Biology | Cell and nuclear division

🧬 Introduction: The Engine of Continuity (Cell and Nuclear Division)

Hello Biologists! Welcome to one of the most fundamental chapters in life science: Cell and Nuclear Division. This topic sits squarely in the "Continuity and Change" section of the syllabus, and for good reason—cell division is the mechanism that ensures life continues, whether through growth, repair, or reproduction.

Why is this important? Every single cell in your body (except your gametes) started as one single cell after fertilization. Cell division is how you grew! It's also how you heal a nasty scrape on your knee. Understanding this process helps us understand everything from human development to aging, and even the causes of diseases like cancer.

Don't worry if terms like chromatids and spindles seem complicated at first; we’ll break them down step-by-step using simple analogies!

1. The Cell Cycle: Life in Phases

The Cell Cycle is the ordered sequence of events that takes place in a eukaryotic cell between one cell division and the next. It’s essentially a life rhythm for the cell, ensuring everything is copied correctly before splitting.

The cycle is divided into two main stages: Interphase (preparation) and the M Phase (division).

1.1 Interphase (The Preparation Phase)

Contrary to popular belief, Interphase is not a resting phase. It is the longest and most active part of the cell cycle, where the cell performs its normal functions, grows, and prepares for division.

The Three Sub-Phases of Interphase:

1. G1 Phase (First Gap/Growth 1):
   

   The cell grows and carries out normal metabolic functions (e.g., protein synthesis). The cell checks its internal and external environment to decide if it should divide.

2. S Phase (Synthesis):
   

   This is the most critical checkpoint before division. The cell replicates its DNA. After the S phase, every chromosome consists of two identical strands called sister chromatids, joined together at a region called the centromere.

   Analogy: If a chromosome is a single sock, after the S phase, it’s a pair of identical socks joined by an elastic band (the centromere).

3. G2 Phase (Second Gap/Growth 2):
   

   The cell continues to grow and synthesizes the necessary proteins and organelles (like microtubules) required for division (Mitosis).

Key Takeaway from Section 1:

Interphase is where the cell grows and, crucially, doubles its genetic material (DNA replication in the S phase) so that it can split into two genetically identical halves.

2. Mitosis: Nuclear Division (PMAT)

Mitosis is the process of nuclear division that results in two daughter nuclei, each genetically identical to the parent nucleus. Its main roles are growth, tissue repair, and asexual reproduction.

Mnemonic: PMAT!

Remember the four stages of Mitosis using this simple acronym: Prophase, Metaphase, Anaphase, Telophase.

Step-by-Step Mitosis Explained:

1. Prophase (P = Preparing)
   • The chromatin (uncondensed DNA) coils and supercoils, making the chromosomes short, fat, and visible under a microscope.
   • The nuclear envelope (membrane surrounding the nucleus) breaks down.
   • Centrosomes (containing centrioles in animal cells) move to opposite poles of the cell and start forming the spindle fibres (microtubules).
2. Metaphase (M = Middle/Metaphase Plate)
   • The spindle fibres attach to the centromere of each chromosome.
   • The chromosomes are pulled into alignment along the cell's equator, forming the metaphase plate.
3. Anaphase (A = Apart)
   • The spindle fibres shorten, pulling the sister chromatids apart.
   • Once separated, each former chromatid is now considered a full, individual chromosome.
   • These new chromosomes move rapidly towards opposite poles of the cell.
   • This step ensures that each daughter cell receives one identical copy of every chromosome.
4. Telophase (T = Two Nuclei Forming)
   • The chromosomes arrive at the poles and begin to uncoil/decondense (returning to chromatin form).
   • A new nuclear envelope reforms around each set of chromosomes at the poles.
   • The spindle fibres disintegrate.
   • Mitosis (nuclear division) is complete, leaving the cell with two separate, identical nuclei.

Key Takeaway from Section 2:

The goal of Mitosis (PMAT) is to precisely separate the duplicated DNA into two identical sets. Anaphase is the key event where sister chromatids separate and move to opposite poles.

3. Cytokinesis: Dividing the Cell Body

Cytokinesis is the division of the cytoplasm, organelles, and cell membrane, which typically occurs concurrently with Telophase. This is the final step where the single parent cell physically splits into two separate, independent daughter cells.

Differences in Cytokinesis (Animals vs. Plants):

Because plant cells have a rigid cell wall, their method of splitting is different from flexible animal cells.

• Animal Cells:

  A ring of contractile protein filaments (actin and myosin) forms inside the cell membrane around the equator. This ring contracts, pinching the cell in half to form a cleavage furrow, like tightening a drawstring bag.

• Plant Cells:

  Since the cell wall prevents pinching, vesicles (carrying materials for the cell wall) gather at the equator. These vesicles fuse to form a temporary partition called the cell plate. The cell plate then develops into a new cell wall and cell membrane separating the two daughter cells.

4. Regulation and Uncontrolled Division (Cancer)

Cell division is far too important to be left unregulated. If cells divide when they shouldn't, tissues can overgrow (leading to tumors); if they don't divide when they should, wounds won't heal.

4.1 Control by Cyclins

The cell cycle is regulated by specialized proteins called cyclins. Cyclins work like a molecular timer, ensuring that the cell only proceeds to the next stage (like S phase or Mitosis) when all previous steps have been successfully completed.

Analogy: Cyclins are the "traffic cops" of the cell cycle, managing checkpoints (G1, G2, Metaphase) to ensure quality control before giving the "go ahead" signal.

Different types of cyclins (e.g., Cyclin D, E, A, B) must reach specific threshold concentrations to activate enzymes called Cyclin-Dependent Kinases (CDKs). The activation of these CDK complexes triggers the transition to the next stage of the cycle.

4.2 Cancer and Tumour Formation

Cancer is essentially a disease of uncontrolled cell division. It occurs when a mutation disrupts the regulation of the cell cycle, causing cells to bypass the normal checkpoints.

When cells divide uncontrollably, they form masses called tumours.

• Primary Tumour: The original site of the tumour.
• Metastasis: If tumour cells break away from the primary tumour and travel through the blood or lymph system to form new tumours elsewhere in the body. This process is how cancer spreads.

Mutations in two main types of genes often lead to cancer:

1. Proto-oncogenes: Normal genes that code for growth-stimulating proteins. When mutated, they become oncogenes, causing excessive cell division.
2. Tumour Suppressor Genes: Genes that inhibit cell division or initiate apoptosis (programmed cell death). Mutations here eliminate the "brakes" on the cell cycle.

Key Takeaway from Section 4:

The cell cycle is controlled by cyclins, ensuring division is orderly. Cancer results from mutations that cause cells to ignore these regulatory signals, leading to uncontrolled mitosis and tumour formation.

5. Mitotic Index (HL/SL Application)

The Mitotic Index is a valuable diagnostic and investigative tool, especially in studying cancer and growth rates.

5.1 Definition and Calculation

The mitotic index is the ratio between the number of cells undergoing mitosis and the total number of observed cells.

\[ \text{Mitotic Index} = \frac{\text{Number of cells in mitosis (P, M, A, T)}}{\text{Total number of cells}} \]

This is usually expressed as a percentage.

5.2 Significance

• Diagnosis: A high mitotic index in tissue samples (like a biopsy) indicates rapid cell growth, which is a key characteristic of malignant (cancerous) tumours.
• Treatment Monitoring: Following chemotherapy or radiation, doctors can measure the mitotic index to assess how effective the treatment was in slowing down the cell division rate of the tumour.
• Research: Scientists use the mitotic index to study how different factors (like temperature or chemicals) affect growth rates in organisms.

Did you know? Some chemotherapy drugs work by targeting and disrupting the spindle fibres during Metaphase, specifically preventing the cancer cells from completing Mitosis. Since cancer cells divide faster than most normal cells, they are more susceptible to these drugs.