CELL CYCLE

Growth, Replication and Division of Cells

Course: Cell Biology | Programme: B.Sc. Zoology |

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By Dr Bhabesh Nath, Assistant Professor,

Department of Zoology, B N College Autonomous , Dhubri

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Learning Objectives

01 Define the cell cycle

02 Explain the phases of the cell cycle

03 Describe events occurring during interphase

04 Explain mitosis and cytokinesis

05 Understand cell cycle regulation

06 Discuss the significance of the cell cycle

Introduction to Cell Cycle

Definition: The cell cycle is an ordered sequence of events through which a cell grows, replicates its DNA and divides into two daughter cells.

Why is it important?

Growth & Development

Tissue Repair

Cell Replacement

Reproduction

Overview of the Cell Cycle

G1 Phase

Cell growth, protein synthesis

S Phase

DNA replication

G2 Phase

Preparation for division

M Phase

Mitosis + Cytokinesis

Interphase – The Longest Phase

Interphase constitutes approximately 90% of the total cell cycle duration

G1 Phase

Gap 1 / Growth

• Cell increases in size
• Active RNA & protein synthesis
• Organelle multiplication
• G1 checkpoint — nutrients & DNA integrity

S Phase

Synthesis

• DNA replication occurs
• Chromosomes duplicate
• Histone proteins synthesized
• DNA content doubles (2N → 4N)

G2 Phase

Gap 2 / Prep

• Continued cell growth
• Spindle protein synthesis
• DNA repair mechanisms
• G2 checkpoint — ensures accuracy

G0 Phase – The Resting Phase

Some cells leave the active cell cycle and enter a resting state called G0. They remain metabolically active but do not replicate.

Examples of G0 Cells

Neurons

Terminally differentiated; rarely re-enter cycle

Cardiac Muscle

Long-lived, highly specialized cells

Hepatocytes

Can re-enter cycle when stimulated

Key Characteristics:

• Metabolically active but non-dividing
• May be reversible (quiescence) or irreversible (senescence)
• Important in tissue homeostasis

Mitotic Phase (M Phase)

Mitosis (nuclear division) + Cytokinesis (cytoplasmic division) → Two genetically identical daughter cells

PROPHASE

Chromosomes condense, spindle forms, nuclear membrane breaks down

METAPHASE

Chromosomes align at the metaphase plate (equatorial plane)

ANAPHASE

Sister chromatids separate and move to opposite poles

TELOPHASE

Nuclear membranes reform, chromosomes decondense

CYTOKINESIS

Cytoplasm divides; cleavage furrow (animals) or cell plate (plants)

Prophase & Metaphase

PROPHASE

• Chromatin condenses into visible chromosomes
• Nucleolus disappears
• Nuclear membrane breaks down
• Spindle fibres begin to form from centrioles

METAPHASE

• Chromosomes align at the equatorial plane (metaphase plate)
• Spindle fibres attach to centromeres (kinetochores)
• Most condensed state of chromosomes
• Ideal stage to study karyotype

Anaphase & Telophase

ANAPHASE

• Sister chromatids separate at centromeres
• Chromatids pulled to opposite poles
• Cell elongates
• Ensures equal genetic material at each pole

TELOPHASE

• Chromosomes reach opposite poles
• Nuclear membrane re-forms around each set
• Nucleolus reappears
• Chromosomes begin to decondense → Two daughter nuclei form

Cytokinesis

Division of the cytoplasm — occurs alongside or after telophase

Animal Cells

Mechanism: Cleavage Furrow

• Actin-myosin ring contracts at cell equator
• Pinches the cell membrane inward
• Results in two separate cells
• Process called furrowing

Plant Cells

Mechanism: Cell Plate

• Vesicles from Golgi fuse at cell center
• Cell plate forms and expands outward
• Grows to form a new cell wall
• Phragmoplast guides vesicle delivery

Cell Cycle Checkpoints

Checkpoints are surveillance mechanisms that prevent damaged or incomplete DNA from being passed to daughter cells.

G1 Checkpoint

Restriction Point

⏱ Late G1

• Cell size adequate?
• Nutrients sufficient?
• DNA undamaged?
• Growth factors present?

G2 Checkpoint

Pre-Mitotic Gate

⏱ End of G2

• DNA fully replicated?
• DNA repair complete?
• Cell large enough?
• Cyclin B/CDK1 active?

M Checkpoint

Spindle Assembly Checkpoint

⏱ During Metaphase

• All chromosomes attached to spindle?
• Kinetochores under tension?
• Prevents premature anaphase

Regulation of the Cell Cycle

Cyclins

Regulatory proteins whose levels fluctuate cyclically. They rise and fall at specific phases.

Examples: Cyclin D (G1), Cyclin E (G1/S), Cyclin A (S/G2), Cyclin B (M)

Cyclin-Dependent Kinases (CDKs)

Kinases activated only when bound to a cyclin. Phosphorylate target proteins to drive cell cycle progression.

Examples: CDK4/6 (G1), CDK2 (S), CDK1 (M)

CDK Inhibitors (CKIs)

Proteins that inhibit cyclin-CDK complexes and apply brakes to the cell cycle.

Examples: p21, p27, p57 — block CDK activity in response to DNA damage

Cell Cycle & Cancer

Cancer = Uncontrolled cell division resulting from loss of cell cycle regulation

Proto-oncogenes → Oncogenes

Mutations convert normal growth-promoting genes into constantly active drivers of division (e.g. RAS, MYC)

Tumor Suppressor Mutations

Loss of genes like p53 (guardian of the genome) or Rb removes cell cycle brakes

Checkpoint Failure

Defective surveillance allows damaged DNA to be replicated and passed on

DNA Repair Defects

Accumulation of mutations in replication/repair genes (e.g., BRCA1/2)

Significance of the Cell Cycle

Growth

Enables organisms to develop from a single fertilized egg to a complex multicellular being

Tissue Repair

Allows replacement of cells lost through injury, wear, and normal turnover

Regeneration

Some organisms regrow lost structures (e.g. lizard tails, liver tissue)

Reproduction

Asexual reproduction in unicellular organisms depends entirely on mitosis

Chromosome Stability

Maintains the correct chromosome number across generations of cells

Genetic Fidelity

Checkpoints and DNA repair ensure accurate transmission of genetic information

Summary

Cell cycle consists of Interphase (G1, S, G2) and the Mitotic Phase (M)

DNA replication occurs exclusively during the S phase

Mitosis produces two genetically identical daughter cells

Three major checkpoints (G1, G2, M) safeguard genomic integrity

Cyclins and CDKs are the molecular engines driving phase transitions

Defects in regulation lead to uncontrolled division — cancer

Thank You

Questions & Discussion

Dr Bhabesh Nath

Assistant Professor

Department of Zoology

References: Alberts et al. — Molecular Biology of the Cell | Karp's Cell and Molecular Biology | Cooper & Hausman: The Cell