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Cellular Reproduction
Beginning with Chromosomes
Duplicated chromosomes(sister chromatids) T
EM
Tight helical fiber
Looped domains
TE
M
Centromere
Nucleosome
“Beads ona string”
Histones
DNA double helix
Figure 8.4
DNA double helix
Chromatin
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• 1. What is a histone?
• 2. What is a nucleosome?
• 3. What is chromatin?
• 4. What is a centromere?
CELL DIVISIONMITOSIS & MEIOSIS
Mitosis = somatic cell divisionhttp://youtu.be/1cVZBV9tD-A
Meiosis = egg and sperm divisionhttp://youtu.be/rB_8dTuh73c
CELL DIVISIONMITOSIS & MEIOSIS
Mitosis = somatic cell divisionhttp://youtu.be/1cVZBV9tD-A
Meiosis = egg and sperm divisionhttp://youtu.be/rB_8dTuh73c
Interphase
Mitotic
Cell Cycle 2 distinct phases
What's the most important event of interphase?
Chromosome duplicationS
G2G1
G0Non reproducing cells
Cell Cycle results in the production of two cells,
each with the same amount of genetic
material and the same genetic information
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?
MITOSIS
• Interphase – this is when DNA is duplicated
(actually, this is not part of mitosis itself)
• Prophase• Metaphase• Anaphase• Telophase
• Interphase – this is when DNA is duplicated
(actually, this is not part of mitosis itself)
• Prophase• Metaphase• Anaphase• Telophase
THE STEPS OF MITOSISTHE STEPS OF MITOSIS
Stages of Mitosis – G2, prophase, metaphase, anaphase, telophase and cytokensis (cytokenesis occurs during telophase)
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.• Mitotic spindle begins to form
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
A SUMMARY OF MITOSIS
Mitosis QuestionsMitosis 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 QuestionsMitosis 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?Why is this duplication of parental DNA necessary?
Something to do with passing on genetic information?
Why undergo mitosis at all?
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 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?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?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 cellsMitosis occurs only in somatic cells
• Gametes are not diploid (2n)• Instead, they are haploid (n)
• 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
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
MEIOSISMitoses 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 spermFertilization is the union of an egg and a sperm
If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?
If the egg and sperm were both diploid, what would the fertilized egg (zygote) be?
It would be a genetic mess!
It would be a genetic mess!
At fertilization, n + n = 23; 23 + 23 = 46!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:Note how mitosis and meiosis differ:
• Number of divisions?• Number of chromosomes?• Number of products?
• Number of divisions?• Number of chromosomes?• Number of products?
Meiosis !Meiosis !
Meiosis is the cell division process that enables the transformation from 2n to nMeiosis 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)
Multicellulardiploid adults(2n 46)
MEIOSIS FERTILIZATION
MITOSIS
2n
and development Key
Sperm cell
n
n
Diploidzygote(2n 46)
Diploid (2n)
Haploid (n)
Egg cell
Haploid gametes (n 23)
Figure 8.12
How is meiosis more complicated than mitosis?
How is meiosis more complicated than mitosis?
Each chromosome has a partner 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 KaryotypeHuman Karyotype
Which pair of chromosomes in us in not homologous?Which pair of chromosomes in us in not homologous?
All our other pairs of chromosomes are homologous and are called autosomes
All our other pairs of chromosomes are homologous and are called autosomes
It’s the 23rd pair in males, the XY pair
Remember what homologous means?
What are the divisions called?What are the divisions called?
How many divisions does meiosis have?
Duplicated chromosome(two sister chromatids)
MITOSIS
Prophase
Chromosome duplication
Chromosomes align at the middle of thecell.
Metaphase
Sister chromatidsseparateduringanaphase.
AnaphaseTelophase
Daughter cellsof mitosis
2n2n
Prophase I
Metaphase I
Anaphase ITelophase I
MEIOSIS
Chromosome duplication
Homologous chromosomes come together in pairs.
MEIOSIS I
Site of crossing overbetween homologous(nonsister) chromatids
Homologous pairsalign at the middle of the cell.
Chromosome with twosister chromatids
Homologous chromosomes separate duringanaphase I;sister chromatidsremain together.
Daughtercells of meiosis I
Sister chromatidsseparate duringanaphase II.
Haploidn 2
MEIOSIS II
Parent cell(before chromosome duplication)
2n 4
Daughter cells of meiosis II n n n 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 start1. One pair of homologues to start
2. DNA is replicated2. DNA is replicated
3. Meiosis 1 = 2n ---> n3. Meiosis 1 = 2n ---> n
4. Meiosis 2 = chromatids separate into 4 products4. Meiosis 2 = chromatids separate into 4 products
In males, how many progeny are produced?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?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
Side by Side Comparision of mitosis and meiosis
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
Genetic Variation
Independent Assortment
Crossing overCrossing 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 SummaryMeiosis 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 SummaryMeiosis Summary
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
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 QuestionsMeiosis 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 QuestionsMeiosis Questions
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
• 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)
Meiosis I
Abnormal gametes
Gametes
Nondisjunction:Pair of homologouschromosomes failsto separate.
NONDISJUNCTION IN MEIOSIS I
Number of chromosomes
Meiosis II
Nondisjunction:Pair of sisterchromatidsfails to separate.
Abnormal gametes Normal gametes
n n n 1n 1 n – 1n 1
NONDISJUNCTION IN MEIOSIS II
n – 1 n – 1
Figure 8.20-3
Resources• http://www.pbslearningmedia.org/asset/lsps07_int_celldivision/
• http://www2.le.ac.uk/departments/genetics/vgec/schoolscolleges/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
Vocabulary• DNA• Chromosome• Chromatin• Histone• Nucleosome• Centromere• Sister chromatids• Mitosis• Meiosis• Homologous
chromsomes• Chromatid• Interphase• Prophase• Prometaphase• Metaphase• Anaphase
TelophaseCytokinesisSpindle fiberDiploidHaploidApoptosisSomaticSex cellGametesZygoteKaryotypeTetradCrossing over
Independent assortmentNondisjunctionDown syndromeTrisomyCancerKlinefelter syndromeTurner syndrome