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Cell Division & Cell Cycle
Comparing Mitosis and Meiosis
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Cell Division•Cell Division-when a parent cell divides into two daughter cells.
•All body cells are somatic cells and divide by mitosis
•Sperm and egg, which are gametes (sex cells) divide by meiosis
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Cell Division• Mitosis-the
nucleus of the cell divides equally
Parent Cell
Two identic
al daughter cells
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Eukaryotic Cell Cycle
Daughter Cells
DNA is Copied
Cells Matur
e
Organelles form & Cells prepares to
Divide
Cell nucleus divides equally
Gap 1 phase
Synthesis phase
Period between cell division
Gap 2 phase
Cytoplasm Divides
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Cell CycleRemember: I P M A T C
• I = Interphase (G1, S, G2)• P = Prophase• M= Metaphase• A= Anaphase• T= Telophase• C= Cytokinesis
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Interphase: Part 1 of Cell Cycle
• Interphase-cell prepares for cell division.
• Genetic Material (DNA)-is in the form of chromatin-uncoiled chromosomes
• Has 3 phases: G1, S, G2
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Eukaryotic Chromosomes• Eukaryote cells
store genetic info.in tightly coiled DNA segments or chromosomes
• Human body cells =46 chromosomes Chromosomes can’t be seen when cells aren’t dividing and are called chromatin
• Chromatin-uncoiled chromosomes
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Chromosome Pictures• Duplicated
chromosomes are called sister chromatids & are held together by the centromere-found at the center of the chromosome
Sister Chromatids
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Mitosis: Part 2 Of Cell Cycle• Mitosis-is
equal division of the nucleus of eukaryote cells
Has four stages:
• P rophase• Metaphase• Anaphase• Telophase
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Prophase• Prophase-first and longest
phase of mitosis• Nucleolus and nuclear
envelope will disappear• Chromosomes are now
visible and are attached to the spindle
• The Spindle-(a fan-like structure that pulls the chromosomes apart) appears.
• Centrioles-(two tiny structures in the cytoplasm that travel toward opposite sides of the nucleus) in animal cells help to form the spindle
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Metaphase• Second, shortest phase of mitosis; lasts a
few minutes• Chromosomes are now lined up at the
equator or center of the cellEquator of Cell
Pole of the Cell
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Anaphase
Sister Chromatid
s being separated
Spindle is pulling the sister chromatids apart Spindle
sister chromatids
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Telophase• Sister chromatids
are at opposite poles
• Spindle breaks apart
• Nuclear envelope forms around each set of sister chromatids
• Nucleolus reappears
• Chromosomes reappear as chromatin
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Cytokinesis• Means division
of the cytoplasm
• Division of cell into two, identical halves called daughter cells
• In plant cells, cell plate forms at the equator to divide cell
• In animal cells, cleavage furrow forms to split cell
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Identical Daughter Cells
Chromosome number is maintained or is the same (Diploid), but cells
smaller than parent cell
What is the 2n or diploid
chromosome number in humans?
4646
46
46
Mitosis-goes from diploid to diploid
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MeiosisFormation of
Gametes (Eggs & Sperm)
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Facts About Meiosis• Interphase occurs before
meiosis; chromosome replication occurs
• Two meiotic divisions-Meiosis I and Meiosis II
• Called Reduction Division-chromosome # is reduced by ½ (4623)
• Original cell is diploid (2n)=46• Four daughter cells produced at
the end of meiosis are monoploid (n) or haploid (chromosomes reduced by ½)
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Facts About Meiosis• Daughter cells contain half (23)
the number of chromosomes as the original cell (46)
• Meiosis produces gametes (eggs & sperm)
• Occurs in the testes in males (Spermatogenesis)
• Occurs in the ovaries in females (Oogenesis)
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Why Do we Need Meiosis?
• It is the fundamental basis of sexual reproduction
• Two haploid (n) gametes are brought together through fertilization to form a diploid (2n) zygote
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Meiosis Forms Haploid Gametes
• Meiosis must reduce the chromosome number by half
• Fertilization then restores the 2n number
meiosis reducesgenetic content
from mom from dad child
toomuch!
The right number!
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Prophase I
Early prophase• Homologs pair.
• Crossing over occurs.
Late prophase• Chromosomes condense.
• Spindle forms.• Nuclear envelope fragments.
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Crossing-Over• Homologous
chromosomes in a tetrad cross over each other
• Pieces of chromosomes or genes are exchanged
• Produces Genetic recombination in the offspring
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Metaphase I
Homologous pairs of
chromosomes align along the equator of the
cell
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Anaphase I
Homologs separate and move to opposite poles.
Sister chromatids remain attached at their centromeres.
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Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
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Meiosis IIOnly one homolog of
each chromosome is present in the cell.
Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.
Sister chromatids carry identical genetic information.
Gene X
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Meiosis II: Reducing Chromosome Number
Prophase II Metapha
se IIAnaphas
e II
Telophase II 4 Identical
haploid cells
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Prophase II
Nuclear envelope fragments.
Spindle forms.
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Metaphase II
Chromosomes align along equator of cell.
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Anaphase II
Sister chromatids separate and move to opposite poles.
Equator
Pole
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Telophase II
Nuclear envelope assembles.
Chromosomes decondense.
Spindle disappears.
Cytokinesis divides cell into two.
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Results of MeiosisGametes (egg & sperm) form
Four haploid cells with one copy of each chromosome
One allele of each gene
Different combinations of alleles for different genes along the chromosome
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Spermatogenesis• Occurs in the
testes• Two divisions
produce 4 spermatids
• Spermatids mature into sperm
• Men produce about 250,000,000 sperm per day
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Spermatogenesis in the Testes
Spermatid
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Oogenesis• Occurs in the ovaries• Two divisions produce 3 polar
bodies that die and 1 functional egg
• Polar bodies die because of unequal division of cytoplasm
• Immature egg is called an oocyte
• Starting at puberty, one oocyte matures into an ovum (egg) every 28 days
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Mitosis MeiosisNumber of
divisions 1 2
Number of daughter cells 2 4
Genetically identical? Yes No
Chromosome # Same as parent Half of parent
Where Somatic (body) cells Germ cells
When Throughout life
At sexual maturity
Role Growth and repair
Sexual reproduction
Comparing Mitosis and Meiosis
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Karyotype• A picture of the
chromosomes from a human cell arranged in pairs by size
• First 22 pairs are called autosomes
• Last pair are the sex chromosomes
• XX female or XY male
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Nondisjunction• When chromosomes fail
to separate during Anaphase I or Anaphase II of meiosis
• Chromosome number can be more (47) or less (45), than the normal chromosome number (46)
• Example: Down’s Syndrome-has extra chromosome on #21 also called Trisomy 21
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Quiz1. If a cat has 38 chromosomes in each
of its body cells, how many chromosomes
will be in each daughter cell after mitosis?
F 11G 19H 38J 76
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Quiz2. If a cat has 38 chromosomes in each
of its cells, how many chromosomes will be in each gamete after meiosis?
F 11G 19H 38J 76
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Quiz3. Which of these symbols represents the normal karyotype of a human female?
F XXYG XXH XOJ XXX
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Quiz4. This human karyotype is unusual because chromosome set —
A 5 has chromosomes of different shapesB 10 is missing genetic materialC 14 has enlarged centromeresD 21 has extra genetic material
Unusual Human Karyotype
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Quiz5. The diagram represents
the chromosomes of a person with a genetic disorder caused by nondisjunction, in which the chromosomes fail to separate properly. Which chromosome set displays nondisjunction?
F 2G 8H 21J 23
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Quiz6.
45
Quiz7.
46
Quiz8.
47
9.
48
Quiz10.