introduction, mitosis, meiosis

1. COMSATS Institute Of Information Technology

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
2

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)
COMSATS Institute Of Information Technology
3

4. Interphase
COMSATS Institute Of Information Technology
4
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)
COMSATS Institute Of Information Technology
5

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)
COMSATS Institute Of Information Technology
6

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)
7

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
COMSATS Institute Of Information Technology
8

9. Metaphase
COMSATS Institute Of Information Technology
9
• 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
COMSATS Institute Of Information Technology
10

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
11

12. Cytokinesis
COMSATS Institute Of Information Technology
12
• 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
13

14. Stages of mitosis
14

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
COMSATS Institute Of Information Technology
15

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
16

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
17

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
18

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
19

20. Stages of Prophase I
20

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)
21

22. Telophase I and Cytokinesis
• Nuclear membrane reorganize
• Nucleoli reappear
• Two nuclei with half chromosome number formed
• Cytoplasm divides (cytokinesis) terminating meiosis I
22

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
23

24. Overview of Meiosis II
24

25. Overview of Meiosis I and II
25

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
26

27. COMSATS Institute Of Information Technology
27