This includes detailed description of the Cell Cycle and Cell Cycle regulation. Courtesy: Campbell Biology Book, And Dr, Rosemary Redfield Lectures, University of British Columbia.

2. Table Of Contents:
1. Introduction to types
2. Mitosis
3. Meiosis
4. Difference b/w Mitosis And Meiosis
By: Rohit Satyam
BT 2nd Yr

3. Two Basic Process In Cell
Reproduction
1. Cell-Growth: Period of synthesis and
duplication of various components
of cell.
2. Cell-Division: Adult cell divides into
two daughter.

4. Fig: 1. Cell Cycle
G0

5. Cell Cycle
Definition: All those changes which occur
during cell growth and cell division are
collectively called Cell Cycle.
Two Parts of Cell Cycle:
1.Interphase
2. M-Phase

6. Cell
cycle
Interphase
G1
Phase
S
Pha
se
G2
Phase
M-phase
Karyoki
nesis
Cytoki
nesis

8. Interphase
• Also known as RESTING PHASE(no visible changes occur)/
Metabolically active phase/ preparatory phase/ energy
phase.
• Three sub phases are:
1. G1/ Gap-I/ Post mitotic/ Pre-synthetic/ First growth
phase:
• Pooling of Amino Acid & Nucleotides for protein and
Nucleic Acid synthesis respectively.
• Energy molecules and enzyme synthesis
• Carbohydrates, Lipids and Protein Synthesis
• Chromosome are Fully Extended

9. 2. S/ Synthetic phase:
• DNA replicates semiconservately forming two sister chromatids
joined at centromere/ primary constriction.
•Histone synthesis
NOTE: In this phase the DNA amount doubles(4N) But the
chromosomes number remain same
3. G2/ Gap-2/ Post Synthetic/ Pre mitotic phase/ second
growth phase: Increase in Nuclear Volume
•Synthesis of:
1. Spindle Proteins 2. Three types of RNA molecules
3. ATP molecules 4. Mitochondria Duplication
5. Damaged DNA is Repaired

10. M-Phase/ Mitotic Phase/ D-phase
• It involves the separation of SISTER CHROMATIDS & their
redistribution into daughter cells.
• Orderly distribution of cell organelles
• It consists of two stages:
1. Karyokinesis: (Karyon: Nucleus And Kinesis: movement): It
involves division of nucleus for which nucleus develops a
constriction in centre and becomes dumbell-shaped.
Constriction divides the parent nucleus into two daughter
nucleus.
2. Cytokinesis: ( kytos= cell): Division of cytoplasm. A
constriction in Plasma membrane develops a constriction
and deepens centripetally and finally divides into two cells.

11. Terminology
1. Homologous Chromosomes: The cell has
two sets of each chromosome; one of the pair
is derived from the mother and the other from
the father. The maternal and paternal
chromosomes in a homologous pair have the
same genes at the same loci, but possibly
different alleles.
2. Kinetochores: On the surface of each
centromere, there are two disc like areas where
the spindle fibres attaches. They are k/a
Kinetochores.

16. Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Chromosomes
duplicate
Sister
chromatids
Diploid cell with
duplicated
chromosomes
Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Sister chromatids
separate
Haploid cells with unduplicated chromosomes
Interphase
Meiosis I
Meiosis II
2
1

17. Mitosis
•It is also called Somatic Cell Division or
Equational Division.
•In this, mature somatic cell divides in such a way
that chromosome number is kept constant in
daughter cell equal to parent cell, so that
daughter cell are quantitatively & qualitatively
similar to Parent Cell, so it is called Equational
Division.

18. •Mitosis produces two
genetically identical
cells.
•Mitosis is referred to in
the following stages:
prophase, metaphase,
anaphase, and telophase.
Mitosis

19. •In prophase, the cell begins the
process of division.
•The chromosomes condense.
Prophase

21. Prophase
• Nuclear envelope disappears.
•Centrioles migrate to opposite poles of the cell.
•Asters and spindle fibers form.

22. Aster
Sister chromatids
Centromere
Centriole
Spindle fibers

23. Metaphase
• The chromosomes line
up at the equator of the
cell (metaphase plate),
with the centrioles at
opposite ends and the
spindle fibers attached
to the centromeres.
Centriole
Centriole
Spindle
fibers
Metaphase
plate

28. Anaphase
• In anaphase, the
centromeres divide.
• At this point, each
chromosome goes from
having 2 sister
chromatids to being 2
separate chromosomes

29. The spindle fibers contract and
the chromosomes are pulledto
opposite poles.

30. Telophase
• In telophase the nucleus
actually divides.
• The chromosomes are at
the poles of the cell.
• The nuclear envelope re-
forms around the two
sets of chromosomes.

31. Cytokinesis
• The division of the
cytoplasm.
• In animal cells, a
Cleavage Furrow
forms and separates
Daughter Cells
Cleavagefurrowin a dividingfrogcell.

32. Cytokinesis
• In plant
cells, a
Cell Plate
forms and
separates
Daughter
Cells.

33. ANIMAL VS. PLANT MITOSIS
• ANIMAL CELL
– Centriole and
aster present
– Daughter cells
separated by
cleavage furrow
• PLANT CELL
– No visible
centriole or aster
– Daughter cells
separated by cell
plate

34. The Stages of Meiosis
• After chromosomes duplicate, two divisions
follow
– Meiosis I (reductional division): homologs pair
up and separate, resulting in two haploid
daughter cells with replicated chromosomes
– Meiosis II (equational division) sister chromatids
separate
• The result is four haploid daughter cells with
unreplicated chromosomes

35. Meiosis
It is called REDUCTIONAL DIVISION
because it involves formation of 4
daughter cells which have half
chromosome number to those in their
parental cell.

37. Crossing Over

38. •Meiosis I is preceded by interphase,
when the chromosomes are duplicated to
form sister chromatids
•The sister chromatids are genetically
identical and joined at the centromere
•The single centrosome replicates,
forming two centrosomes

39. Division in meiosis I occurs in
four phases
–Prophase I
–Metaphase I
–Anaphase I
–Telophase I and
cytokinesis

40. Prophase I
•Prophase I typically occupies more
than 90% of the time required for
meiosis
•Chromosomes begin to condense
•In synapsis, homologous
chromosomes loosely pair up, aligned
gene by gene

41. Sister chromatids
of one duplicated
chromosome
Key
Maternal set of
chromosomes (n  3)
Paternal set of
chromosomes (n  3)
Key
2n  6
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

54. 1. Leptotene/ Leptonema (leptos= Thin tene= Thread)
• Also called THIN THREADED STAGE
• Volume of nucleas increases.
• Formation of Aster
• Condensation of nuclear chromatin by dehydration and
spiralization .
2. Zygotene/ Zygonema:
• Pairing( k/a Synapsis or Syndesis) of homologous chromosomes
to form Bivalents. It occurs due to forces of attraction between
alleles on Homologous Chromosomes
• Pairing of Homologous chromosomes in Zipper Fashion
No of Bivalents= ½ of total no of chromosomes in a diploid cell.
• Further condensation of chromosomes and moving away of asters.
• Under EM, a filamentous ladder-like nucleoproteinous complex
called Synaptonemal Complex is observed.

56. 3. Pachytene/ Pachynema/ Thick Thread stage:
• Further condensation of chromosome. Sister chromatids
are clearly visible and are joined at centromere and is called
Dyad. Each bivalent has 2 dyad and is called a tetrad(i.e. 4
chromatids).
• Recombination sometimes takes place wherein exchange of
genes or crossing over b/w two non sister chromatids of
Homologous Chromosomes occurs at the points called
Recombination Nodules. This is enzimetically controlled
process and is regulated by recombinase enzyme.
• Moving away of Asters.

57. Crossing Over
• Crossing over produces recombinant
chromosomes, which combine DNA
inherited from each parent
• Crossing over begins very early in
prophase I, as homologous chromosomes
pair up gene by gene

58. •In crossing over, homologous
portions of two nonsister chromatids
trade places
•Crossing over contributes to genetic
variation by combining DNA from two
parents into a single chromosome

59. 4. Diplotene/ Diplonema:
• Nuclear membrane disappears and Nucleoli start
disappearing.
• Desynapsis- Is separation of Homologous
Chromosomes begins due to dissolution of
synaptonemal complex.
• Chismata are Visible & Terminilization.
5. Diakinesis
• Terminilization Completed. But still Homologous
chromosomes are attached at the ends.
•Formation of Spindle & Complete disappearance of
spindles.

61. Metaphase I
• In metaphase I, tetrads line up at the
metaphase plate, with one chromosome
facing each pole
• Microtubules from one pole are attached
to the kinetochore of one chromosome of
each tetrad
• Microtubules from the other pole are
attached to the kinetochore of the other
chromosome

62. Kinetochore
Orientation
Is Random

64. Anaphase I
•In anaphase I, pairs of homologous
chromosomes separate
•One chromosome moves toward each pole,
guided by the spindle apparatus
•Sister chromatids remain attached at the
centromere and move as one unit toward the
pole

65. Telophase I and Cytokinesis
•In the beginning of telophase I, each
half of the cell has a haploid set of
chromosomes; each chromosome still
consists of two sister chromatids
•Cytokinesis usually occurs
simultaneously, forming two haploid
daughter cells

66. •In animal cells, a cleavage furrow forms;
in plant cells, a cell plate forms.
•No chromosome replication occurs
between the end of meiosis I and the
beginning of meiosis II because the
chromosomes are already replicated.

67. Prophase I Metaphase I Anaphase I Telophase I and
Cytokinesis
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Homologous
chromosomes
Fragments
of nuclear
envelope
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Centromere
(with kinetochore)
Metaphase
plate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
Sister chromatids
remain attached
Homologous
chromosomes
separate
Each pair of homologous
chromosomes separates.
Cleavage
furrow
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.

68. Meiosis II
Division in meiosis II also occurs in four
phases
•Prophase II
•Metaphase II
•Anaphase II
•Telophase II and cytokinesis
•Meiosis II is very similar to mitosis

69. Prophase II
•In prophase II, a spindle
apparatus forms
•In late prophase II, chromosomes
(each still composed of two
chromatids) move toward the
metaphase plate

70. Metaphase II
•In metaphase II, the sister chromatids
are arranged at the metaphase plate
•Because of crossing over in meiosis I,
the two sister chromatids of each
chromosome are no longer genetically
identical
•The kinetochores of sister chromatids
attach to microtubules extending from
opposite poles

71. Anaphase II
•In anaphase II, the sister
chromatids separate.
•The sister chromatids of each
chromosome now move as two
newly individual chromosomes
toward opposite poles.

72. Telophase II and Cytokinesis
•In telophase II, the chromosomes
arrive at opposite poles.
•Nuclei form, and the chromosomes
begin decondensing.

73. •Cytokinesis separates the cytoplasm.
•At the end of meiosis, there are four
daughter cells, each with a haploid set
of unreplicated chromosomes.
•Each daughter cell is genetically distinct
from the others and from the parent cell.

75. Prophase II Metaphase II Anaphase II
Telophase II and
Cytokinesis
Sister chromatids
separate
Haploid daughter
cells forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.

76. MEIOSIS I: Separates homologous chromosomes
Prophase I Metaphase I Anaphase I
Telophase I and
Cytokinesis
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Homologous
chromosomes
Fragments
of nuclear
envelope
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Centromere
(with kinetochore)
Metaphase
plate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
Sister chromatids
remain attached
Homologous
chromosomes
separate
Each pair of homologous
chromosomes separates.
Cleavage
furrow
Two haploid cells
form; each chromosome
still consists of two
sister chromatids.
MEIOSIS I: Separates sister chromatids
Prophase II Metaphase II Anaphase II
Telophase II and
Cytokinesis
Sister chromatids
separate
Haploid daughter
cells forming
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.

78. Figure 13.11-5
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes

79. • Three events are unique to meiosis, and all
three occur in meiosis l
• Synapsis and crossing over in prophase I:
Homologous chromosomes physically
connect and exchange genetic information
• At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead
of individual replicated chromosomes
• At anaphase I, it is homologous
chromosomes, instead of sister chromatids,
that separate

80. Figure 13.9
Prophase
Duplicated
chromosome
MITOSIS
Chromosome
duplication
Parent cell
2n  6
Metaphase
Anaphase
Telophase
2n 2n
Daughter cells
of mitosis
MEIOSIS
MEIOSIS I
MEIOSIS II
Prophase I
Metaphase I
Anaphase I
Telophase I
Haploid
n  3
Chiasma
Chromosome
duplication Homologous
chromosome pair
Daughter
cells of
meiosis I
Daughter cells of meiosis II
n n n n
SUMMARY
Property Mitosis Meiosis
DNA
replication
Number of
divisions
Synapsis of
homologous
chromosomes
Number of
daughter cells
and genetic
composition
Role in the
animal body
Occurs during interphase before
mitosis begins
One, including prophase, metaphase,
anaphase, and telophase
Does not occur
Two, each diploid (2n) and genetically
identical to the parent cell
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
Occurs during interphase before meiosis I begins
Two, each including prophase, metaphase, anaphase,
and telophase
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Produces gametes; reduces number of chromosomes
by half and introduces genetic variability among the