Cell Cycle NEET Notes: Interphase, M Phase, G₀ Phase Explained
10.2 Phases of Cell Cycle – (NEET Level)
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| Cell Cycle Mind Map showing Interphase, G1, S, G2, M Phase, G₀ Phase and important NEET revision points. |
- Dr.Sanjaykumar Pawar
Original Line 1
"A typical eukaryotic cell cycle is illustrated by human cells in culture."
Easy Notes
- A eukaryotic cell has a true nucleus and membrane-bound organelles.
- Human cells are commonly used to study the cell cycle because they are easy to grow in laboratories (cell culture).
- The human cell cycle is considered a standard example.
NEET Point: Human cells are commonly used as the model for studying the eukaryotic cell cycle.
Original Line 2
"These cells divide once in approximately every 24 hours."
Easy Notes
- A typical human body cell completes one full cell cycle in about 24 hours.
- This includes growth, DNA replication, and cell division.
NEET Fact: Average human cell cycle = 24 hours.
Original Line 3
"However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type."
Easy Notes
- Not all cells divide at the same speed.
- The duration depends on:
- Type of organism.
- Type of cell.
Examples
- Skin cells divide frequently.
- Nerve cells divide very rarely.
NEET Point: Cell cycle duration is not the same in all organisms.
Original Line 4
"Yeast for example, can progress through the cell cycle in only about 90 minutes."
Easy Notes
- Yeast cells divide much faster than human cells.
- Their cell cycle takes only 90 minutes.
NEET Fact
- Human cell = 24 hours
- Yeast cell = 90 minutes
Original Line 5
"The cell cycle is divided into two basic phases:"
Easy Notes
The cell cycle has 2 main phases:
- Interphase
- M Phase (Mitotic Phase)
Memory Trick
I → M
Interphase → Mitosis
Original Line 6
"Interphase"
Easy Notes
- Longest phase of the cell cycle.
- Cell grows.
- DNA replicates.
- Cell prepares for division.
NEET Point: Interphase is the preparation phase.
Original Line 7
"M Phase (Mitosis phase)"
Easy Notes
- M phase is the division phase.
- The nucleus divides.
- The cytoplasm divides.
- Two daughter cells are formed.
NEET Point: M phase = Actual cell division.
Original Line 8
"The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases."
Easy Notes
- Actual division happens during M phase.
- Interphase is the time between two mitotic divisions.
- The cell prepares for the next division during interphase.
Memory
Interphase = Preparation
M phase = Division
Original Line 9
"It is significant to note that in the 24 hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour."
Easy Notes
- Total cell cycle = 24 hours
- Actual cell division takes only 1 hour.
NEET Fact
- Cell division (M phase) ≈ 1 hour
Original Line 10
"The interphase lasts more than 95% of the duration of cell cycle."
Easy Notes
- Interphase occupies most of the cell cycle.
- More than 95% of the total time is spent in interphase.
NEET Fact
- Interphase = >95%
- M phase = <5%
Original Line 11
"The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis)."
Easy Notes
M phase has two main events:
1. Karyokinesis
- Division of nucleus.
- Chromosomes separate.
2. Cytokinesis
- Division of cytoplasm.
- Two daughter cells are formed.
Word Meaning
- Karyokinesis = Nuclear division
- Cytokinesis = Cytoplasmic division
NEET Point
Karyokinesis occurs before cytokinesis.
Original Line 12
"The interphase, though called the resting phase, is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner."
Easy Notes
- Interphase is NOT a true resting phase.
- The cell is highly active.
- During interphase:
- Cell grows.
- DNA replicates.
- Proteins are made.
- Organelles increase.
NEET Point
Interphase is metabolically the most active phase.
Original Line 13
"The interphase is divided into three further phases:"
Easy Notes
Interphase has 3 stages:
- G₁ phase
- S phase
- G₂ phase
Memory Trick
G₁ → S → G₂
Original Line 14
"G₁ phase (Gap 1)"
Easy Notes
- First growth phase.
- Cell increases in size.
- Proteins and RNA are produced.
- DNA is not copied.
NEET Point
- Cell is metabolically active.
- DNA replication does not occur.
Original Line 15
"S phase (Synthesis)"
Easy Notes
- DNA replication occurs.
- Every chromosome forms two sister chromatids.
- DNA amount doubles.
NEET Point
- S = DNA synthesis.
Original Line 16
"G₂ phase (Gap 2)"
Easy Notes
- Second growth phase.
- Cell prepares for mitosis.
- Proteins needed for cell division are made.
- Cell continues to grow.
Original Line 17
"G₁ phase corresponds to the interval between mitosis and initiation of DNA replication."
Easy Notes
- G₁ comes after mitosis.
- DNA replication has not started yet.
- Cell prepares for DNA synthesis.
Original Line 18
"During G₁ phase, the cell is metabolically active and continuously grows but does not replicate its DNA."
Easy Notes
During G₁:
- Cell is active.
- Cell grows.
- Proteins are synthesized.
- DNA is not replicated.
Original Line 19
"S or synthesis phase marks the period during which DNA synthesis or replication takes place."
Easy Notes
- S phase is the only phase in which DNA replication occurs.
NEET Fact
DNA replication occurs only in the S phase.
Original Line 20
"During this time the amount of DNA per cell doubles."
Easy Notes
- DNA content becomes double.
DNA Content
- Before S phase = 2C
- After S phase = 4C
Original Line 21
"If the initial amount of DNA is denoted as 2C then it increases to 4C."
Easy Notes
DNA Quantity:
Before replication = 2C
↓
After replication = 4C
NEET Point Only DNA amount doubles.
Original Line 22
"However, there is no increase in the chromosome number."
Easy Notes
- Chromosome number remains the same.
- Only DNA content doubles.
NEET Point Chromosome number does not change during S phase.
Original Line 23
"If the cell had diploid or 2n number of chromosomes at G₁, even after S phase the number of chromosomes remains the same, i.e., 2n."
Easy Notes
| Stage | Chromosome Number | DNA Amount |
|---|---|---|
| G₁ | 2n | 2C |
| After S | 2n | 4C |
Very Important NEET Question
✔ Chromosome number = Same
✔ DNA amount = Double
Original Line 24
"In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm."
Easy Notes
During S phase:
- DNA replicates inside the nucleus.
- Centrioles also duplicate in animal cells.
NEET Fact Centrioles duplicate during S phase.
Original Line 25
"During the G₂ phase, proteins are synthesised in preparation for mitosis while cell growth continues."
Easy Notes
During G₂:
- Proteins required for mitosis are synthesized.
- Cell continues growing.
- Cell becomes fully prepared for division.
Flow Chart (Easy to Remember)
CELL CYCLE
│
├── Interphase (>95%)
│ │
│ ├── G1
│ │ ├── Cell growth
│ │ ├── Protein synthesis
│ │ └── No DNA replication
│ │
│ ├── S
│ │ ├── DNA replication
│ │ ├── DNA: 2C → 4C
│ │ ├── Chromosome: 2n → 2n
│ │ └── Centriole duplicates
│ │
│ └── G2
│ ├── Cell growth
│ ├── Protein synthesis
│ └── Preparation for mitosis
│
└── M Phase (<5%)
├── Karyokinesis
└── Cytokinesis
NEET Quick Revision
- Human cell cycle = 24 hours
- Yeast cell cycle = 90 minutes
- Interphase = More than 95%
- M phase = About 1 hour
- Interphase = G₁ + S + G₂
- DNA replication occurs only in S phase
- DNA amount: 2C → 4C
- Chromosome number: 2n → 2n
- Centrioles duplicate during S phase
- Karyokinesis occurs before cytokinesis
- G₂ prepares the cell for mitosis.
10.2 Phases of Cell Cycle (G₀ Phase & Exceptions) (NEET Level)
Original Line 1
"Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death."
Easy Notes
- Not all cells divide throughout life.
- Some adult cells permanently stop dividing.
- Example: Heart (cardiac muscle) cells rarely divide.
- Some cells divide only when needed.
- They replace damaged or dead cells after injury.
Examples
- Heart cells: Almost never divide.
- Liver cells: Divide only after injury.
- Skin cells: Divide regularly to replace old cells.
NEET Point
- Heart cells are an example of non-dividing cells.
- Skin and bone marrow cells divide continuously.
Original Line 2
"These cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G₀) of the cell cycle."
Easy Notes
- Some cells leave the normal cell cycle after G₁ phase.
- They enter a special stage called the G₀ phase.
- G₀ is also called the quiescent stage.
- In this stage, cells stop dividing.
Word Meaning
- Exit G₁ = Leave the cell cycle after the first growth phase.
- Quiescent = Inactive for cell division.
Flow
G₁
↓
G₀ (Quiescent Stage)
↓
No Cell Division
NEET Point
- G₀ = Resting (quiescent) phase outside the cell cycle.
Original Line 3
"Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism."
Easy Notes
- Cells in G₀ are alive and active.
- They continue normal functions such as respiration and protein synthesis.
- However, they do not divide.
- Some G₀ cells can re-enter the cell cycle if the body needs more cells (for example, during tissue repair).
Word Meaning
- Metabolically active = Performing normal life processes.
- Proliferate = Divide and increase in number.
Examples
- Liver cells can return from G₀ to divide after injury.
- Heart cells generally remain in G₀ permanently.
NEET Point
- G₀ cells are metabolically active but non-dividing.
Original Line 4
"In animals, mitotic cell division is only seen in the diploid somatic cells."
Easy Notes
- In animals, mitosis usually occurs only in diploid (2n) body cells.
- These body cells are called somatic cells.
Word Meaning
- Diploid (2n) = Two sets of chromosomes.
- Somatic cells = All body cells except reproductive cells.
Examples
- Skin cells
- Bone cells
- Liver cells
- Muscle cells (when capable of division)
NEET Point
- Mitosis normally occurs in diploid somatic cells.
Original Line 5
"However, there are few exceptions to this where haploid cells divide by mitosis, for example, male honey bees."
Easy Notes
- There are a few exceptions.
- In male honey bees, cells are haploid (n).
- Even though they are haploid, they still divide by mitosis.
Word Meaning
- Haploid (n) = One set of chromosomes.
NEET Fact (Very Important)
- Male honey bee (Drone) = Haploid (n)
- Shows mitosis
This is a frequently asked NEET fact.
Original Line 6
"Against this, the plants can show mitotic divisions in both haploid and diploid cells."
Easy Notes
- Plants are different from animals.
- In plants, mitosis occurs in:
- Diploid cells (2n)
- Haploid cells (n)
Reason
Plants have alternation of generations, so both generations grow by mitosis.
NEET Point
- Plants show mitosis in both haploid and diploid stages.
Original Line 7
"From your recollection of examples of alternation of generations in plants identify plant species and stages at which mitosis is seen in haploid cells."
Easy Notes
- In plants, the gametophyte (haploid generation) grows by mitosis.
- Haploid spores divide repeatedly by mitosis to form the gametophyte.
Examples
- Mosses (Bryophytes): Haploid gametophyte grows by mitosis.
- Ferns (Pteridophytes): Prothallus (gametophyte) grows by mitosis.
- Flowering plants (Angiosperms):
- Pollen grain (male gametophyte) divides by mitosis.
- Embryo sac (female gametophyte) develops by mitosis.
NEET Point
- Haploid gametophytes in plants divide by mitosis.
Summary Table
| Topic | Easy Point |
|---|---|
| Non-dividing cells | Heart cells rarely divide |
| Occasionally dividing cells | Liver cells divide after injury |
| G₀ Phase | Resting (quiescent) stage |
| Activity in G₀ | Metabolically active but non-dividing |
| Animal mitosis | Occurs mainly in diploid somatic cells |
| Exception | Male honey bee (haploid) shows mitosis |
| Plant mitosis | Occurs in both haploid and diploid cells |
| Haploid plant stage | Gametophyte divides by mitosis |
Flow Chart (Easy to Remember)
Cell after G₁
│
├── Continues Cell Cycle
│ │
│ └── DNA Replication → Mitosis
│
└── Enters G₀ Phase
│
├── Metabolically active
├── No cell division
├── Performs normal functions
└── May re-enter cell cycle (some cells)
NEET Quick Revision
- G₀ phase = Quiescent (resting) stage
- Cells in G₀ are alive but non-dividing
- Heart cells usually remain in G₀ permanently
- Liver cells can leave G₀ and divide after injury
- Animal mitosis usually occurs in diploid somatic cells
- Male honey bee (drone) is the important exception: it is haploid (n) and still undergoes mitosis
- Plants show mitosis in both haploid and diploid cells
- The gametophyte is the haploid stage that grows by mitosis in plants.
CBSE Class 11 Biology – Cell Cycle (10.1 & 10.2) Question Bank with Answers
Section A: MCQs (1 Mark)
1. Cell cycle is the sequence of events involving:
A. Cell death only B. Cell growth, DNA replication and cell division C. Protein synthesis only D. Respiration only
Answer: B
2. Which phase occupies more than 95% of the cell cycle?
A. G₁ B. S C. M D. Interphase
Answer: D
3. DNA replication occurs during:
A. G₁ B. G₂ C. S D. M
Answer: C
4. The average duration of the human cell cycle is:
A. 12 hours B. 24 hours C. 48 hours D. 90 minutes
Answer: B
5. Which phase is known as the synthesis phase?
A. G₁ B. G₂ C. S D. M
Answer: C
6. DNA content doubles during:
A. G₁ B. S C. G₂ D. M
Answer: B
7. Chromosome number after S phase remains:
A. n B. 4n C. 2n D. Doubled
Answer: C
8. Centriole duplication occurs during:
A. G₁ B. G₂ C. S D. M
Answer: C
9. Quiescent stage is called:
A. G₁ B. G₂ C. G₀ D. S
Answer: C
10. Heart cells usually remain in:
A. S phase B. M phase C. G₀ phase D. G₂ phase
Answer: C
Section B: Very Short Answer Questions (1 Mark)
1. Define cell cycle.
Answer: The cell cycle is the sequence of events in which a cell grows, replicates its DNA, and divides into two daughter cells.
2. Name the phase in which DNA replication occurs.
Answer: S phase.
3. Which phase is called the resting phase?
Answer: G₀ phase.
4. What is karyokinesis?
Answer: Division of the nucleus.
5. What is cytokinesis?
Answer: Division of the cytoplasm.
6. Name the two major phases of the cell cycle.
Answer:
- Interphase
- M phase
7. Which phase prepares the cell for mitosis?
Answer: G₂ phase.
8. Which cells generally do not divide in adults?
Answer: Heart muscle cells.
Section C: Short Answer Questions (2–3 Marks)
1. Differentiate between Interphase and M phase.
| Interphase | M Phase |
|---|---|
| Cell grows | Cell divides |
| DNA replicates | Chromosomes separate |
| >95% of cycle | About 1 hour |
| G₁, S, G₂ phases | Karyokinesis & Cytokinesis |
2. Write the functions of G₁, S and G₂ phases.
Answer
G₁
- Cell growth
- Protein synthesis
S
- DNA replication
- Centriole duplication
G₂
- Protein synthesis
- Preparation for mitosis
3. Why is interphase called the preparatory phase?
Answer
During interphase:
- Cell grows
- DNA replicates
- Organelles increase
- Proteins are synthesized
- Cell prepares for mitosis
4. What is G₀ phase?
Answer
- Quiescent stage
- Cell leaves G₁
- Metabolically active
- Does not divide
- Can re-enter cycle in some cells
Section D: Long Answer Questions (5 Marks)
1. Explain the phases of the cell cycle.
Answer
Cell cycle has two phases:
Interphase
G₁
- Cell growth
- Protein synthesis
- No DNA replication
S
- DNA replication
- DNA doubles (2C → 4C)
- Chromosome number remains 2n
G₂
- Cell growth
- Protein synthesis
- Preparation for mitosis
M Phase
- Karyokinesis
- Cytokinesis
- Two daughter cells formed
2. Explain the significance of the cell cycle.
Answer
- Growth
- Repair of tissues
- Replacement of damaged cells
- Reproduction
- Maintains chromosome number
- Ensures genetic continuity
Section E: Assertion & Reason
1.
Assertion: DNA replication occurs in S phase.
Reason: DNA amount doubles during S phase.
Answer: ✅ Both Assertion and Reason are true, and Reason is the correct explanation.
2.
Assertion: Interphase is called the resting phase.
Reason: No metabolic activity occurs during interphase.
Answer: ❌ Assertion is true but Reason is false.
3.
Assertion: Heart cells usually remain in G₀ phase.
Reason: They generally do not divide after birth.
Answer: ✅ Both are true and Reason correctly explains Assertion.
4.
Assertion: Chromosome number doubles during S phase.
Reason: DNA replication occurs.
Answer: ❌ Assertion is false but Reason is true.
Section F: Fill in the Blanks
- DNA replication occurs during the ______ phase.
Answer: S
- Human cell cycle lasts approximately ______ hours.
Answer: 24
- Nuclear division is called ______.
Answer: Karyokinesis
- Cytoplasmic division is called ______.
Answer: Cytokinesis
- The resting stage is called ______ phase.
Answer: G₀
- DNA content changes from ______ to ______ during S phase.
Answer: 2C, 4C
- Centriole duplicates during the ______ phase.
Answer: S
Section G: Match the Columns
| Column A | Column B |
|---|---|
| A. G₁ | 1. DNA replication |
| B. S | 2. Cell growth |
| C. G₂ | 3. Preparation for mitosis |
| D. G₀ | 4. Quiescent stage |
Answer
A → 2
B → 1
C → 3
D → 4
Section H: Statement-Based Questions
Statement 1
Interphase occupies more than 95% of the cell cycle.
Statement 2
Actual cell division occurs during M phase.
Choose:
A. Both statements are true.
B. Statement 1 true, Statement 2 false.
C. Statement 1 false, Statement 2 true.
D. Both false.
Answer: A
Statement 2
Statement I
DNA amount doubles during S phase.
Statement II
Chromosome number also doubles during S phase.
Answer: Statement I is true.
Statement II is false.
Section I: Case Study Questions
Case Study
A scientist observed a human cell under the microscope. The cell was actively growing, synthesizing proteins, and replicating DNA. Later, the chromosomes separated, and finally the cytoplasm divided to produce two daughter cells.
Questions
1. Which phase occupies the maximum time?
Answer: Interphase
2. During which phase was DNA replication observed?
Answer: S phase
3. What is the division of nucleus called?
Answer: Karyokinesis
4. What is the division of cytoplasm called?
Answer: Cytokinesis
5. Which phase represents actual cell division?
Answer: M phase
Section J: Higher Order Thinking Questions (HOTS)
1. Why does DNA double during S phase but chromosome number remains the same?
Answer
Each chromosome replicates to form two identical sister chromatids joined at the centromere. Therefore, DNA content doubles (2C → 4C), but the chromosome number remains unchanged (2n).
2. Why are heart cells unable to divide continuously?
Answer
Heart cells enter the G₀ (quiescent) phase after development. They remain metabolically active but generally lose the ability to undergo further cell division.
CBSE Important Diagram-Based Questions
- Draw and label the cell cycle.
- Draw G₁, S, G₂, and M phases.
- Represent DNA content (2C → 4C) during the S phase.
- Show the sequence:
- G₁ → S → G₂ → M → G₁
One-Mark Revision Facts
- Human cell cycle = 24 hours
- Yeast cell cycle = 90 minutes
- Interphase = >95%
- M phase = ~1 hour
- DNA replication = S phase
- DNA content = 2C → 4C
- Chromosome number = 2n → 2n
- G₀ = Quiescent stage
- Heart cells = Non-dividing
- Male honey bee = Haploid but undergoes mitosis
- Plants show mitosis in both haploid and diploid cells
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