CELL DIVISION MITOSIS & MEIOSIS
CELL DIVISION MITOSIS & MEIOSIS
Interphase
Mitotic
Cell Cycle 2 distinct phases
What's the most important
event of interphase?
Chromosome duplication
S
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 blastula
Telophase 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?
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?
SOMATIC CELLS
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?
46
23 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?
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!
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 pairs
One 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 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
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?
Meiosis Questions
Cyclin
• protein
• regulates the timing of the cell
cycle in eukaryotic cells
Regulatory Proteins
• Two types
1. Internal Regulators
2. External Regulators
Internal Regulators
• proteins that respond to events
inside the cell
• signals cell cycle to continue
ONLY WHEN OTHER THINGS
ARE COMPLETE
External Regulator
• proteins that respond to events
outside the cell
• direct cells to speed up or slow
down the cell cycle
• example: growth regulators
Uncontrolled Cell Growth
• Cancer
– body loses ability to control
growth
– does not respond to the regulator
proteins
– result = large masses of cells
Tumors • masses of cells
• damages the surrounding tissues
• cells break off and spread
throughout the body
Cause of cancer • Brought to you by carcinogens
–Cigarette smoke
–Burnt meat
–Preservatives, etc
• Radiation exposure
• Viral infection
• Others
Review Questions
1. Why is it important for cells to have
regulated cell growth?
2. What is the purpose of the protein
cyclin?
3. Compare and contrast internal
regulators and external regulators.
4. What is cancer? What causes
cancer?