Cell Cycle and Cell Division

CELL CYCLE

Prepared By Dr Bhabesh Nath

Assistant Professor

Department of Zoology

B N College (A) Dhubri

Learning Objectives

· Define the cell cycle and explain its major phases.

· Describe the important events of G₁, S and G₂ phases.

· Explain the stages of mitosis and cytokinesis.

· Understand the role of cyclins, cyclin dependent kinases and cell cycle checkpoints.

· Explain the significance of the G₀ phase and p53 mediated regulation.

· Relate abnormal cell cycle regulation to cancer.

1. Introduction

The cell cycle is the ordered sequence of molecular and cellular events through which a cell grows, duplicates its genetic material and divides to produce daughter cells. It is essential for growth, development, tissue maintenance, regeneration and reproduction. In eukaryotic cells, the cell cycle is broadly divided into interphase and the M phase.

2. Major Phases of the Cell Cycle

The cell cycle consists of interphase, which includes G₁, S and G₂ phases, followed by the M phase. The M phase includes mitosis and cytokinesis. Some cells leave the active cycle and enter G₀.

Phase	Major Events	Important Point
G₁ phase	Cell growth, RNA and protein synthesis, organelle production	Preparation for DNA replication
S phase	DNA replication and histone synthesis	DNA amount doubles, chromosome number remains unchanged
G₂ phase	Growth, DNA checking and synthesis of mitotic proteins	Preparation for mitosis
M phase	Mitosis and cytokinesis	Nuclear and cytoplasmic division
G₀ phase	Non proliferative state	Cells may remain temporarily or permanently outside the active cycle

3. Interphase

Interphase is the period between successive cell divisions. It is not a resting period because the cell performs intense metabolic and biosynthetic activities. Interphase consists of G₁, S and G₂ phases.

3.1 G₁ Phase

· The cell increases in size and carries out normal metabolic activities.

· RNA and proteins are synthesized.

· Cellular organelles are produced or increased.

· The cell prepares the molecular machinery required for DNA replication.

· The G₁ checkpoint evaluates cell size, nutrients, growth signals and DNA integrity.

3.2 S Phase

· DNA replication takes place.

· Histone proteins are synthesized to package newly replicated DNA.

· Each chromosome produces two sister chromatids joined at the centromere.

· The centrosome is duplicated in animal cells.

· DNA content doubles while chromosome number remains the same.

3.3 G₂ Phase

· The cell continues to grow.

· Proteins needed for mitosis are synthesized.

· The replicated DNA is checked for errors and damage.

· The G₂ checkpoint determines whether the cell is ready to enter mitosis.

4. M Phase

The M phase is the division phase. Mitosis distributes duplicated chromosomes between two daughter nuclei, while cytokinesis divides the cytoplasm.

4.1 Prophase

· Chromatin condenses into visible chromosomes.

· Each chromosome contains two sister chromatids.

· The nucleolus becomes inconspicuous.

· The mitotic spindle begins to form.

· Centrosomes move toward opposite poles in animal cells.

4.2 Prometaphase

· The nuclear envelope breaks down.

· Spindle microtubules enter the nuclear region.

· Microtubules attach to chromosomes through kinetochores.

· Chromosomes begin movement toward the equatorial region.

4.3 Metaphase

· Chromosomes align at the metaphase plate.

· Sister chromatids remain attached.

· Kinetochores are connected to spindle microtubules from opposite poles.

· The spindle assembly checkpoint verifies proper attachment before chromosome separation.

4.4 Anaphase

· Sister chromatids separate.

· Each separated chromatid becomes an independent daughter chromosome.

· Daughter chromosomes move toward opposite poles.

4.5 Telophase

· Chromosomes reach the poles.

· Chromosomes decondense.

· Nuclear envelopes reform.

· Nucleoli reappear and the spindle disassembles.

5. Cytokinesis

Cytokinesis is the division of the cytoplasm. In animal cells, an actin and myosin contractile ring forms beneath the plasma membrane. Constriction of this ring produces a cleavage furrow that progressively separates the parent cell into two daughter cells.

6. Cell Cycle Regulation

Cell cycle progression is controlled by regulatory proteins and checkpoint systems. Cyclins bind to cyclin dependent kinases, or CDKs, and activate them. The resulting complexes phosphorylate target proteins and promote progression through specific stages of the cell cycle.

· Cyclins are regulatory proteins whose concentrations rise and fall during the cycle.

· CDKs are protein kinases that require cyclins for specific cell cycle functions.

· Cyclin CDK complexes help control transitions such as G₁ to S and G₂ to M.

7. Cell Cycle Checkpoints

G₁ checkpoint

Checks cell size, nutrients, growth signals and DNA integrity before DNA replication.

G₂ checkpoint

Checks whether DNA replication is complete and whether significant DNA damage is present before mitosis.

Spindle assembly checkpoint

Ensures that chromosomes are appropriately attached to spindle microtubules before anaphase.

8. G₀ Phase

G₀ is a non proliferative state in which a cell is outside the active cell cycle. Some cells can re enter the cycle when stimulated, whereas highly differentiated cells may remain in G₀ for long periods or permanently.

9. Role of p53

p53 is an important tumor suppressor protein that responds to cellular stress and DNA damage. It can slow or stop cell cycle progression, allowing DNA repair. If damage is severe, p53 dependent pathways can contribute to programmed cell death. Loss of normal p53 function can permit cells carrying damaged DNA to continue dividing.

10. Cell Cycle and Cancer

· Cancer cells frequently show abnormal control of cell proliferation.

· Mutations can activate proteins that promote cell division.

· Tumor suppressor pathways may become inactive.

· Checkpoint control can be impaired.

· Cells may continue dividing despite DNA damage.

11. Biological Significance

· Growth and development of multicellular organisms.

· Replacement of old or damaged cells.

· Tissue repair and regeneration.

· Maintenance of chromosome number during mitotic division.

· Accurate transmission of genetic information.

· Asexual reproduction in many organisms.

12. Frequently Asked Questions

What is the cell cycle?
It is the ordered sequence of growth, DNA replication and cell division events in a cell.

Which phase is responsible for DNA replication?
The S phase of interphase.

Does chromosome number double during S phase?
No. DNA content doubles, but chromosome number remains unchanged because each chromosome forms two sister chromatids.

What are the stages of mitosis?
Prophase, prometaphase, metaphase, anaphase and telophase.

What is cytokinesis?
It is the division of the cytoplasm that produces separate daughter cells.

What is G₀?
It is a non proliferative state outside the active cell cycle.

What are cyclins?
They are regulatory proteins that control cell cycle progression through interactions with CDKs.

What are CDKs?
They are cyclin dependent kinases that phosphorylate target proteins and regulate cell cycle transitions.

Why are checkpoints important?
They prevent progression when conditions are unsuitable or when DNA or chromosome attachment is defective.

What is the role of p53?
p53 helps respond to DNA damage by promoting cell cycle arrest, repair and, when necessary, programmed cell death.

13. Multiple Choice Questions

1. DNA replication occurs during which phase?

· A. G₁

· B. S

· C. G₂

· D. M

Answer: B

2. Which phase follows S phase?

· A. G₁

· B. G₀

· C. G₂

· D. Anaphase

Answer: C

3. Chromosomes align at the metaphase plate during:

· A. Prophase

· B. Metaphase

· C. Anaphase

· D. Telophase

Answer: B

4. Separation of sister chromatids occurs during:

· A. Prophase

· B. Metaphase

· C. Anaphase

· D. Telophase

Answer: C

5. Cytokinesis in animal cells involves formation of a:

· A. Cell plate

· B. Cleavage furrow

· C. Spore

· D. Nucleosome

Answer: B

6. Cyclin dependent kinases are activated by association with:

· A. Histones

· B. Cyclins

· C. Ribosomes

· D. Lipids

Answer: B

7. Which protein is an important tumor suppressor?

· A. p53

· B. Actin

· C. Tubulin

· D. Keratin

Answer: A

8. The spindle assembly checkpoint operates primarily to ensure:

· A. RNA synthesis

· B. DNA transcription

· C. Correct chromosome attachment

· D. Protein digestion

Answer: C

9. The G₀ state is best described as:

· A. DNA replication

· B. Non proliferative state

· C. Chromosome alignment

· D. Cytoplasmic division

Answer: B

10. During S phase, the amount of DNA:

· A. Decreases

· B. Remains unchanged

· C. Doubles

· D. Becomes zero

Answer: C

14. Question Bank

A. Very Short Answer Questions

1. Define the cell cycle.

2. Name the phases of interphase.

3. What happens during S phase?

4. What is G₀ phase?

5. What is cytokinesis?

6. What is a kinetochore?

7. What is a cyclin?

8. What is a CDK?

9. What is the metaphase plate?

10. Name an important tumour suppressor protein involved in cell cycle control.

B. Short Answer Questions

11. Describe the major events of G₁ phase.

12. Explain the significance of S phase.

13. Describe the G₂ phase.

14. Write a short note on cell cycle checkpoints.

15. Explain the role of cyclins and CDKs.

16. Describe cytokinesis in animal cells.

17. Write a note on the G₀ phase.

18. Explain the role of p53 in cell cycle regulation.

19. Differentiate between chromosome number and DNA content during the cell cycle.

20. Explain the biological significance of mitosis.

C. Long Answer Questions

21. Describe the cell cycle in detail with suitable headings and explain the events of interphase and M phase.

22. Explain the stages of mitosis and discuss the major structural changes occurring during each stage.

23. Discuss the molecular regulation of the cell cycle with special reference to cyclins, CDKs and checkpoints.

24. Explain the relationship between cell cycle dysregulation and cancer.

25. Describe DNA replication and chromosome behaviour in relation to the different phases of the cell cycle.

15. Important Distinctions

Feature	Interphase	M phase
Main purpose	Growth, DNA replication and preparation	Chromosome segregation and cell division
Subdivisions	G₁, S and G₂	Mitosis and cytokinesis
DNA replication	Occurs during S phase	Does not occur
Chromosome segregation	Does not occur	Occurs during mitosis

16. Examination Tips

· Remember the sequence G₁, S, G₂, M.

· Write DNA replication specifically under S phase.

· Mention that chromosome number does not double during S phase.

· For mitosis, remember prophase, prometaphase, metaphase, anaphase and telophase.

· Mention cytokinesis separately from nuclear division.

· For regulation questions, include cyclins, CDKs and checkpoints.

· For cancer related questions, discuss abnormal proliferation and defective checkpoint control.

17. References

1.  Alberts, B. et al. Molecular Biology of the Cell. Garland Science.
2.  Cooper, G. M. and Hausman, R. E. The Cell: A Molecular Approach. Oxford University Press.
3.  Lodish, H. et al. Molecular Cell Biology. W. H. Freeman.
4.  Campbell, N. A. et al. Campbell Biology. Pearson.
5.  Raven, P. H. et al. Biology. McGraw Hill.
6.  NCBI Bookshelf. Cell cycle and molecular cell biology resources. National Center for Biotechnology Information.

Quick Revision

Cell cycle = G₁ + S + G₂ + M. G₁ is mainly growth and preparation. S is DNA replication. G₂ is preparation for mitosis. M includes mitosis and cytokinesis. Cyclins and CDKs regulate progression, while checkpoints monitor cellular conditions, DNA integrity and chromosome attachment.

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