The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle

2. Cell Cycle
The sequence of events by which a cell
duplicates its genome, synthesizes the
other constituents of the cell and
eventually divides into two daughter
cells is termed cell cycle.

3. Phases of Cell Cycle
A typical eukaryotic cell divides once in approximately every
24 hours.
The cell cycle is divided into two basic phases:
l Interphase
l M Phase (Mitosis phase)

4. Phases of Cell Cycle
Interphase

5. M Phase
• Phase when the actual cell division or mitosis occurs
• Starts with the nuclear division, corresponding to the separation of
daughter chromosomes
• Ends with division of cytoplasm (cytokinesis)
• Cell division proper lasts for only about an hour

6. Interphase
• Represents the phase between two successive M phases
• Lasts more than 95% of the duration of cell cycle
• It is the time during which the cell is preparing for division by
undergoing both cell growth and DNA replication

7. Interphase
Interphase is divided into three further phases:
l G1 phase (Gap 1)
l S phase (Synthesis)
l G2 phase (Gap 2)

8. Interphase: G1 phase
• 1st growth stage
• Interval between mitosis and initiation of DNA replication
• Cell is metabolically active and continuously grows but does not
replicate its DNA

9. Interphase: S phase
• DNA synthesis or replication takes place
• Amount of DNA per cell doubles
• However, there is no increase in the
chromosome number

10. Interphase: G2 phase
• 2nd Growth Stage
• Proteins are synthesized in preparation for mitosis
• Cell growth continues
*Some cells in the adult animals do not appear to exhibit division (e.g.,
heart cells) and some divide only occasionally
*These cells exit G1 phase to enter quiescent stage (G0) of the cell
cycle which is an inactive stage of the cell cycle

11. Mitosis
• The process of cell division which results in the production of two
daughter cells from a single parent cell
• The daughter cells are identical to one another and to the original
parent cell since the chromosomes are same
• Mitosis is divided into four stages:-
• Prophase
• Metaphase
• Anaphase
• Telophase

12. Mitosis : Phases
Prophase Metaphase
Anaphase
Telophase

13. Mitosis: Prophase
• Chromosomal material
condenses to form compact
mitotic chromosomes
• Centrioles begins to move
towards opposite poles of the
cell

14. Mitosis: Metaphase
• Spindle fibres attach to kinetochores
of chromosomes
• Chromosomes line up at the center
of the cell
• Chromosomes are moved to spindle
equator and get aligned along
metaphase

15. Mitosis: Anaphase
• Centromeres split and
chromatids separate
• Chromatids move to opposite
poles

16. Mitosis: Telophase
• Chromosomes cluster at opposite
spindle
• Nuclear envelope assembles
around the chromosome clusters
• Nucleolus, Golgi complex and ER
reform.

17. Cytokinesis
• 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

18. Significance of Mitosis
• The growth of multicellular organisms is due to mitosis
• Significant role in cell repair
• Mitotic divisions in the meristematic tissues result in a continuous
growth of plants throughout their life

19. Meiosis
•Specialized kind of cell division that reduces the
chromosome number by half
•Results in production of haploid daughter cells
•It involves
• two cycles of nuclear and cell division(Meiosis I & II )
• single cycle of DNA replication
• pairing of homologous chromosomes and recombination
between them.

20. Meiosis: Phases
• Meiotic events can be grouped under the following phases:
Meiosis I Meiosis II
Prophase I Prophase II
Metaphase I Metaphase II
Anaphase I Anaphase II
Telophase I Telophase II

21. Meiosis I: Reduction Division
Early
Prophase I
(Chromosome
number
doubled)
Late
Prophase I
Metaphase I Anaphase I
Telophase I
(diploid)

22. Meiosis I: Prophase I
• Further subdivided into the following five phases based on
chromosomal behavior
1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis
Early prophase Late prophase

23. Meiosis I: Prophase I
Leptotene stage:
• The compaction of chromosomes occurs which continues throughout
leptotene
Zygotene:
• The chromosomes start pairing together
• Paired chromosomes are called homologous chromosomes
• Formation of complex structure called synaptonemal complex
• The complex formed by a pair of synapsed homologous chromosomes is
called a bivalent or a tetrad

24. Meiosis I: Prophase I
Pachytene:
• Bivalent chromosomes now appears as tetrads
• Crossing over occurs between non-sister chromatids of the homologous
chromosomes leading to recombination between homologous chromosomes

25. Meiosis I: Prophase I
Diplotene:
• Dissolution of the synaptonemal complex
• homologous chromosomes of the bivalents separate from each other except
at the sites of crossovers
• This forms X-shaped structures, are called chiasmata
Diakinesis:
• Terminalisation of chiasmata
• Chromosomes are fully condensed
• It represents transition to metaphase

26. Meiosis I: Metaphase I
• Bivalent chromosomes align on the equatorial plate
• The microtubules from the opposite poles of the spindle attach to
the pair of homologous chromosomes.
Homologous pairs of
chromosomes align
along the equator of
the cell

27. Meiosis I: Anaphase I
• The homologous chromosomes
separate
• Sister chromatids remain
associated at their centromeres

28. Meiosis I: Telophase I
• The nuclear membrane and
nucleolus reappear
• Spindle disappears
• Cytokinesis follows

29. Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
4 Identical
haploid cells

30. Meiosis II
Prophase II: The nuclear
membrane disappears &
chromosomes again become
compact
Meiosis II resembles a normal mitosis
Nuclear
envelope
fragments

31. Meiosis II
Metaphase II:
• Chromosomes align at
the equator
• Microtubules from
opposite poles of the
spindle get attached to
the kinetochores of
sister chromatids.

32. Meiosis II
Anaphase II: Sister
chromatids separate
and move to
opposite poles
Equator
Pole

33. Meiosis II
Telophase II:
• Nuclear envelope assembles
• Chromosomes decondense
• Spindle disappears
• Cytokinesis divides cell into
two.

34. Significance of Meiosis
• Conservation of specific chromosome number of each species is
achieved across generations by meiosis
• Reduction of chromosome number by half
• Increases the genetic variability in the population of organisms from
one generation to the next

35. Mitosis & Meiosis : A comparison
Mitosis Meiosis
Number of divisions 1 2
Number of daughter
cells
2 4
Genetically identical? YES NO
Chromosome no. Same as parent Half of parent
Where Somatic cells Germ cells
When Throughout life At sexual maturity
Role Growth and repair Sexual reproduction

36. Thankyou
Presented by:
Nivedita Dubey
Class 11th (Biology)
Roll no. 32