Biology (9700) | Replication and division of nuclei and cells

Welcome to the World of Cell Division!

Hello future Biologists! This chapter might seem a bit detailed, but it is one of the most fundamental topics in A-Level Biology. Why? Because cell division is how you grew from a single fertilised egg into the amazing organism you are today!

We are focusing on the mitotic cell cycle (Topic 5), which is the process eukaryotic cells use to divide their nucleus (mitosis) and then their cytoplasm (cytokinesis) to produce two genetically identical daughter cells. This is essential for growth, repair, and asexual reproduction.

Key Takeaway from the Introduction:

Mitosis ensures that when cells divide, the two new cells are genetically identical to the parent cell. Think of it as making a perfect photocopy of the cell's genetic material.

5.1 Replication and Division of Nuclei and Cells

Understanding the Chromosome Structure

Before a cell divides, the genetic material must be organised perfectly. The entire length of DNA in a cell is packaged into structures called chromosomes. Let’s break down the components:

Components of a Chromosome:

• DNA: This is the long, linear molecule containing the genetic instructions.
• Histone Proteins: The DNA is tightly wound around specialised proteins called histones. This complex structure (DNA wrapped around histones) is called chromatin.
• Centromere: This is the crucial, condensed region where the two identical copies of the chromosome are held together after replication. It looks like a waist on an 'X' shape.
• Sister Chromatids: Once the DNA has replicated (copied itself), the two identical copies held together by the centromere are called sister chromatids.
• Telomeres: These are repetitive sequences of non-coding DNA found at the ends of chromosomes.

Did You Know? The Role of Telomeres (5.1.4)

Telomeres act like the plastic tips on shoelaces. Every time DNA replicates, the very end of the linear chromosome cannot be fully copied. Without telomeres, vital genes would be lost with every cell division. Telomeres prevent this, protecting the important genes from degradation or loss during DNA replication.

The Mitotic Cell Cycle Outline (5.1.3)

The cell cycle is the continuous process of growth and division. It is divided into two main sections: Interphase (the preparation time) and the Mitotic (M) Phase (actual division).

1. Interphase (The Long Preparation Phase)

Interphase is when the cell grows, checks its environment, and prepares for division. Don't worry, it’s not resting! This is where the cell spends most of its life, busy doing its job (like making hormones or producing energy).

Interphase is broken down into three sub-phases:

G₁ Phase (First Growth Phase):
The cell grows in size and synthesises proteins and new organelles (like mitochondria and ribosomes).

S Phase (Synthesis Phase):
DNA replication occurs. This is the most critical step, ensuring that each chromosome is duplicated, producing sister chromatids. The nucleus now contains twice the amount of DNA.

G₂ Phase (Second Growth Phase):
The cell continues to grow and synthesises proteins and enzymes necessary for mitosis (e.g., tubulin, which forms the spindle fibres).

2. The M Phase (Division Phase)

• Mitosis: The division of the nucleus (P, M, A, T stages).
• Cytokinesis: The division of the cytoplasm, resulting in two separate daughter cells.

The Importance of Mitosis (5.1.2, 5.1.5)

Mitosis is vital because it creates new cells that are genetically identical to the original parent cell. This is crucial for several biological functions:

1. Growth of Multicellular Organisms:
A fertilised egg divides repeatedly through mitosis to form the many trillions of cells in an adult body.

2. Repair of Tissues/Replacement of Cells:
When you get a cut or burn, mitosis quickly produces new, identical skin cells to heal the wound. Red blood cells, which only live for about 120 days, are constantly replaced by cells produced through mitosis in the bone marrow.

3. Asexual Reproduction:
Organisms like yeast or simple plants reproduce by mitosis, creating offspring that are exact clones of the parent.

The Role of Stem Cells (5.1.5)

Mitosis is especially important for stem cells. Stem cells are undifferentiated cells (cells that haven't decided what they want to be yet). They continuously divide by mitosis to produce more stem cells (self-renewal) and cells that can differentiate into specialised tissues (like muscle or nerve cells). This process drives tissue repair and replacement throughout life.

5.2 Chromosome Behaviour in Mitosis

Mitosis itself is a continuous process, but scientists divide it into four main stages to make it easier to understand: Prophase, Metaphase, Anaphase, and Telophase (PMAT).

Don't worry if this seems tricky at first! Focus on where the chromosomes are and what the nuclear envelope is doing.

Stage 1: Prophase

• Chromosome Behaviour: The chromatin fibres condense and coil, becoming visible as distinct chromosomes (each made of two sister chromatids joined at the centromere).
• Associated Behaviour: The nuclear envelope breaks down. In animal cells, centrioles move to opposite poles and begin to form the spindle fibres (a framework of microtubules).

Analogy: Think of Prophase as winding up tangled yarn into neat, visible balls (chromosomes) and clearing the room (breaking down the nuclear envelope) for the next step.

Stage 2: Metaphase

• Chromosome Behaviour: The centromeres of all the chromosomes line up exactly along the cell's equator or centre plane. This line is called the metaphase plate.
• Associated Behaviour: The spindle fibres, which radiate from the poles, attach to the centromeres of the sister chromatids.

Stage 3: Anaphase

This is often the shortest, but most crucial, stage.

• Chromosome Behaviour: The spindle fibres contract, pulling the centromeres apart. The sister chromatids separate, instantly becoming individual chromosomes. These new individual chromosomes are pulled rapidly towards opposite poles of the cell (V-shaped or J-shaped when viewed under a microscope).
• Associated Behaviour: This movement ensures that each pole receives an identical set of genetic information.

Stage 4: Telophase

Telophase essentially reverses the events of prophase.

• Chromosome Behaviour: The identical sets of chromosomes arrive at the poles and begin to uncoil, returning to their diffuse chromatin state.
• Associated Behaviour: New nuclear envelopes form around the two separate groups of chromosomes at each pole, resulting in two genetically identical nuclei within one cell. The spindle fibres break down.

Cytokinesis (Division of the Cell)

Cytokinesis usually overlaps with the late stages of mitosis (Anaphase and Telophase).

• Animal Cells: The cell surface membrane pinches inwards around the centre of the cell, forming a cleavage furrow until the cell splits completely into two daughter cells.
• Plant Cells: Due to the rigid cell wall, a new cell plate forms across the middle of the cell, which then develops into new cell walls and cell surface membranes, separating the two daughter cells.

When Cell Division Goes Wrong: Tumours (5.1.6)

Cell division is normally tightly controlled by various genes (proto-oncogenes promote division; tumour suppressor genes prevent it).

Sometimes, mutations occur in these control genes, causing the cell cycle checkpoints to fail. This leads to uncontrolled cell division.

If a cell begins to divide relentlessly without responding to the usual signals (like contact inhibition or internal checkpoints), it forms a mass of cells known as a tumour. The formation and progression of a tumour is the basic mechanism of cancer.

Key Takeaway: Control is Everything

Mitosis is a controlled, precise process that yields genetically identical cells for specific biological needs (growth, repair). A loss of this precise control leads to the formation of tumours.