11bio10

Cell Cycle and Cell Division

Explore the fundamental processes of cell growth, DNA replication, and division that enable life to continue from one generation to the next.

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Chapter Overview

Brief Introduction

All organisms begin life as a single cell, which grows and divides to form complex structures. This chapter explores the cell cycle - the sequence of events by which a cell duplicates its genome, synthesizes cellular components, and divides into two daughter cells. Understanding these processes is crucial for comprehending growth, development, and reproduction in living organisms.

Learning Objectives

• Understand the phases and significance of the cell cycle
• Learn about mitosis and its stages
• Comprehend meiosis and its importance in sexual reproduction
• Differentiate between mitosis and meiosis
• Explore the regulation of cell cycle and its importance

Key Topics Covered

• Phases of Cell Cycle (Interphase and M Phase)
• Mitosis and its stages (Prophase to Telophase)
• Cytokinesis in plant and animal cells
• Significance of Mitosis
• Meiosis I and Meiosis II
• Prophase I stages (Leptotene to Diakinesis)
• Significance of Meiosis
• Differences between mitosis and meiosis

Interactive Chapter Index

Cell Cycle

Explore the sequence of events from cell formation to its division into daughter cells.

Mitosis

Learn about the equational division process that produces genetically identical daughter cells.

Meiosis

Understand the reduction division that produces haploid gametes for sexual reproduction.

Significance

Discover the importance of mitosis and meiosis in growth, repair, and genetic diversity.

Key Differences

Compare and contrast mitosis and meiosis in terms of process and outcomes.

Chapter Summary

Review the key concepts and takeaways from this chapter.

Full Chapter Notes

10.1 Cell Cycle

The cell cycle is the sequence of events by which a cell duplicates its genome, synthesizes other cellular components, and eventually divides into two daughter cells. It is essential for growth, development, and reproduction of organisms.

Key Features of Cell Cycle:

• Ensures accurate division and formation of progeny cells with intact genomes
• DNA synthesis occurs only during one specific stage (S phase)
• Cell growth (cytoplasmic increase) is continuous
• Replicated chromosomes are distributed to daughter nuclei during division
• All processes are under genetic control

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.

10.1.1 Phases of Cell Cycle

The cell cycle is divided into two main phases: Interphase and M Phase (Mitosis phase). In human cells with a 24-hour cycle, cell division lasts about 1 hour while interphase lasts more than 95% of the cycle.

Figure: Phases of the Cell Cycle

Interphase: The phase between two successive M phases where the cell prepares for division through growth and DNA replication. Divided into:

• G₁ phase (Gap 1): Cell grows and carries out normal metabolism
• S phase (Synthesis): DNA replication occurs (2C → 4C)
• G₂ phase (Gap 2): Proteins are synthesized for mitosis

Q: A cell in onion root tip has 16 chromosomes at G₁ phase. What will be its chromosome number at S phase and after M phase?

A: The chromosome numbers will be:

• At G₁ phase: 16 chromosomes (2C DNA content)
• After S phase: 16 chromosomes (DNA content doubles to 4C but chromosome number remains same)
• After M phase: 16 chromosomes (each daughter cell gets identical chromosome number as parent cell)

G₀ Phase (Quiescent Stage)

Some cells in adult animals exit the G₁ phase to enter an inactive stage called G₀ where they remain metabolically active but do not proliferate unless required. Examples include heart cells and neurons.

10.2 M Phase (Mitosis)

Mitosis is the most dramatic period of the cell cycle, involving major reorganization of cellular components. It is called equational division because the chromosome number in parent and progeny cells remains the same.

Stages of Mitosis (Karyokinesis):

1. Prophase: Chromosomes condense, nucleolus disappears, mitotic spindle forms
2. Metaphase: Chromosomes align at equator, spindle fibers attach to kinetochores
3. Anaphase: Centromeres split, chromatids separate and move to opposite poles
4. Telophase: Chromosomes decondense, nuclear envelope reforms

Figure: Stages of Mitosis

10.2.5 Cytokinesis

Cytokinesis is the division of cytoplasm that completes cell division. It differs between animal and plant cells:

Feature	Animal Cells	Plant Cells
Process	Cleavage furrow forms and deepens	Cell plate forms from center outward
Mechanism	Actin-myosin contractile ring	Vesicle fusion forms cell plate
Result	Two separate daughter cells	New cell wall forms between cells

Q: Why is cytokinesis different in plant and animal cells?

A: The differences exist because:

• Animal cells: Have flexible plasma membranes that can form a cleavage furrow through contraction of actin filaments
• Plant cells: Have rigid cell walls that prevent furrowing, so they form a cell plate from vesicles that fuse at the equator to create new cell walls
• The presence of chloroplasts and large vacuoles in plant cells also influences the cytokinesis mechanism

10.3 Significance of Mitosis

Mitosis plays crucial roles in growth, maintenance, and repair of organisms:

Key Significance:

• Growth: Enables multicellular organisms to grow from a single cell
• Maintenance: Maintains nucleo-cytoplasmic ratio
• Cell Repair: Replaces damaged or dead cells (e.g., skin, gut lining)
• Genetic Stability: Produces genetically identical daughter cells
• Vegetative Reproduction: Enables asexual reproduction in plants
• Regeneration: Allows regeneration of lost body parts in some organisms

Note: While mitosis is typically restricted to diploid cells, some exceptions exist:

• Haploid cells in male honey bees divide by mitosis
• Plants can show mitotic divisions in both haploid and diploid cells

10.4 Meiosis

Meiosis is a specialized cell division that reduces chromosome number by half, producing haploid gametes. It involves two sequential divisions (Meiosis I and II) but only one DNA replication.

Meiosis: A type of cell division that reduces the chromosome number by half, resulting in the production of haploid daughter cells. Essential for sexual reproduction.

Key Features of Meiosis:

• Involves two divisions (Meiosis I and II) but one DNA replication
• Homologous chromosomes pair and undergo recombination
• Produces four genetically distinct haploid cells
• Essential for sexual reproduction and genetic diversity

10.4.1 Meiosis I

Meiosis I is the reductional division where homologous chromosomes separate:

Figure: Stages of Meiosis I

Prophase I (Longest and Most Complex Phase):

1. Leptotene: Chromosomes start condensing
2. Zygotene: Synapsis of homologous chromosomes forms bivalents
3. Pachytene: Crossing over occurs between non-sister chromatids
4. Diplotene: Synaptonemal complex dissolves, chiasmata visible
5. Diakinesis: Terminalization of chiasmata, spindle formation

Key Terms:

• Synapsis: Pairing of homologous chromosomes
• Bivalent: Pair of synapsed homologous chromosomes (tetrad)
• Chiasmata: X-shaped points where crossing over occurs

Metaphase I to Telophase I:

• Metaphase I: Bivalents align at equator with spindle fibers attached
• Anaphase I: Homologous chromosomes separate (sister chromatids remain attached)
• Telophase I: Nuclear membrane may reform, cytokinesis occurs forming two haploid cells

10.4.2 Meiosis II

Meiosis II is similar to mitosis but occurs in haploid cells:

Figure: Stages of Meiosis II

Stages of Meiosis II:

• Prophase II: Chromosomes condense, spindle forms
• Metaphase II: Chromosomes align at equator
• Anaphase II: Centromeres divide, sister chromatids separate
• Telophase II: Nuclear envelope reforms, four haploid cells result

Q: What is the significance of crossing over during Prophase I of meiosis?

A: Crossing over is significant because:

• Creates new combinations of genes on chromosomes
• Increases genetic variation in offspring
• Helps in proper segregation of homologous chromosomes
• Provides raw material for evolution through genetic recombination
• Ensures genetic uniqueness of each gamete

10.5 Significance of Meiosis

Meiosis is crucial for sexual reproduction and maintaining genetic diversity:

Key Significance:

• Chromosome Number Maintenance: Reduces diploid to haploid for sexual reproduction
• Genetic Diversity: Through independent assortment and crossing over
• Evolution: Provides variation for natural selection to act upon
• Repair: Can help eliminate harmful mutations through recombination
• Life Cycle: Essential for alternation of generations in plants

Key Differences: Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Number of divisions	One	Two (Meiosis I and II)
DNA replication	Once per division	Once for two divisions
Synapsis	Does not occur	Occurs in Prophase I
Crossing over	Rare	Regular occurrence in Prophase I
Chromosome number	Maintained (2n → 2n)	Reduced by half (2n → n)
Daughter cells	Two, genetically identical	Four, genetically different
Function	Growth, repair, asexual reproduction	Gamete formation, sexual reproduction

Q: How can you distinguish between anaphase of mitosis and anaphase I of meiosis?

A: Key differences:

• Anaphase of Mitosis:
  • Sister chromatids separate
  • Centromeres divide
  • Single chromatids move to poles
• Anaphase I of Meiosis:
  • Homologous chromosomes separate
  • Centromeres do not divide
  • Whole chromosomes (with two chromatids) move to poles

Chapter Summary

Key Takeaways:

• The cell cycle consists of interphase (G₁, S, G₂) and M phase (mitosis)
• Mitosis produces two genetically identical diploid cells through prophase, metaphase, anaphase, and telophase
• Cytokinesis differs in plant (cell plate) and animal (cleavage furrow) cells
• Meiosis reduces chromosome number by half through two divisions (Meiosis I and II)
• Prophase I of meiosis is complex with synapsis and crossing over
• Mitosis maintains genetic stability while meiosis creates genetic diversity
• Both processes are essential for growth, development, and reproduction

NCERT Solutions

Question 1: What is the average cell cycle span for a mammalian cell?

The average cell cycle span for a mammalian cell is approximately 24 hours. However, this duration can vary:

• Human cells in culture typically divide once every 24 hours
• Yeast cells can complete the cell cycle in about 90 minutes
• The duration varies between organisms and cell types
• In the 24-hour cycle, M phase lasts about 1 hour while interphase takes more than 95% of the time

Question 2: Distinguish cytokinesis from karyokinesis.

Karyokinesis	Cytokinesis
Division of the nucleus	Division of the cytoplasm
Occurs during M phase	Follows karyokinesis
Involves chromosome separation	Involves cytoplasmic division
Same in plant and animal cells	Differs between plant and animal cells
Includes prophase, metaphase, anaphase, telophase	Involves cleavage furrow or cell plate formation

Question 3: Describe the events taking place during interphase.

Interphase is the phase between two successive M phases where the cell prepares for division. It consists of three sub-phases:

1. G₁ phase (Gap 1):
   • Cell is metabolically active and grows
   • Carries out normal cellular functions
   • No DNA replication occurs
   • Most organelles duplicate during this phase
2. S phase (Synthesis):
   • DNA replication occurs
   • Amount of DNA per cell doubles (2C → 4C)
   • Chromosome number remains the same
   • In animal cells, centriole duplicates in cytoplasm
3. G₂ phase (Gap 2):
   • Proteins required for mitosis are synthesized
   • Cell growth continues
   • Cell prepares for division
   • Final checkpoint before entering M phase

Question 4: What is G₀ (quiescent phase) of cell cycle?

The G₀ phase (quiescent stage) is an inactive stage that cells may enter from the G₁ phase of the cell cycle. Key features:

• Cells in G₀ do not proliferate but remain metabolically active
• They have exited the active cell cycle
• May re-enter the cell cycle if required by the organism
• Examples include:
  • Heart cells (cardiomyocytes)
  • Neurons
  • Mature muscle cells
• Some cells in G₀ divide only occasionally to replace lost cells (e.g., liver cells)
• Represents a state of cell cycle arrest

Question 5: Why is mitosis called equational division?

Mitosis is called equational division because:

• The chromosome number in the daughter cells remains equal to that of the parent cell
• Diploid parent cell (2n) produces two diploid daughter cells (2n)
• There is no reduction in chromosome number
• Sister chromatids separate during anaphase, ensuring each daughter cell receives an identical set of chromosomes
• Maintains genetic stability across cell generations
• Contrasts with meiosis (reductional division) where chromosome number is halved

Question 6: Name the stage of cell cycle at which one of the following events occur:

1. Chromosomes are moved to spindle equator: Metaphase (both mitosis and meiosis II) or Metaphase I (in meiosis)
2. Centromere splits and chromatids separate: Anaphase (mitosis and meiosis II)
3. Pairing between homologous chromosomes takes place: Zygotene stage of Prophase I (meiosis)
4. Crossing over between homologous chromosomes takes place: Pachytene stage of Prophase I (meiosis)

Question 7: Describe the following: (a) synapsis (b) bivalent (c) chiasmata. Draw a diagram to illustrate your answer.

(a) Synapsis:

• The pairing of homologous chromosomes during meiosis
• Occurs during zygotene stage of Prophase I
• Facilitated by the formation of synaptonemal complex
• Brings homologous chromosomes in close apposition
• Essential for crossing over

(b) Bivalent:

• A pair of synapsed homologous chromosomes
• Also called a tetrad (as it contains four chromatids)
• Formed during zygotene and visible during pachytene
• Site where crossing over occurs
• Held together by chiasmata in diplotene

(c) Chiasmata:

• X-shaped structures where homologous chromosomes remain attached after synaptonemal complex dissolves
• Represent sites where crossing over has occurred
• Visible during diplotene stage of Prophase I
• Help hold homologous chromosomes together until anaphase I
• Undergo terminalization (move toward chromosome ends) during diakinesis

Figure: Synapsis forms bivalents, and chiasmata are visible at crossing over sites

Question 8: How does cytokinesis in plant cells differ from that in animal cells?

Feature	Animal Cells	Plant Cells
Process	Cleavage furrow forms by constriction of actin filaments	Cell plate forms from fusion of Golgi vesicles at equator
Mechanism	Contractile ring of actin and myosin filaments	Vesicle fusion forms new cell wall material
Structure involved	Actin-myosin contractile ring	Phragmoplast (microtubule structure guiding vesicles)
Result	Two separate cells with complete membranes	New cell wall divides the parent cell into two
Reason for difference	Flexible plasma membrane allows furrowing	Rigid cell wall prevents furrowing, requires new wall formation

Question 9: Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Equal-sized daughter cells:

• Spermatogenesis in males produces four equal-sized sperm cells
• Microsporogenesis in plants produces four equal pollen grains
• Meiosis in many fungi and algae produces four equal spores

Unequal-sized daughter cells:

• Oogenesis in females produces one large ovum and three small polar bodies
• Megasporogenesis in plants produces one functional megaspore and three degenerating ones

The size difference occurs due to unequal cytoplasmic division during meiosis, where one cell retains most of the cytoplasm while others get minimal cytoplasm.

Question 10: Distinguish anaphase of mitosis from anaphase I of meiosis.

Feature	Anaphase of Mitosis	Anaphase I of Meiosis
Separation occurs between	Sister chromatids	Homologous chromosomes
Centromere behavior	Centromeres divide	Centromeres do not divide
Moving units	Single chromatids move to poles	Whole chromosomes (with two chromatids) move to poles
Genetic content	Identical genetic material to each pole	Different genetic material to each pole (due to crossing over)
Result	Maintains chromosome number	Reduces chromosome number by half

Question 11: List the main differences between mitosis and meiosis.

Feature	Mitosis	Meiosis
Number of divisions	One	Two (Meiosis I and II)
DNA replication	Once per division	Once for two divisions
Synapsis	Does not occur	Occurs in Prophase I
Crossing over	Rare	Regular occurrence in Prophase I
Chromosome number	Maintained (2n → 2n)	Reduced by half (2n → n)
Daughter cells	Two, genetically identical	Four, genetically different
Function	Growth, repair, asexual reproduction	Gamete formation, sexual reproduction
Prophase	Simple, shorter	Complex, longer (5 substages in Prophase I)
Anaphase	Sister chromatids separate	Homologous chromosomes separate (Anaphase I), sister chromatids separate (Anaphase II)

Question 12: What is the significance of meiosis?

Meiosis is significant because:

1. Chromosome Number Maintenance:
   • Reduces diploid number (2n) to haploid (n) in gametes
   • Ensures fertilization restores diploid number
   • Maintains constant chromosome number across generations
2. Genetic Diversity:
   • Independent assortment produces new combinations of chromosomes
   • Crossing over creates new combinations of genes on chromosomes
   • Random fertilization adds further variation
3. Evolution:
   • Provides genetic variation for natural selection to act upon
   • Allows adaptation to changing environments
   • Drives evolutionary change
4. Repair:
   • Can help eliminate harmful mutations through recombination
   • Allows masking of deleterious recessive alleles
5. Life Cycle:
   • Essential for alternation of generations in plants
   • Enables sexual reproduction in eukaryotes

Question 16: Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell (ii) amount of DNA content (C) per cell

Stage	Chromosome Number (N)	DNA Content (C)
G₁ phase	2n (diploid)	2C
S phase	2n (DNA replicates but chromosome number doesn't change)	2C → 4C (doubles)
G₂ phase	2n	4C
Mitosis (M phase)	2n → 2n (remains same in daughter cells)	4C → 2C (halved in each daughter cell)
Meiosis I	2n → n (reduced by half)	4C → 2C (halved)
Meiosis II	n → n (remains haploid)	2C → C (halved again)

Key Points:

• Chromosome number changes only during meiosis I (reductional division)
• DNA content changes during S phase (replication) and during each division (halving)
• Mitosis maintains ploidy level while meiosis reduces it
• In meiosis, two divisions halve both chromosome number and DNA content

Practice Questions

1. Which phase of the cell cycle is the longest in most cells?

a) M phase b) G₁ phase c) S phase d) G₂ phase

Correct Answer: b) G₁ phase

Explanation: The G₁ phase is typically the longest phase of the cell cycle in most cells. In a standard 24-hour cell cycle of human cells, G₁ may last about 11 hours, while S phase takes about 8 hours, G₂ about 4 hours, and M phase only about 1 hour.

2. Crossing over occurs during which stage of meiosis?

a) Leptotene b) Zygotene c) Pachytene d) Diplotene

Correct Answer: c) Pachytene

Explanation: Crossing over occurs during the pachytene stage of Prophase I in meiosis. This is when homologous chromosomes are fully paired (synapsed) and recombination nodules facilitate the exchange of genetic material between non-sister chromatids.

3. The stage of mitosis where chromosomes align at the equator is called:

a) Prophase b) Metaphase c) Anaphase d) Telophase

Correct Answer: b) Metaphase

Explanation: Metaphase is characterized by the alignment of chromosomes at the metaphase plate (equator) of the cell, with spindle fibers attached to their kinetochores. This stage is ideal for studying chromosome morphology.

1. What is the significance of chiasmata in meiosis?

Answer:

Chiasmata are significant in meiosis because:

1. They represent the physical manifestation of crossing over between homologous chromosomes
2. Help hold homologous chromosomes together until anaphase I
3. Ensure proper segregation of homologous chromosomes
4. Contribute to genetic recombination and variation
5. Provide mechanical stability to bivalents during meiosis I

2. Differentiate between cytokinesis in plant and animal cells.

Answer:

Animal Cells:

• Form a cleavage furrow that constricts the cell
• Involves actin-myosin contractile ring
• Plasma membrane pinches inward

Plant Cells:

• Form a cell plate that grows outward to meet the cell wall
• Involves fusion of Golgi-derived vesicles
• New cell wall material is deposited
• Phragmoplast guides vesicle movement

The differences exist because plant cells have rigid cell walls that prevent furrowing.

1. Describe the various stages of Prophase I of meiosis with suitable diagrams.

Answer:

Prophase I of meiosis is the longest and most complex phase, divided into five substages:

1. Leptotene:

• Chromosomes start condensing and become visible
• Chromosome compaction continues throughout this stage
• Chromosomes appear as long, thin threads

2. Zygotene:

• Homologous chromosomes begin pairing (synapsis)
• Synaptonemal complex forms between homologs
• Paired chromosomes are called bivalents or tetrads

3. Pachytene:

• Chromosomes fully paired and appear as tetrads
• Crossing over occurs between non-sister chromatids
• Recombination nodules mark crossover sites
• Chiasmata become visible by end of this stage

4. Diplotene:

• Synaptonemal complex dissolves
• Homologs begin to separate but remain attached at chiasmata
• Chromosomes may decondense slightly
• In some organisms (e.g., human oocytes), this stage can last for years

5. Diakinesis:

• Chromosomes fully condense again
• Chiasmata undergo terminalization (move toward ends)
• Nuclear envelope breaks down
• Spindle fibers begin to form
• Transition to metaphase I

Figure: The five substages of Prophase I of meiosis

2. Explain how mitosis maintains genetic stability while meiosis promotes genetic diversity.

Answer:

Mitosis maintains genetic stability because:

1. It produces two genetically identical daughter cells
2. DNA replicates precisely during S phase
3. Sister chromatids separate equally during anaphase
4. No recombination occurs between homologous chromosomes
5. Chromosome number remains constant (2n → 2n)
6. Essential for growth and repair where identical cells are needed

Meiosis promotes genetic diversity through:

1. Crossing over: Exchange of genetic material between non-sister chromatids during Prophase I creates new gene combinations
2. Independent assortment: Random orientation of homologous chromosome pairs during Metaphase I leads to 2²³ possible combinations in humans
3. Random fertilization: Any sperm can fuse with any egg, further increasing variation
4. Reduction division: Halving chromosome number allows for restoration during fertilization, preventing doubling each generation

These differences make mitosis ideal for growth and repair (where genetic consistency is needed), while meiosis is essential for sexual reproduction (where genetic variation is advantageous for evolution).

Interactive Flashcards

What are the four stages of mitosis?

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The four stages of mitosis are:

1. Prophase: Chromosomes condense, spindle forms
2. Metaphase: Chromosomes align at equator
3. Anaphase: Sister chromatids separate
4. Telophase: Nuclear envelopes reform

Followed by cytokinesis (cytoplasmic division).

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