All Cells Arise from Other Cells

What does ‘All cells arise from other cells’ mean?

Every cell in your body (about 37 trillion) ultimately came from a single fertilised egg. That egg divided into two cells, those cells divided again, and so on through many rounds of cell division. Every new cell arose from a pre-existing cell dividing. This is what the idea “all cells arise from other cells” means.

The Cell Cycle

Before a cell can divide, it must prepare carefully. The cell cycle describes the full sequence of events in a cell’s life, from its formation by the division of a parent cell to its own division into two daughter cells.

The cell cycle has two main stages:

• Interphase: the cell grows and replicates its DNA. This is the longest part of the cycle.
• M phase: the cell divides, first the nucleus (mitosis), then the cytoplasm (cytokinesis).

Interphase

During interphase, chromosomes are decondensed and not visible under a light microscope. It is subdivided into three stages:

M phase (Mitosis)

Once interphase is complete, the cell enters the M phase, starting with mitosis, where the nucleus divides, distributing one copy of each chromosome to each end of the cell.

M phase (Cytokinesis)

After the cell has completed mitosis (M phase) the cytoplasm divides, producing two separate daughter cells. Note: Cytokinesis is not part of mitosis, as it occurs after telophase.

Stages of Mitosis

Mitosis produces genetically identical daughter cells and is used for growth, repair, and asexual reproduction.

Sexual reproduction uses meiosis instead (covered in Genetic Diversity: Mutation and Meiosis), which produces genetically varied gametes.

Step 1: Prophase

• Chromosomes condense (shorten and thicken), becoming visible under a microscope
• The nuclear envelope breaks down
• Spindle fibres form at opposite poles of the cell, and in late prophase they begin to attach to the centromeres

Step 2: Metaphase

• Spindle fibres are attached to the centromeres of each chromosome
• Chromosomes are moved to the equator (middle) of the cell, which is called the metaphase plate
• Each chromosome is held under tension, pulled equally toward both poles

Step 3: Anaphase

• The centromeres divide (split)
• Spindle fibres shorten, pulling the two sister chromatids of each chromosome to opposite poles
• Each chromatid is now called a chromosome in its own right
• The cell elongates as chromosomes move apart

Step 4: Telophase

• Chromosomes arrive at the poles and begin to decondense
• A nuclear envelope reforms around each group of chromosomes
• Two new nuclei are now present in a single cell

Cytokinesis

• The cytoplasm divides to produce two separate daughter cells
• In animal cells: the membrane pinches inward at the equator
• In plant cells: a new cell plate forms at the equator, which becomes the new cell wall

Required Practical 2: Root Tip Squash

Root tip cells are ideal for studying mitosis because the root tip meristem (zone of actively dividing cells) contains many cells in different stages of the cell cycle.

The method

1. Cut the terminal ~5 mm from a root tip (this is the meristem region)
2. Place in hydrochloric acid (HCl) for ~10 minutes
3. Rinse in distilled water, then add toluidine blue stain (stains chromosomes purple)
4. Place on a slide with a coverslip and gently squash the tissue
5. Observe under an optical microscope

Calculating the mitotic index

The mitotic index is a measure of how many cells in a sample are actively dividing:

$$\text{Mitotic index} = \frac{\text{number of cells in mitosis}}{\text{total number of cells observed}}$$

When looking at diagrams you can assume if we can see visible chromosomes the cell is in mitosis.

A higher mitotic index means a greater proportion of cells are dividing. Cancer cells typically have a high mitotic index; mature, differentiated cells have a low one.

Cancer (Uncontrolled Cell Division)

The cell cycle is normally controlled cell division. There are certain proteins that check whether the cell is ready to progress from one phase to the next (e.g. whether DNA has been correctly replicated before mitosis begins).

Mutations in the genes involved in regulating the cell cycle can lead to uncontrolled cell division, which leads to the formation of tumours and cancers.

Examples of genes that control the cell cycle (mutations in these can lead to cancer, covered in detail in Unit 8):

• Tumour suppressor genes
• Proto-oncogenes (which can mutate into oncogenes)

Cancer Treatments

Many cancer treatments work by targeting rapidly dividing cells, for example chemotherapy drugs that block mitosis or DNA replication.

This is why cancer treatments also affect healthy rapidly dividing cells (e.g. hair follicle cells, gut lining cells).

Binary Fission in Prokaryotes

Prokaryotic cells (bacteria) have no nucleus and no linear chromosomes. They cannot undergo mitosis (which depends on spindle fibres and a nuclear envelope). Instead, bacteria divide by binary fission, a simpler and faster process. This is also exploited in in vivo cloning to amplify DNA.

Steps of binary fission

1. The circular DNA replicates (using DNA polymerase and helicase, exactly as in semi-conservative replication)
2. Plasmids also replicate independently (number of copies passed to daughter cells varies)
3. The cell elongates, and the two DNA copies move to opposite ends
4. A new cell wall and membrane forms across the middle
5. The cell splits into two daughter cells, each with
   • One copy of the circular DNA
   • Variable number of plasmids

Because prokaryotes can divide in as little as 20 minutes under ideal conditions, binary fission allows bacteria to grow into very large populations extremely rapidly.

Binary fission vs mitosis

Viral Replication

Viruses are acellular and non-living

Viruses are not cells. They can be described as ‘acellular’ (not made of cells, as they have no cytoplasm and no organelles).

They are ‘non-living’ because they have no ribosomes, so they cannot make proteins or replicate on their own. Instead they must hijack the machinery of a host cell (e.g. HIV uses T helper cells as its host cell).

Steps of viral replication

1. Attachment proteins on the virus bind to complementary receptors on the host cell surface
2. The viral nucleic acid (DNA or RNA) enters the host cell
3. The nucleic acid is replicated inside the cell using host cell enzymes
4. The host cell’s ribosomes produce viral proteins (translation)
5. New virus particles are assembled and released by either lysis (bursting the cell) or budding. This often kills the host cell.

The HIV page covers a detailed example. HIV is a retrovirus that uses reverse transcriptase to convert its RNA into DNA, which is then incorporated into the host cell’s genome.

Exam Question Practice

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