The document summarizes the processes of mitosis and meiosis in eukaryotic cells. It describes the stages of the cell cycle including interphase and mitosis. Mitosis is divided into prophase, metaphase, anaphase and telophase where chromosomes are aligned and separated. Meiosis produces gametes through two cell divisions and a single DNA replication. This reduces the chromosome number by half to maintain genetic diversity between generations. Crossing over and random chromosome segregation in meiosis lead to genetic variation important for evolution.

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2. Contents
• Introduction to Cell Cycle
• Interphase
• Mitosis
• Mitotic Apparatus and Karyokinesis
• Phases of Mitosis and Cytokinesis
• Significance of Mitosis
• Meiosis
• Phases of Meiosis I, Meiosis II and Cytokinesis
• Significance of Meiosis COMSATS Institute Of Information Technology
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3. Introduction
• The cell undergoes sequence of changes, which involves period of
growth, DNA replication and cell division and this process is called
cell cycle
• It involves 2 phases;
 Interphase (period between two consecutive cell divisions,
misleadingly called resting phase)
 Mitotic phase (cell division period)
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4. Interphase
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Period of great biochemical activity, divided into 3 phases;
 G1 (Gap 1):
• Period of extensive metabolic activity
• Cell grows in size
• Specific enzymes synthesized
• DNA bases accumulate for DNA synthesis
 S (synthesis) phase:
• DNA synthesis
• Chromosome number doubles
 G2 (pre-mitotic) phase:
• Preparing cell for division
• Energy storage for chromosome movement
• Mitosis specific proteins synthesis
• Microtubule subunit synthesis

5. Mitosis
• Type of cell division that ensures same number of chromosomes in
daughter cells as that in parent cells
• Takes place in haploid and diploid cells in all body parts whenever
required
• It is a continuous process
• For understanding, divided into 2 phases
 Karyokinesis (division of nucleus)
 Cytokinesis (division of whole cell)
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6. Mitotic Apparatus and Karyokinesis
• At beginning, partition of two pair of centrioles (duplicated in interphase)
occurs and they migrate towards opposite sides of nucleus
• Three microtubule sets originate from each centriole pair
• One set is astral microtubule that radiate outward and form aster (polar
microtubules)
• Other two sets form the spindle (kinetochore microtubules)
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7. Mitotic Apparatus and Karyokinesis
• Spindle fibers attach to chromosomes at kinetochore
• Astral microtubule forms connection with other astral
microtubules from opposite pole
• Spindle and aster together forms mitotic apparatus
• Designed to attach, capture, align and separate chromosomes to
ensure equal distribution
• Karyokinesis divided into 4 phases (prophase, metaphase,
anaphase, telophase)
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8. Prophase
• Chromatin material condense by folding and chromosomes appear as thin threads
(0.25µm-50μm) at beginning of prophase
• Chromosomes become more visible having 2 sister chromatids attached at centromere
• At the end; nuclear envelope disappears, nuclear material released in cytoplasm,
nucleoli disappear, mitotic apparatus organize (as already described) and cytoplasm
becomes more viscous
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9. Metaphase
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• Each metaphase chromosome is duplicated structure consisting 2 sister
chromatids attached at centromere
• Centromere has specialized area called kinetochore, here kinetochore fibers of
spindle attach
• After attachment, they align chromosomes at the equator of spindle forming
equatorial plate or metaphase plate
• Each kinetochore gets 2 fibers from each pole

10. Anaphase
• Most critical phase that ensures equal distribution of chromatids in daughter
cells
• Kinetochore fibers contract towards respective poles
• Polar microtubules elongate and exert force
• Sister chromatids separate from centromere
• Ultimately half sister chromatids migrate towards each pole
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11. Telophase
• Anaphase terminates after sister chromatids migrate towards opposite
poles and telophase starts
• Chromosome decondense by unfolding
• Ultimately disappear as chromatin
• Mitotic apparatus disorganize
• Nuclear membrane and nucleoli reorganize
• Resulting in two nuclei at two poles of cell COMSATS Institute Of Information Technology
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12. Cytokinesis
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• During late telophase, astral microtubules send signals to equatorial
region of cell
• Then actin and myosin activates and form contractile ring
• This will cleave the parent cell into 2 daughter cells

13. Stages of mitosis
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14. Stages of mitosis
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15. Importance of mitosis
• Hereditary material is equally distributed in daughter cells
• Genetic information remains unchanged from parent to daughter cells
since there is no crossing over, ensuring continuity of same information
• Regeneration, wound healing and older cells are replaced are all gifts of
mitosis (asexual reproduction)
• Multicellular organisms depend on controlled mitosis for development
and growth
• Tissue culture and cloning seek help through mitosis
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16. Meiosis
• Special type of cell division in which chromosome number in daughter
cells reduced to half, as compared to parent cell
• Takes place in diploid cells only (gamete formation in animals and spore
formation in plants)
• Each diploid cell forms 4 haploid cells due to two consecutive divisions
after single replication of DNA
• Two divisions are meiosis I and meiosis II
• Meiosis I is reduction division
• Meiosis II is just like mitosis
• Both divisions are further divided into prophase I, metaphase I, anaphase
I, telophase I and same names used for meiosis II
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17. Prophase I
• A very prolonged phase different from prophase of mitosis as chromosomes
behave as homologous pairs
• Each diploid cell contains two chromosomes as homologous pairs, one
from each parent due to male and female gamete fusion
• Each chromosome contains 2 chromatids, since chromosomes have been
replicated during interphase
• These similar (not necessarily identical) chromosomes are called homologous
chromosomes
• Prophase I further contains 5 stages:
 Leptotene
 Zygotene
 Pachytene
 Diplotene
 Diakinesis
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18. Stages of Prophase I
 Leptotene
• Chromosomes become visible, shorten and thick
• Nucleus size increases
• Homologous chromosomes start getting closer
 Zygotene
• Critical phase as pairing of homologous chromosomes (synapsis)
• Pairing is highly specific and pointed
• Each paired but not fused complex structure is called bivalent or tetrad (4
chromatids)
 Pachytene
• Pairing completed and each bivalent wrap around each other
• Chromosomes become more thick
• Non-sister chromatids exchange segments by chiasmata formation
(crossing over)
• Reshuffling of genetic material producing recombinations
• This phase lasts for days, weeks or years while former two lasts for few
hours
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19. Stages of Prophase I
 Diplotene
• Paired chromosomes repel and separate, except where chiasmata
occurred
 Diakinesis
• Chromosome condensation is maximum
• Separation started during diplotene is completed (still united where
chiasmata occurred)
• Nucleoli disappear
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20. Stages of Prophase I
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21. Meiosis I
 Metaphase I
• Nuclear membrane disorganizes
• Spindle fibers originate
• Kinetochore fibers attach at kinetochore of
homologous chromosome and arrange bivalents at
equator
 Anaphase I
• Kinetochore fibers contract
• Spindle or pole fibers elongate
• Individual chromosome (containing 2
chromatids) are pulled towards their respective
poles
• Note that in contrast to anaphase of mitosis, sister
chromatids are not separated
• This is reduction phase (as each pole receives half
of total number of chromosomes)
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22. Telophase I and Cytokinesis
• Nuclear membrane reorganize
• Nucleoli reappear
• Two nuclei with half chromosome number formed
• Cytoplasm divides (cytokinesis) terminating meiosis I
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23. Meiosis II
• After telophase I, 2 daughter cells undergo interphase but there is no
replication of chromosomes
• Stages of meiosis II (prophase II, metaphase II, anaphase II and
telophase II) are similar to meiosis I
• Hence form 4 nuclei at respective poles of two daughter cells
• Cytokinesis takes place
• Ultimately 4 haploid cells form having single chromatid each
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24. Overview of Meiosis II
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25. Overview of Meiosis I and II
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26. Importance of Meiosis
• Two important events of meiosis are crossing over and random
assortment
• Crossing over between parental segments leads to large number of
recombinations
• Assortment of homologous chromosomes is random during anaphase
• This leads to wide variety of gametes
• Both events give rise to evolution
• Make every individual specific and unique in characteristics
• Even progeny of same parents are not identical
• Maintains constant number of chromosomes generation after generation
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