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CELL DIVISION 
MITOSIS & MEIOSIS
Cell Cycle 2 distinct phases 
S 
Interphase 
Mitotic 
What's the most important 
event of interphase? 
Chromosome duplication 
G2
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?
THE STEPS OF MITOSIS 
• Interphase 
(actually, this is not part of mitosis itself) 
• Prophase 
• Metaphase 
• Anaphase 
• Telophase
Interphase 
Onion root tip Whitefish blastula 
Mitosis 
• 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 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
Double-chromatid chromosomes
Metaphase Mitosis 
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
Anaphase Mitosis 
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 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
Returning to Interphase Mitosis 
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
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?
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). 
At fertilization, n + n = 23; 23 + 23 = 46! 
Egg and sperm both 
have exactly half the 
number of 
chromosomes of 
somatic cells
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
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
How many divisions does meiosis have? 
What are the divisions called?
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
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?

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Cell reproduction mitosis&meosis

  • 2. Cell Cycle 2 distinct phases S Interphase Mitotic What's the most important event of interphase? Chromosome duplication G2
  • 3. 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?
  • 4. Some terminology • Chromatin • Homologous chromosomes • Chromatid • Double-chromatid chromosomes • Single-chromatid chromosomes When do each of these occur?
  • 5. THE STEPS OF MITOSIS • Interphase (actually, this is not part of mitosis itself) • Prophase • Metaphase • Anaphase • Telophase
  • 6. Interphase Onion root tip Whitefish blastula Mitosis • 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
  • 7. 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
  • 9. Metaphase Mitosis 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
  • 10. Anaphase Mitosis 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
  • 11. 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
  • 12. Returning to Interphase Mitosis Onion root tip Whitefish blastula • Cytokinesis completes • Chromosomes ‘disappear’ as interphase chromatin reforms • 2 new daughter cells enter G1 phase of the cell cycle
  • 13. A SUMMARY OF MITOSIS
  • 14. 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?
  • 15. 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?
  • 16. Why is this duplication of parental DNA necessary? Something to do with passing on genetic information?
  • 17. Why undergo mitosis at all? Something to do with cells getting damaged, old, lost? Something to do with the organism growing, infant to adult?
  • 18. 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
  • 19. 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
  • 20. 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)
  • 21. Our haploid (n) number is 23 So our eggs and sperm have how many chromosomes? Half the number Why?
  • 22. 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?
  • 23. It would be a genetic mess! Instead, gametes are haploid (n). At fertilization, n + n = 23; 23 + 23 = 46! Egg and sperm both have exactly half the number of chromosomes of somatic cells
  • 24. Note how mitosis and meiosis differ: • Number of divisions? • Number of chromosomes? • Number of products?
  • 25. 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
  • 26. 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
  • 28. 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
  • 29. How many divisions does meiosis have? What are the divisions called?
  • 30. 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
  • 31.
  • 32. In males, how many progeny are produced? Typically 4 viable sperm are produced following each Meiosis 2
  • 33. In females, how many progeny are produced? Just one viable ovum (egg) is produced, plus 3 small polar bodies
  • 34. 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?
  • 35. 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
  • 36. 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
  • 37. 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?
  • 38. 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?