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CELL DIVISIONMITOSIS & MEIOSIS
Interphase
Mitotic
Cell Cycle 2 distinct phases
What's the most important event of interphase?
Chromosome duplicationS
G2
All chromosomes are duplicated
Do they contain identical genes?
What is significant about DNA in the S and G2 phases?
That means: two copies of each chromosome
What are the copies called?
Sister chromatids
Yes, but…
What about meiosis?
Some terminology
• Double-chromatid chromosomes
• Single-chromatid chromosomes
• Chromatin
• Homologous chromosomes
• Chromatid
When do each of these occur?
• Interphase (actually, this is not part of mitosis itself)
• Prophase• Metaphase• Anaphase• Telophase
THE STEPS OF MITOSIS
Onion root tip Whitefish blastula
Interphase
• Nuclear envelop intact
• Chromosomes duplicated but not apparent
• 2 nucleoli visible in onion root tip
• Nucleoli not visible in whitefish
blastula
Mitosis
• DNA loose, uncondensed, called chromatin
Onion root tip Whitefish blastula
Prophase Mitosis
• Double-chromatid chromosomes evident
• Chromatin becomes super-coiled & compact
• Nuclear envelop breaks down
• Nucleoli disappear in onion root tip
• Centrioles migrate toward poles of cell forming the spindle
Double-chromatid chromosomes
Onion root tip Whitefish blastula
Metaphase Mitosis
• Double-chromatid chromosomes line up on equatorial plate of cell
• The spindle fibers attach to the chromosomes at the centromeres
Onion root tip Whitefish blastula
Anaphase Mitosis
• Centromeres are pulled apart
• Groups of single-chromatid chromosomes move opposite poles of the cell
• Spindle fibers pull chromatids apart
Onion root tip Whitefish blastulaTelophase Mitosis
• Cytokinesis begins with appearance of cleavage furrow
• Cytokinesis begins with appearance of cell plate
• Groups of single-chromatid chromosomes reach poles of cell
• Nuclear envelop begins to reform
• Nucleoli reform
• 2 new daughter cells formed
Onion root tip Whitefish blastula
Returning to Interphase Mitosis
• 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?
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?
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 al l?
Something to do with cells getting damaged, old, lost?
Something to do with the organism growing, infant to adult?
SOMATIC CELLS
What is the process of somatic cell duplication called?What is the process of somatic cell duplication called?
What are typical body cells called?
These cells divide continuously
The new cells receive an exact copy of all the parent cell’s:
Mitosis
DNA
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?
4623 pairs
Mitosis occurs only in somatic cells
• Gametes are not diploid (2n)• Instead, they are haploid (n)
What about sex cells?
Called gametes
Eggs and sperm
Produced in ovaries or testes
Our haploid (n) number is 23
So our eggs and sperm have how many chromosomes?
Half the number
Why?
Ferti l ization is the union of an egg and a sperm
If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?
I t would be a genetic mess!
At fertilization, n + n = 23; 23 + 23 = 46!
Instead, gametes are haploid (n).
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 !
Meiosis is the cell division process that enables the transformation from 2n to n
Somehow somatic cells (2n) in our ovaries or testes must produce
gametes (n)
How is meiosis more complicated than mitosis?
Each chromosome has a partner
Gametes must contain precisely half the diploid number of chromosomes
They must contain one of each homologous pair of chromosomes
Remember karyotypes?
They come in pairsOne from mom
One from dad
Human Karyotype
Which pair of chromosomes in us in not homologous?
All our other pairs of chromosomes are homologous
It’s the 23rd pair in males, the XY pair
Remember what homologous means?
What are the divisions called?
How many divisions does meiosis have?
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 2Typically 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 bodiesJust 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 1a. 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
1. Meiosis 1a. 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 Summary2. 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
2. Meiosis 2a. 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
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?
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?
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?
Meiosis Questions
