Biology (9700) | The mitotic cell cycle

Welcome to Topic 5: The Mitotic Cell Cycle

Hello Biologists! This chapter is fundamental because it explores how cells divide to create new, genetically identical copies. Whether you're growing taller, healing a cut, or replacing millions of old blood cells, the process of mitosis is responsible. Understanding the cell cycle is key to grasping concepts like growth, repair, and even diseases like cancer.

Don't worry if all the terminology seems confusing at first—we will break down the process into easy, memorable steps!

1. The Structure of a Chromosome

Before a cell can divide, the genetic material needs to be perfectly organised and packaged into structures called chromosomes. Think of a chromosome as a highly organised spool of thread (DNA).

Key Components (5.1.1)

1. DNA: This is the genetic information itself, the molecule that carries the instructions for the cell's structure and function.

2. Histone Proteins: The long DNA molecule wraps tightly around small basic proteins called histones. This packaging allows the massive length of DNA to fit inside the tiny nucleus.

3. Sister Chromatids: Once the DNA has been replicated (copied) but before division, the chromosome consists of two identical strands, known as sister chromatids. (Analogy: They are like identical twin arms, attached at the elbow.)

4. Centromere: This is the narrow region where the two sister chromatids are joined. It is essential for the separation process during mitosis.

5. Telomeres: These are repetitive, non-coding sequences of DNA found at the ends of linear chromosomes. (Analogy: Think of the plastic caps on the ends of shoelaces—they prevent the main body of the shoelace from fraying.)

Did you know? (5.1.4)

The primary role of telomeres is preventing the loss of genes from the ends of chromosomes during DNA replication. Each time DNA replicates, a small section at the very end cannot be fully copied, leading to shortening. Telomeres buffer this loss, protecting the important coding regions.

2. Outlining the Mitotic Cell Cycle (5.1.3)

The mitotic cell cycle is the sequence of events that takes place in a cell between one cell division and the next. It has two main phases: Interphase and the Mitotic (M) Phase.

2.1 Interphase: The Preparation Phase

Interphase is often incorrectly called the "resting phase," but it is actually a period of intense metabolic activity and preparation! The cell spends most of its life here.

Interphase is divided into three sub-phases:

1. G₁ Phase (Gap 1):

• The cell grows and synthesises new organelles (like mitochondria and ribosomes).
• Proteins required for DNA synthesis are produced.
• The cell checks its internal and external environment to ensure conditions are right for division.

2. S Phase (Synthesis):

• The most crucial step: DNA replication occurs.
• Each chromosome replicates its DNA, resulting in two identical sister chromatids.
• At the end of S phase, the cell has double the amount of DNA, but the chromosome number remains the same (e.g., 46 chromosomes, each now duplicated).

3. G₂ Phase (Gap 2):

• Continued growth and energy stores are increased.
• Proteins and materials necessary for the actual cell division (mitosis and cytokinesis) are synthesised, particularly microtubules for the spindle.
• Final checks are made to ensure DNA replication was successful and complete.

2.2 The M Phase

The M phase is the division stage, consisting of two major events:

1. Mitosis: The division of the nucleus (nuclear division), ensuring each daughter nucleus receives an identical set of chromosomes.

2. Cytokinesis: The division of the cytoplasm and cell membrane, resulting in two separate daughter cells.

3. Mitosis: The PMAT Stages (5.2.1, 5.2.2)

Mitosis is a continuous process, but biologists divide it into four stages for easier study. Remember the mnemonic: PMAT (Prophase, Metaphase, Anaphase, Telophase).

3.1 Prophase

The "Packing Up" stage:

• The chromosomes, which were long and thread-like (chromatin) during interphase, begin to shorten, thicken, and become visible under a light microscope. This process is called condensation.

• The nuclear envelope (membrane) breaks down into small vesicles.

• The nucleolus disappears.

• In animal cells, centrioles move to opposite poles of the cell and begin forming the spindle apparatus (made of microtubules).

3.2 Metaphase

The "Middle Alignment" stage:

• The replicated chromosomes line up individually along the equator (centre) of the cell. This central plane is known as the metaphase plate.

• Spindle fibres, extending from the poles, attach to the centromere of each sister chromatid pair.

This alignment is crucial—it ensures that when they split, the copies are evenly distributed.

3.3 Anaphase

The "Apart/Away" stage:

• The centromere of each chromosome splits.

• The sister chromatids are now considered individual chromosomes (unreplicated) and are pulled rapidly apart towards opposite poles of the cell by the shortening spindle fibres.

• This movement is often described as a 'V' shape as the centromere leads the way, dragging the arms behind.

3.4 Telophase

The "Tidy Up" stage (the reverse of Prophase):

• The chromosomes arrive at the poles and begin to uncoil and lengthen (decondense), becoming less distinct.

• A new nuclear envelope forms around each set of chromosomes at the poles, resulting in two new nuclei.

• The spindle apparatus breaks down.

3.5 Cytokinesis

• This is the final step: the division of the cytoplasm and the cell surface membrane to form two separate, genetically identical daughter cells.

• In animal cells, the membrane pinches inwards, forming a cleavage furrow.

• In plant cells (which have rigid cell walls), a cell plate forms in the middle, developing into a new cell wall.

4. The Importance and Control of Mitosis

4.1 Why is Mitosis Important? (5.1.2)

Mitosis produces two daughter cells that are genetically identical to the parent cell. This genetic uniformity is vital for several processes:

1. Growth of Multicellular Organisms: From zygote to adult, mitosis increases the number of cells in the body.

2. Replacement of Damaged or Dead Cells: Mitosis ensures that new cells produced (like skin or intestinal cells) function identically to the cells they replace.

3. Repair of Tissues: If you cut yourself, mitosis rapidly produces new cells to bridge the wound and repair the tissue.

4. Asexual Reproduction: For single-celled organisms (like Yeast) or certain plants, mitosis is the method of reproduction, creating clones.

4.2 The Role of Stem Cells (5.1.5)

Mitosis is closely linked to the function of stem cells.

• Stem cells are unspecialised (undifferentiated) cells capable of dividing by mitosis indefinitely.

• They can then differentiate (specialise) into various cell types (e.g., muscle, nerve, or blood cells).

• Their role in cell replacement and tissue repair is fundamental because they provide the continuous supply of new cells needed to maintain tissues that experience high wear-and-tear (like skin or the lining of the gut).

4.3 Uncontrolled Cell Division and Tumours (5.1.6)

The cell cycle is tightly controlled by complex mechanisms (checkpoints) to prevent errors and ensure division only happens when appropriate.

• If these control mechanisms fail, the cell may start dividing uncontrollably, ignoring the normal signals to stop or pause.

• This continuous, uncontrolled cell division results in a mass of abnormal cells called a tumour.

• Tumour formation represents the simplest definition of cancer at the cellular level—a disease resulting from disruptions to the mitotic cell cycle regulatory system.

5. Interpreting Mitotic Images (5.2.2)

You must be able to interpret diagrams, photomicrographs, and microscope slides to identify the stages of the cell cycle (Interphase and PMAT).

How to Identify the Stages

1. Interphase:

• The nucleus is usually large, clear, and defined.
• Chromosomes are invisible as distinct structures (they are long, decondensed chromatin).

2. Prophase:

• Chromosomes become thick and distinct (look like spaghetti strands).
• Nuclear envelope starts to break up (may look blurry or absent).

3. Metaphase:

• Chromosomes are clearly lined up single-file along the centre of the cell (the metaphase plate).
• Look for an organised, straight line.

4. Anaphase:

• Chromosomes are moving away from the centre towards the poles.
• Look for two distinct groups of chromosomes that appear V-shaped, separated by a gap.

5. Telophase/Cytokinesis:

• Two distinct clusters of chromosomes are visible at the poles.
• New nuclear membranes start to form.
• The cell surface membrane begins to pinch inwards (cleavage furrow) or a cell plate starts to form in the middle.