cycle
2 distinct phases
S
Chromosome duplication
G0 Non reproducing cells
G2
G1
What's the most important event of interphase?
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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?
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Some terminology

• Chromatin

• Homologous chromosomes

• Chromatid

• Double-chromatid chromosomes

• Single-chromatid chromosomes

When do each of these occur?
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MITOSIS
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THE STEPS OF MITOSIS

• Interphase
• (actually, this is not part of mitosis itself)
• Prophase
• Metaphase
• Anaphase
• Telophase

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Mitosis
Interphase
Whitefish blastula
Onion root tip

• 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

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Prophase
Mitosis
Onion root tip
Whitefish blastula

• Double-chromatid chromosomes evident

• Chromatin becomes super-coiled compact

• Nuclear envelop breaks down

• Centrioles migrate toward poles of cell forming
  the spindle

• Nucleoli disappear in onion root tip

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Double-chromatid chromosomes
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Mitosis
Metaphase
Whitefish blastula
Onion root tip

• Double-chromatid chromosomes line up on
  equatorial plate of cell

• The spindle fibers attach to the chromosomes at
  the centromeres

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Mitosis
Anaphase
Whitefish blastula
Onion root tip

• Spindle fibers pull chromatids apart

• Centromeres are pulled apart

• Groups of single-chromatid chromosomes move
  opposite poles of the cell

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Telophase
Mitosis
Onion root tip
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 cleavage
  furrow

• Cytokinesis begins with appearance of cell plate

• Nucleoli reform

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Mitosis
Returning to Interphase
Whitefish blastula
Onion root tip

• Cytokinesis completes

• Chromosomes disappear as interphase chromatin
  reforms

• 2 new daughter cells enter G1 phase of the cell
  cycle

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A SUMMARY OF MITOSIS
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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?

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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?

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Why is this duplication of parental DNA necessary?
Something to do with passing on genetic
information?
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Why undergo mitosis at all?
Something to do with cells getting damaged, old,
lost?
Something to do with the organism growing, infant
to adult?
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SOMATIC CELLS
What are typical body cells called?
These cells divide continuously
The new cells receive an exact copy of all the
parent cells
DNA

• What is the process of somatic cell duplication
  called?

Mitosis
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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?
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23 pairs
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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)

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Our haploid (n) number is 23
So our eggs and sperm have how many chromosomes?
Half the number
Why?
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MEIOSIS
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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?

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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!

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Note how mitosis and meiosis differ

• Number of divisions?
• Number of chromosomes?
• Number of products?

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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

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Haploid gametes (n ? 23)
Egg cell
n
n
Sperm cell
MEIOSIS
FERTILIZATION
Multicellular diploid adults (2n ? 46)
Diploid zygote (2n ? 46)
2n
MITOSIS
and development
Key
Haploid (n)
Diploid (2n)
Figure 8.12
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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
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Human Karyotype
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Which pair of chromosomes in us in not homologous?
Its the 23rd pair in males, the XY pair
Remember what homologous means?

• All our other pairs of chromosomes are homologous

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How many divisions does meiosis have?

• What are the divisions called?

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MITOSIS
MEIOSIS
Prophase I
Prophase
MEIOSIS I
Chromosome duplication
Chromosome duplication
Duplicated chromosome (two sister chromatids)
Parent cell (before chromosome duplication) 2n ?
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Homologous chromosomes come together in pairs.
Site of crossing over between homologous (nonsiste
r) chromatids
Metaphase
Metaphase I
Chromosomes align at the middle of the cell.
Homologous pairs align at the middle of the
cell.
Anaphase Telophase
Anaphase I Telophase I
Chromosome with two sister chromatids
Sister chromatids separate during anaphase.
Homologous chromosomes separate during anaphase
I sister chromatids remain together.
Haploid n ? 2
Daughter cells of meiosis I
2n
2n
MEIOSIS II
Daughter cells of mitosis
Sister chromatids separate during anaphase II.
n
n
n
n
Daughter cells of meiosis II
Figure 8.15
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1. One pair of homologues to start

1. DNA is replicated

1. Meiosis 1 2n ---gt n

1. Meiosis 2 chromatids separate into 4 products

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In males, how many progeny are produced?

• Typically 4 viable sperm are produced following
  each Meiosis 2

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In females, how many progeny are produced?

• Just one viable ovum (egg) is produced, plus 3
  small polar bodies

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Side by Side Comparision of mitosis and meiosis
http//www.pbs.org/wgbh/nova/baby/divi_flash.html

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Genetic Variation
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Independent Assortment
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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?
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Meiosis Summary

• Meiosis 1
• DNA replication takes place
• A parent cell produces two daughter cells each
  with one member of each original pair of
  homologous chromosomes (to create haploid
  daughter cells)
• Crossing over may occur

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Meiosis Summary

• Meiosis 2
• There is no more DNA replication
• The chromatids of each chromosome separate and
  each daughter cell divides
• 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

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Meiosis Questions

• The cells produced in meiosis are _____ (haploid
  or diploid?)
• Sex cells are called ________.
• _____ are produced by males, ____ by females.
• 3. Whats crossing over and why is it important?

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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?

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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.

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When Meiosis doesnt work

• 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)

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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
Number of chromosomes
n
n 1
n 1
n 1
n
n ? 1
n ? 1
n ? 1
Abnormal gametes
Abnormal gametes
Normal gametes
Figure 8.20-3
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Resources

• http//www2.le.ac.uk/departments/genetics/vgec/sch
  oolscolleges/topics/cellcycle-mitosis-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
Telophase Cytokinesis Spindle fiber Diploid Haploi
d Apoptosis Somatic Sex cell Gametes Zygote Karyot
ype Tetrad Crossing over
Independent assortment Nondisjunction Down
syndrome Trisomy Cancer Klinefelter
syndrome Turner syndrome

• DNA
• Chromosome
• Chromatin
• Histone
• Nucleosome
• Centromere
• Sister chromatids
• Mitosis
• Meiosis
• Homologous chromsomes
• Chromatid
• Interphase
• Prophase
• Prometaphase
• Metaphase
• Anaphase