1. Introduction

Cell division is one of the fundamental processes of life. It allows organisms to grow, replace damaged cells, reproduce, and maintain genetic continuity.

In eukaryotic organisms, two major forms of nuclear division are mitosis and meiosis.

Mitosis generally produces daughter cells that retain the chromosome number of the parent cell, whereas meiosis is a specialized division that reduces chromosome number and generates genetically diverse reproductive cells.

The basic relationship can be represented as:

Cell growth → DNA replication → Chromosome segregation → Cell division

Although both processes involve chromosome condensation, spindle formation, and chromosome segregation, their biological purposes are different.

Mitosis → Growth, tissue maintenance, cell replacement

Meiosis → Gamete formation, chromosome-number reduction, genetic variation

2. Cell Cycle and Cell Division

Before understanding mitosis and meiosis, it is important to understand the cell cycle.

The cell cycle consists broadly of:

1. G1 phase
2. S phase
3. G2 phase
4. M phase

During G1, the cell grows and performs normal metabolic functions.

During S phase, DNA is replicated.

During G2, the cell prepares for division.

The M phase includes chromosome segregation and cytokinesis.

A simplified sequence is:

G1 → S → G2 → M → G1

Meiosis also begins after a period of DNA replication, but the replicated chromosomes undergo two successive nuclear divisions.

3. DNA Replication Before Division

DNA replication occurs before mitosis and before meiosis.

During S phase, each chromosome is replicated to produce two sister chromatids.

Thus:

One chromosome → DNA replication → two sister chromatids

The sister chromatids remain physically associated until they are separated during chromosome segregation.

A crucial difference is:

• In mitosis, sister chromatids separate during the single chromosome-segregation division.
• In meiosis I, homologous chromosomes separate while sister chromatids remain together.
• In meiosis II, sister chromatids separate.

4. Mitosis: Definition

Mitosis is a form of nuclear division in which duplicated chromosomes are segregated so that each daughter nucleus generally receives one copy of each chromosome.

Mitosis is important for:

• Growth
• Development
• Tissue maintenance
• Cell replacement
• Asexual reproduction in some organisms
• Genome stability

The overall outcome is usually:

One parent cell → Two genetically similar daughter cells

5. Major Stages of Mitosis

Mitosis is conventionally divided into:

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase

Cytokinesis usually follows or overlaps with the final stages of mitosis.

6. Prophase

During prophase:

• Chromatin begins to condense.
• Chromosomes become increasingly visible.
• The mitotic spindle begins to form.
• Centrosomes move apart in animal cells.
• The nucleolus becomes less prominent.

Chromosome condensation is essential because compact chromosomes are easier to move and segregate without becoming excessively entangled.

7. Prometaphase

Prometaphase begins with breakdown of the nuclear envelope.

The spindle microtubules gain access to chromosomes.

Specialized protein structures called kinetochores assemble at centromeric regions.

Spindle microtubules attach to kinetochores.

Thus:

Nuclear-envelope breakdown → Kinetochore exposure → Microtubule attachment

8. Kinetochore Attachment

Each replicated chromosome contains two sister chromatids.

Their kinetochores are positioned so that sister chromatids can attach to spindle microtubules originating from opposite spindle poles.

This arrangement is called bipolar attachment.

Correct bipolar attachment is essential for accurate chromosome segregation.

9. Metaphase

During metaphase, chromosomes become highly organized near the center of the spindle.

They form the metaphase plate.

At this stage, cells use surveillance mechanisms to ensure that chromosomes are appropriately attached to spindle microtubules.

The major regulatory system involved is the spindle assembly checkpoint.

10. Spindle Assembly Checkpoint

The spindle assembly checkpoint prevents premature chromosome segregation.

It monitors whether kinetochores have established appropriate interactions with spindle microtubules.

If chromosomes are not properly attached, the cell delays anaphase.

This provides time for correct spindle attachment to be established.

11. Anaphase

Anaphase begins when sister chromatids separate.

The sister chromatids are then considered individual daughter chromosomes.

They move toward opposite spindle poles.

A simplified sequence is:

Sister chromatid cohesion → Cohesion removal → Chromatid separation → Poleward movement

12. Telophase

During telophase:

• Chromosomes reach opposite poles.
• Chromosomes begin to decondense.
• Nuclear envelopes reform.
• Nucleoli reappear.
• The mitotic spindle disassembles.

Two daughter nuclei are thereby established.

13. Cytokinesis

Cytokinesis is the physical division of the cytoplasm.

In animal cells, cytokinesis occurs through formation of a contractile ring containing:

• Actin
• Myosin

The contractile ring produces a cleavage furrow that divides the cell.

In plant cells, cytokinesis occurs through formation of a cell plate.

14. Outcome of Mitosis

The general outcome of mitosis is:

One parent cell → Two daughter cells

The daughter cells generally maintain the same chromosome number as the parent cell.

For example:

Diploid cell → mitosis → two diploid daughter cells

Mitosis therefore maintains chromosome number across successive cell generations.

15. Biological Importance of Mitosis

Mitosis is essential for:

15.1 Growth

Multicellular organisms increase cell number through repeated cell divisions.

15.2 Tissue Repair

Damaged cells can be replaced through cell proliferation.

15.3 Development

Embryonic development depends on repeated mitotic divisions.

15.4 Tissue Maintenance

Many tissues continuously replace old or damaged cells.

15.5 Asexual Reproduction

Some organisms use mitotic division as part of asexual reproduction.

16. Meiosis: Definition

Meiosis is a specialized form of cell division that reduces chromosome number by half and generates genetically diverse reproductive cells.

Meiosis consists of two successive nuclear divisions:

1. Meiosis I
2. Meiosis II

DNA replication occurs once before meiosis I, but there is no second round of DNA replication between meiosis I and meiosis II.

The overall process can be represented as:

DNA replication → Meiosis I → Meiosis II → Four haploid products

17. Purpose of Meiosis

The major functions of meiosis include:

• Reduction of chromosome number.
• Production of haploid reproductive cells.
• Generation of genetic variation.
• Separation of homologous chromosomes.
• Facilitation of sexual reproduction.

Meiosis therefore connects chromosome inheritance with genetic diversity.

18. Meiosis I

Meiosis I is called the reductional division because homologous chromosomes separate from one another.

The major stages are:

1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I

19. Prophase I

Prophase I is the longest and most complex stage of meiosis.

It is characterized by:

• Homologous chromosome pairing.
• Synapsis.
• Formation of the synaptonemal complex.
• Crossing over.
• Recombination.
• Formation of chiasmata.
• Chromosome condensation.

Prophase I is divided into:

1. Leptotene
2. Zygotene
3. Pachytene
4. Diplotene
5. Diakinesis

20. Leptotene

During leptotene:

• Chromosomes begin to condense.
• Chromosomal axes become visible.
• DNA has already been replicated.
• Homologous chromosomes begin the process of pairing.

Each chromosome consists of two sister chromatids, although these chromatids may not yet be individually distinguishable under the light microscope.

21. Zygotene

During zygotene, homologous chromosomes begin to pair closely.

This process is called synapsis.

The synaptonemal complex develops between homologous chromosomes and facilitates close alignment.

22. Synapsis

Synapsis is the pairing of homologous chromosomes during prophase I.

The paired homologues form a structure known as a bivalent or tetrad.

A tetrad contains:

2 homologous chromosomes × 2 sister chromatids = 4 chromatids

Synapsis is essential for proper meiotic recombination and chromosome segregation.

23. Synaptonemal Complex

The synaptonemal complex is a proteinaceous structure that forms between paired homologous chromosomes.

Its major functions include:

• Promoting homolog alignment.
• Supporting synapsis.
• Facilitating meiotic recombination.
• Organizing chromosome interactions.

The synaptonemal complex is especially prominent during the synapsed stages of prophase I.

24. Pachytene

During pachytene:

• Homologous chromosomes are fully synapsed.
• Crossing-over events are established through meiotic recombination.
• Genetic exchange occurs between non-sister chromatids.

Pachytene is therefore an important stage for generating genetic diversity.

25. Crossing Over

Crossing over is the reciprocal exchange of DNA between homologous chromosomes.

It usually occurs between non-sister chromatids of homologous chromosomes.

The simplified concept is:

Maternal chromosome + paternal chromosome

↓

DNA exchange

↓

Recombinant chromosomes

Crossing over creates new combinations of genetic variants.

26. Chiasmata

The visible manifestations of crossover-associated connections between homologues are called chiasmata.

Chiasmata help maintain physical connections between homologous chromosomes after the synaptonemal complex begins to disassemble.

They are important for:

• Proper homolog alignment.
• Accurate segregation.
• Genetic recombination.

27. Diplotene

During diplotene:

• The synaptonemal complex begins to disassemble.
• Homologous chromosomes begin to separate.
• Chiasmata become clearly visible.

The homologues remain connected at chiasmata.

28. Diakinesis

Diakinesis is the final stage of prophase I.

During this stage:

• Chromosomes become highly condensed.
• Chiasmata move toward chromosome ends in a process called terminalization.
• The nuclear envelope breaks down.
• Spindle formation progresses.

The cell is now prepared for metaphase I.

29. Metaphase I

During metaphase I, homologous chromosome pairs align at the metaphase plate.

The orientation of each homologous pair is random.

This phenomenon contributes to independent assortment.

Maternal and paternal homologues can therefore be distributed into daughter cells in different combinations.

30. Independent Assortment

Independent assortment refers to the random orientation and segregation of homologous chromosome pairs during meiosis I.

For a species with multiple chromosome pairs, different combinations of maternal and paternal chromosomes can enter gametes.

Independent assortment is therefore an important source of genetic variation.

31. Anaphase I

During anaphase I:

• Homologous chromosomes separate.
• Sister chromatids remain associated.
• Homologous chromosomes move toward opposite poles.

This is the major chromosome-number-reducing step of meiosis.

The key principle is:

Homologues separate, sister chromatids remain together.

32. Telophase I

During telophase I:

• Homologous chromosomes reach opposite poles.
• Nuclear envelopes may reform depending on organism and cell type.
• Chromosomes may partially decondense.

Cytokinesis can occur.

The result is two cells containing one chromosome from each homologous pair.

33. Interkinesis

The period between meiosis I and meiosis II is sometimes called interkinesis.

An important feature is:

No DNA replication occurs during interkinesis.

The chromosomes remain replicated and proceed directly toward meiosis II.

34. Meiosis II

Meiosis II resembles mitosis in several respects because sister chromatids are separated.

Its stages are:

1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II

35. Prophase II

During prophase II:

• Chromosomes condense if necessary.
• Spindle formation occurs.
• Nuclear envelopes, if present, break down.
• Kinetochores interact with spindle microtubules.

36. Metaphase II

Chromosomes align individually at the metaphase plate.

Each chromosome contains two sister chromatids.

The kinetochores of sister chromatids become attached to opposite spindle poles.

37. Anaphase II

During anaphase II:

• Sister chromatids separate.
• Each chromatid becomes an independent chromosome.
• Chromosomes move toward opposite poles.

Thus, meiosis II completes the separation of sister chromatids.

38. Telophase II

During telophase II:

• Chromosomes reach the poles.
• Nuclear envelopes reform.
• Chromosomes decondense.
• Cytokinesis occurs.

The final result is generally:

Four haploid cells

39. Outcome of Meiosis

The general sequence is:

One diploid cell

↓

DNA replication

↓

Meiosis I

↓

Two haploid cells with replicated chromosomes

↓

Meiosis II

↓

Four haploid cells

These products are genetically different because of crossing over and independent assortment.

40. Mitosis and Meiosis: Major Comparison

Feature	Mitosis	Meiosis
Number of divisions	One	Two
DNA replication	One round before division	One round before meiosis I
Daughter cells	Usually 2	Usually 4
Chromosome number	Generally maintained	Reduced by half
Homolog pairing	Absent	Present
Crossing over	Not a normal feature	Characteristic of prophase I
Genetic similarity	Generally similar	Genetically diverse
Homolog separation	Not the defining event	Occurs in meiosis I
Sister chromatid separation	Anaphase	Meiosis II
Major function	Growth and maintenance	Sexual reproduction and variation

41. Regulation of Cell Division

Cell division must be tightly regulated to prevent:

• DNA damage transmission.
• Incorrect chromosome segregation.
• Excessive cell proliferation.
• Chromosome-number abnormalities.

Major regulators include:

• Cyclins
• Cyclin-dependent kinases
• APC/C
• Spindle assembly checkpoint
• Cohesin
• Separase
• Condensin
• DNA-damage checkpoints

42. Cyclins and Cyclin-Dependent Kinases

Cyclin-dependent kinases (CDKs) are protein kinases that regulate progression through the cell cycle.

CDKs become active when associated with specific cyclins.

Major examples include:

• Cyclin D–CDK4/6
• Cyclin E–CDK2
• Cyclin A–CDK2
• Cyclin A/B–CDK1

The precise combinations and timing differ among organisms and cell types.

43. CDK1 and Entry into Mitosis

CDK1 is a central regulator of entry into mitosis.

Its association with mitotic cyclins promotes processes such as:

• Chromosome condensation.
• Nuclear-envelope breakdown.
• Spindle formation.
• Reorganization of cellular structures.

The activity of CDK1 is controlled through phosphorylation and dephosphorylation.

44. Cyclin B–CDK1 Complex

The Cyclin B–CDK1 complex is a major driver of mitotic entry.

Its activation triggers the transition from G2 phase to M phase.

When Cyclin B–CDK1 activity rises:

Chromosome condensation + spindle assembly + nuclear-envelope breakdown

are promoted.

45. Regulation of CDK Activity

CDKs are regulated by:

• Cyclin binding
• Activating phosphorylation
• Inhibitory phosphorylation
• CDK inhibitors
• Protein degradation

This provides multiple levels of control over cell-cycle progression.

46. CDK Inhibitors

CDK inhibitors prevent inappropriate CDK activity.

Important families include:

• INK4 proteins
• CIP/KIP proteins

These proteins can slow or stop cell-cycle progression under conditions such as:

• DNA damage
• Differentiation
• Growth-factor withdrawal
• Cellular stress

47. DNA Damage Checkpoint

Before a cell enters or progresses through division, it must ensure that its DNA is sufficiently intact.

DNA damage activates signaling pathways involving proteins such as:

• ATM
• ATR
• CHK1
• CHK2
• p53

These pathways can delay cell-cycle progression and promote DNA repair.

48. p53 and Cell-Cycle Control

p53 is an important tumor-suppressor protein involved in cellular responses to DNA damage.

When DNA damage is detected, p53 can promote expression of cell-cycle inhibitory proteins such as p21.

The resulting CDK inhibition can temporarily stop cell-cycle progression.

If damage is severe, p53-dependent pathways may also contribute to cellular senescence or apoptosis.

49. Spindle Assembly Checkpoint Regulation

The spindle assembly checkpoint prevents the cell from entering anaphase until chromosomes are correctly attached to the spindle.

Important checkpoint components include:

• MAD proteins
• BUB proteins
• Kinetochore-associated factors

The checkpoint inhibits premature activation of the machinery that triggers sister chromatid separation.

50. APC/C

The anaphase-promoting complex/cyclosome (APC/C) is a large ubiquitin ligase complex that promotes the destruction of specific cell-cycle proteins.

APC/C activation contributes to:

• Anaphase initiation
• Cyclin degradation
• Mitotic exit

It is therefore a major regulator of the transition from metaphase to anaphase and subsequent exit from mitosis.

51. Securin and Separase

Before anaphase, the enzyme separase is kept inactive by a protein called securin.

When the APC/C becomes appropriately activated:

Securin degradation → Separase activation → Cohesin cleavage → Sister chromatid separation

This is one of the central molecular switches controlling anaphase.

52. Cohesin

Cohesin is a protein complex that holds sister chromatids together after DNA replication.

It is essential for:

• Sister chromatid cohesion.
• Chromosome organization.
• Accurate segregation.

In mitosis, cohesin is removed from chromosome arms earlier, while centromeric cohesion is protected until anaphase.

53. Separase

Separase is a protease that cleaves a key cohesin component.

Its activation results in loss of sister chromatid cohesion.

Thus:

Separase activation → Cohesin cleavage → Chromatid separation

This mechanism ensures that sister chromatids remain together until the correct stage of division.

54. Shugoshin and Centromeric Protection

During meiosis I, sister chromatids must remain together while homologous chromosomes separate.

A protein system involving shugoshin protects centromeric cohesin from premature removal.

This is a crucial difference between meiosis I and mitosis.

55. Regulation of Meiosis I

Meiosis I requires mechanisms that are different from ordinary mitosis.

During meiosis I:

• Homologous chromosomes pair.
• Recombination occurs.
• Cohesin is protected at centromeres.
• Homologues separate.
• Sister chromatids remain together.

Thus, meiosis I requires specialized regulation of chromosome cohesion and spindle attachment.

56. Meiotic Recombination Regulation

Meiotic recombination begins with programmed DNA double-strand breaks.

These breaks are generated by specialized meiotic machinery, including the protein Spo11.

The breaks are repaired through homologous chromosome interactions, producing either:

• Crossovers
• Non-crossovers

Crossovers create chiasmata that contribute to proper homolog segregation.

57. Meiotic Double-Strand Breaks

Programmed DNA double-strand breaks may initially appear harmful, but they are essential for normal meiotic recombination.

The general sequence is:

Spo11-mediated break

↓

DNA-end processing

↓

Homology search

↓

Strand invasion

↓

DNA synthesis and repair

↓

Crossover or non-crossover product

58. Crossover Formation

Crossover formation is tightly regulated because too few or improperly positioned crossovers can interfere with chromosome segregation.

Crossovers are generally distributed along chromosome arms rather than occurring randomly without constraints.

Their number and distribution are influenced by:

• Chromosome structure
• Recombination machinery
• Cellular regulation
• Chromatin environment

59. Regulation of Meiosis II

Meiosis II is more similar to mitosis than meiosis I.

During meiosis II:

• Chromosomes align individually.
• Sister kinetochores become attached to opposite spindle poles.
• Centromeric cohesion is removed.
• Sister chromatids separate.

Thus, meiosis II completes the reduction process initiated in meiosis I.

60. Differences in Chromosome Behavior

A central distinction between mitosis and meiosis is the behavior of homologous chromosomes.

Mitosis

Sister chromatids separate

Meiosis I

Homologous chromosomes separate

Meiosis II

Sister chromatids separate

This can be summarized as:

Mitosis → sister separation

Meiosis I → homolog separation

Meiosis II → sister separation

61. Regulation of Mitosis vs Meiosis

Regulatory feature	Mitosis	Meiosis
DNA replication	Once	Once before meiosis I
Number of divisions	One	Two
Homolog pairing	No	Yes
Recombination	Not characteristic	Essential feature
Centromeric cohesion	Removed at anaphase	Protected during meiosis I
APC/C regulation	Controls mitotic transitions	Controls meiotic transitions
Separase	Separates sister chromatids	Functions at different stages to separate homolog-associated and sister cohesion
Spindle attachment	Sister kinetochores orient oppositely	Meiosis I uses homolog-oriented attachment

62. Checkpoints in Cell Division

Cell-cycle checkpoints help prevent errors.

Major checkpoints include:

1. G1/S checkpoint
2. G2/M checkpoint
3. Spindle assembly checkpoint

These checkpoints assess:

• DNA integrity
• DNA replication
• Chromosome attachment
• Cellular conditions

63. G1/S Checkpoint

The G1/S checkpoint determines whether the cell is ready to enter DNA replication.

It responds to:

• Growth signals
• Nutrient availability
• DNA damage
• Cellular stress

If conditions are unfavorable, cell-cycle progression can be delayed.

64. G2/M Checkpoint

The G2/M checkpoint ensures that DNA replication has been completed properly before the cell enters mitosis.

If DNA remains damaged or incompletely replicated, entry into mitosis can be delayed.

65. Spindle Assembly Checkpoint

The spindle assembly checkpoint operates during mitosis and meiosis.

It prevents chromosome separation until appropriate kinetochore-spindle attachments are established.

This checkpoint is essential for preventing chromosome missegregation.

66. Chromosome Missegregation

Failure of accurate chromosome segregation can produce cells with abnormal chromosome numbers.

This condition is known as aneuploidy.

Aneuploidy can arise through:

• Nondisjunction
• Improper kinetochore attachment
• Checkpoint failure
• Cohesion defects

Such abnormalities can have major biological consequences.

67. Nondisjunction

Nondisjunction occurs when chromosomes fail to separate properly during cell division.

It can involve:

• Homologous chromosomes during meiosis I.
• Sister chromatids during meiosis II.
• Sister chromatids during mitosis.

Nondisjunction can produce daughter cells with abnormal chromosome numbers.

68. Mitosis and Genetic Stability

Accurate mitosis ensures that daughter cells receive appropriate chromosome complements.

This depends on:

• Accurate DNA replication.
• Correct spindle formation.
• Proper kinetochore attachment.
• Functional checkpoints.
• Controlled cohesin removal.

Defects in these mechanisms can lead to genomic instability.

69. Meiosis and Genetic Variation

Meiosis generates genetic diversity through at least two major mechanisms:

69.1 Crossing Over

DNA exchange between homologous chromosomes creates recombinant chromosomes.

69.2 Independent Assortment

Homologous chromosomes segregate into gametes in different combinations.

These processes create genetically diverse reproductive cells.

70. Meiosis in Spermatogenesis

In males of many animal species, meiosis occurs during spermatogenesis.

A simplified sequence is:

Spermatogonium → Primary spermatocyte → Meiosis I → Secondary spermatocytes → Meiosis II → Spermatids

The spermatids subsequently differentiate into sperm cells.

71. Meiosis in Oogenesis

In females of many animal species, meiosis occurs during oogenesis.

A simplified sequence is:

Oogonium → Primary oocyte → Meiosis I → Secondary oocyte + polar body → Meiosis II

The details of meiotic timing and completion differ substantially between species.

In humans, for example, oocytes begin meiosis during fetal development and progress through specific arrest points.

72. Regulation of Meiotic Arrest

Oocytes can remain arrested at particular stages of meiosis for extended periods.

The maintenance and release of these arrests involve complex signaling networks involving:

• Cyclins
• CDKs
• Protein kinases
• Phosphatases
• Calcium signaling

This specialized regulation distinguishes oocyte meiosis from the more continuously proliferative divisions of many somatic cells.

73. Mitotic Exit

Mitotic exit requires coordinated reduction of mitotic CDK activity.

A major mechanism is degradation of mitotic cyclins by APC/C-dependent ubiquitination.

This allows:

• Chromosome decondensation.
• Nuclear-envelope reformation.
• Spindle disassembly.
• Cytokinesis.
• Return to interphase organization.

74. Meiotic Exit

Meiotic progression also depends on regulated changes in CDK activity and protein degradation.

The two meiotic divisions must be coordinated so that:

One DNA replication event → Two chromosome segregation events

This unusual cell-cycle architecture is a defining characteristic of meiosis.

75. Mitosis and Meiosis in the Life Cycle

Mitosis primarily maintains chromosome number during organismal growth and tissue development.

Meiosis reduces chromosome number before sexual reproduction.

Fertilization subsequently restores the diploid chromosome complement.

The cycle can be represented as:

Diploid organism

↓

Meiosis

↓

Haploid gametes

↓

Fertilization

↓

Diploid zygote

↓

Mitosis

↓

Multicellular organism

76. Evolutionary Significance of Meiosis

Meiosis provides an important mechanism for generating genetic variation.

Through:

• Recombination
• Independent assortment
• Random combination of gametes during fertilization

meiosis contributes to genetic diversity within populations.

This variation provides raw material on which evolutionary processes can act.

77. Functional Significance of Mitosis

Mitosis provides genetic continuity within multicellular organisms.

It allows:

• Embryonic development.
• Tissue growth.
• Tissue repair.
• Cell replacement.
• Maintenance of cell populations.

Its high accuracy is essential because errors can be propagated to many descendant cells.

78. Mitosis, Meiosis and Chromatin

Chromatin must undergo dramatic structural changes during cell division.

During mitosis and meiosis:

• Chromosomes condense.
• Histone modifications change.
• Chromatin-associated proteins reorganize.
• Spindle attachment sites become accessible.
• Chromosome architecture becomes specialized for segregation.

Thus, chromosome structure and cell-cycle regulation are tightly interconnected.

79. Conceptual Flowchart of Mitosis

G1

↓

DNA replication

↓

G2

↓

Prophase

↓

Prometaphase

↓

Metaphase

↓

Anaphase

↓

Telophase

↓

Cytokinesis

↓

Two daughter cells

80. Conceptual Flowchart of Meiosis

DNA replication

↓

Prophase I

↓

Synapsis + Recombination

↓

Metaphase I

↓

Homolog separation

↓

Telophase I

↓

Meiosis II

↓

Sister chromatid separation

↓

Four haploid products