All cells arise from other cells

Welcome to "All Cells Arise from Other Cells"

Hello! Today we are looking at one of the most fundamental rules of Biology: Biogenesis. This is the fancy way of saying that cells don't just appear out of thin air. Whether it is a bacterium, a plant, or a human, every cell you see came from a "parent" cell that divided.

Don't worry if cell division feels like a lot of steps at first. We will break it down into a clear story, from how your body grows to how viruses "hijack" cells to make more of themselves.

1. The Eukaryotic Cell Cycle

In complex organisms like us (eukaryotes), cells go through a regular cycle of growth and division called the cell cycle. Think of it like a clock where most of the time is spent preparing, and only a small slice is spent actually dividing.

Interphase: The Preparation Phase

Before a cell can divide, it needs to get ready. This long period is called interphase.
During this time:
1. The cell grows and makes more organelles (like mitochondria).
2. DNA replication occurs. The cell makes a perfect copy of all its DNA so that both new cells will have the full set of instructions.

Common Mistake to Avoid: Many students think mitosis is the whole cell cycle. It's not! Interphase is the longest part; mitosis is just the final split.

Mitosis: The Division Phase

Mitosis is the part of the cycle where the nucleus divides. The goal is to produce two genetically identical daughter cells.

Memory Aid (The Mnemonic): To remember the stages of mitosis in order, just remember PMAT:
Prophase
Metaphase
Anaphase
Telophase

Step-by-Step Mitosis:

1. Prophase: The chromosomes become visible (they condense and get "fat"). The nuclear envelope (the "bag" holding the DNA) disappears.
2. Metaphase: Think M for Middle. The chromosomes line up along the center of the cell. Spindle fibres attach to the centromere (the "button" holding the chromosome copies together).
3. Anaphase: Think A for Away. The spindle fibres shorten and pull the chromatids (identical halves of the chromosome) to opposite ends of the cell.
4. Telophase: Two new nuclear envelopes form around the two sets of DNA. The chromosomes start to uncoil and disappear.

Cytokinesis: The Final Cut

Once the DNA is divided, the cytoplasm itself must split. This is called cytokinesis. The cell membrane pinches in, and voila!—you have two separate, identical cells.

Quick Review: The Cell Cycle = Interphase (Growth & DNA copy) + Mitosis (DNA split) + Cytokinesis (Physical split).

2. When Division Goes Wrong: Cancer

Usually, the cell cycle is strictly controlled by genes. However, if a mutation (a change in the DNA) occurs in these "control genes," the cell might start dividing out of control.

- Tumours: A mass of abnormal cells caused by uncontrolled division.
- Cancer Treatments: Many medicines work by disrupting the cell cycle. For example, some drugs stop DNA from replicating, and others stop the spindle fibres from working in metaphase.

Did you know? Because cancer drugs target any rapidly dividing cells, they often affect healthy cells that divide fast, too—like hair follicles! This is why some treatments cause hair loss.

3. Binary Fission: How Bacteria Divide

Bacteria are prokaryotes. They don't have a nucleus, so they don't do mitosis. Instead, they use a simpler process called binary fission.

The Process:
1. The circular DNA and the plasmids (small loops of DNA) replicate.
2. The cell gets bigger, and the DNA loops move to opposite poles.
3. The cytoplasm begins to divide, and a new cell wall forms.
4. The cell splits into two.

Important Point: In binary fission, the main DNA loop is copied exactly once, but plasmids can be copied many times. This means the two new daughter cells might have a variable number of plasmids.

4. Viruses: The Non-Living Replicators

Viruses are acellular (not made of cells) and non-living. Because they aren't alive, they cannot divide by themselves. Instead, they act like biological pirates.

How they replicate:
1. They use attachment proteins to "lock onto" a host cell.
2. They inject their nucleic acid (DNA or RNA) into the host cell.
3. The host cell is tricked into using its own "machinery" (ribosomes and enzymes) to build new virus particles.
4. The new viruses burst out of the cell to find new victims.

5. Required Practical & Math Skills

You will often be asked to look at cells from a plant root tip under a microscope. We use root tips because that is where the most growth (and mitosis) is happening!

Calculating the Mitotic Index

The Mitotic Index tells us what percentage of cells in a sample are currently dividing.
\( \text{Mitotic Index} = \frac{\text{Number of cells with visible chromosomes}}{\text{Total number of cells observed}} \times 100 \)

Calculating Actual Size

When looking through a microscope, remember this simple formula:
\( \text{Actual Size} = \frac{\text{Size of Image}}{\text{Magnification}} \)

Top Tip: Always make sure your measurements for "Image Size" and "Actual Size" are in the same units (usually micrometers \( \mu m \)) before you do the math!

Summary Key Takeaways

- Eukaryotic cells use the cell cycle (Interphase, Mitosis, Cytokinesis) to create identical copies.
- Mitosis has four main stages: Prophase, Metaphase, Anaphase, and Telophase (PMAT).
- Prokaryotic cells (bacteria) divide by Binary Fission.
- Viruses are non-living and must hijack a host cell to replicate.
- Cancer is the result of uncontrolled cell division caused by mutations.
- Mitotic Index is a ratio showing how many cells are dividing vs. the total number of cells.