Biology (9700) | Chromosome behaviour in mitosis

Welcome to Chromosome Behaviour in Mitosis!

Have you ever wondered how a single fertilised egg turns into a complex human body with trillions of cells, or how a cut on your finger heals perfectly? The answer lies in mitosis!

Mitosis is nuclear division that produces two daughter cells genetically identical to the parent cell. This chapter, 5.2, focuses on the choreography—the precise, step-by-step movement of chromosomes—that makes this perfect duplication possible. Don't worry if the stages seem confusing; we will use clear steps and mnemonics to master this vital process!

Quick Review: What Are We Dividing?

Before the M phase (Mitosis) even begins, the cell is in Interphase (G₁, S, and G₂ phases). By the time mitosis starts, the DNA has already replicated in the S phase.

• Chromosome Structure: Each chromosome consists of two identical strands of DNA called sister chromatids.
• Centromere: This is the region where the two sister chromatids are joined together. It is also where the spindle fibres attach.
• Genetic Goal: Ensure that each new daughter cell receives one exact copy of every chromosome.

The Four Stages of Mitosis (P.M.A.T.)

Mitosis is a continuous process, but scientists divide it into four main stages based on the appearance and location of the chromosomes. Use the mnemonic P-M-A-T to remember the order: Prophase, Metaphase, Anaphase, Telophase.

1. Prophase (P = Prepare or Pack up)

This is the "preparation" stage where the cell gets everything ready for the big split.

A. Chromosome Behaviour

• The long, diffuse chromatin threads start to coil up and condense, becoming shorter and thicker. This makes them visible as discrete structures (the familiar X shape) when viewed under a microscope.
• They appear as two sister chromatids joined at the centromere.

B. Associated Structures (What else is happening?)

• Nuclear Envelope: The nuclear envelope (membrane) breaks down into small vesicles. This allows the chromosomes to be accessed by the spindle.
• Nucleolus: The nucleolus disappears.
• Spindle: The spindle fibres (made of microtubules) begin to form. In animal cells, the centrioles (which duplicate during interphase) move to opposite poles of the cell, forming the focus points for the spindle.

2. Metaphase (M = Middle or Metaphase Plate)

This is a quick and critical stage for organization.

A. Chromosome Behaviour

• The chromosomes line up individually along the centre plane of the cell, known as the metaphase plate (or equatorial plate).
• They line up single file, perpendicular to the spindle axis.

B. Associated Structures

• Spindle: The fully formed spindle apparatus is now visible, stretching from pole to pole.
• Attachment: Spindle fibres (microtubules) attach to the centromere region of each chromosome. Crucially, fibres from opposite poles attach to opposite sister chromatids.
• Did you know? This precise alignment ensures that when the chromatids separate, each daughter cell receives exactly the right complement of genetic material. If this stage goes wrong, it can lead to genetically non-identical cells.

3. Anaphase (A = Away or Apart)

This is the stage of active movement and physical separation.

A. Chromosome Behaviour

• The centromeres divide, separating the sister chromatids.
• Once separated, each chromatid is considered a full, independent daughter chromosome.
• These daughter chromosomes are pulled rapidly apart (or away) towards opposite poles of the cell.

B. Associated Structures

• Spindle: The movement is caused by the shortening of the spindle fibres (microtubules), which literally reel in the chromosomes.
• The cell often elongates during this phase.

4. Telophase (T = Two Nuclei and Tidy Up)

Telophase essentially reverses the processes of prophase.

A. Chromosome Behaviour

• The daughter chromosomes arrive at the poles and begin to decondense (uncoil), reverting back to long, thin chromatin threads.
• They become less visible under the microscope.

B. Associated Structures

• Nuclear Envelope: A new nuclear envelope forms around each set of daughter chromosomes at the two poles, creating two distinct nuclei.
• Nucleolus: The nucleoli reappear in each new nucleus.
• Spindle: The spindle fibres disintegrate and break down.

The Division of the Cell: Cytokinesis

Cytokinesis is the division of the cytoplasm and the cell surface membrane (and cell wall, if applicable) to form two separate daughter cells. It usually begins during late anaphase or telophase.

Differences between Animal and Plant Cell Cytokinesis

• Animal Cells: The cell surface membrane pinches inwards along the metaphase plate, forming a cleavage furrow. This furrow deepens until the cell separates into two.
• Plant Cells: Plant cells have a rigid cell wall, so they cannot form a cleavage furrow. Instead, vesicles containing cell wall material gather at the centre of the cell (where the metaphase plate was) and fuse to form a cell plate. The cell plate eventually extends outwards and fuses with the existing cell wall, dividing the cell.

Identifying Mitotic Stages (Practical Skill 5.2.2)

You must be able to identify these stages from diagrams, photomicrographs, and prepared slides (often of root tip cells, as they divide rapidly). Focus on the location and appearance of the chromosomes:

Quick Review Box: How to Identify Each Phase

Remember: Mitosis ensures genetic uniformity—the two new cells are clones of the parent cell, crucial for growth and tissue repair.