Welcome to "All Cells Arise from Other Cells"
In this chapter, we are going to explore one of the most important rules in Biology: cells don't just appear out of thin air! Every single cell in your body (and in every living thing around you) came from a cell that existed before it. This is a big part of what scientists call Cell Theory.
We will look at how eukaryotic cells (like yours) divide using a cycle, how prokaryotic cells (bacteria) do it much more simply, and why viruses are the "rule-breakers" of the biological world. Don't worry if it seems like a lot of steps at first—we will break it down piece by piece!
1. The Eukaryotic Cell Cycle
Most cells in a multicellular organism (like a human or a plant) have the ability to divide. However, they don't divide all the time. They follow a specific "life insurance" plan called the cell cycle. This ensures that when one cell becomes two, the new cells have everything they need to survive.
Interphase: The Preparation Phase
Think of Interphase as the "behind-the-scenes" preparation for a big performance. The cell isn't just resting; it is incredibly busy!
Key Event: During Interphase, DNA replication occurs. This is vital because if the cell is going to split into two, it needs two identical sets of genetic instructions so both new cells know what to do.
Mitosis: The Division Phase
Mitosis is the part of the cycle where the nucleus actually divides. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell. This means they have the exact same copies of DNA.
To remember the stages of mitosis in order, use the mnemonic: PMAT.
1. Prophase: The chromosomes (which were copied during Interphase) become visible as they thicken and shorten. The nuclear envelope (the "bag" holding the DNA) disappears.
2. Metaphase: Think M for Middle. The chromosomes line up along the center (equator) of the cell. Spindle fibres attach to the centromere of each chromosome.
3. Anaphase: Think A for Away. The spindle fibres shorten and pull the chromatids (the two halves of the chromosome) apart to opposite ends of the cell.
4. Telophase: The chromatids reach the ends of the cell and are now called chromosomes again. Two new nuclear envelopes form around each set.
Cytokinesis: The Final Split
After the nucleus has divided (Mitosis), the rest of the cell needs to catch up. Cytokinesis is the division of the cytoplasm. This physical splitting creates the two separate daughter cells.
Quick Review Box:
- DNA is copied in Interphase.
- Mitosis is the division of the nucleus.
- Cytokinesis is the division of the rest of the cell.
Key Takeaway: The cell cycle ensures genetic continuity. Because the DNA is copied exactly, the new cells can do the same job as the old one.
2. When Division Goes Wrong: Cancer
Mitosis is a very controlled process. The cell has "checkpoints" to make sure everything is going correctly. However, if a mutation occurs in the genes that control this process, the cell can start dividing uncontrollably.
This uncontrolled cell division leads to the formation of tumours and cancers. Many treatments for cancer work by controlling the rate of cell division—essentially trying to hit the "brakes" on the cell cycle.
Did you know? Some chemotherapy drugs work by preventing the spindle fibres from forming. If the cell can't use spindle fibres, it can't finish Metaphase or Anaphase, and the division stops!
3. Binary Fission: How Bacteria Divide
Prokaryotic cells (like bacteria) are much simpler than our cells. They don't have a nucleus, so they don't do mitosis. Instead, they use a process called binary fission.
Step-by-Step Binary Fission:
1. The circular DNA molecule and any plasmids (small loops of DNA) replicate.
2. The cell gets bigger, and the DNA loops move to opposite ends.
3. The cytoplasm begins to divide.
4. A new cell wall forms, producing two daughter cells. Each cell has one copy of the circular DNA, but they can have a variable number of copies of the plasmids.
Common Mistake to Avoid: Don't say bacteria do mitosis! They don't have a nucleus or linear chromosomes, so they must use binary fission.
4. Viruses: The Non-Living Replicators
Viruses are described as non-living and acellular (not made of cells). Because they aren't cells, they cannot undergo cell division like mitosis or binary fission.
Instead, they are like biological pirates. They "hijack" a host cell. A virus injects its nucleic acid (DNA or RNA) into a healthy host cell. The host cell is then forced to use its own "machinery" (like ribosomes and enzymes) to build and assemble new virus particles.
Key Takeaway: Bacteria divide themselves (Binary Fission), but viruses must use a host cell to replicate.
5. Required Practical 2: Looking at Mitosis
In your lab work, you will likely prepare a root tip squash. We use root tips because that is where the plant is growing quickly, so lots of cells are in mitosis!
Calculating the Mitotic Index
The Mitotic Index is just a way of saying "what percentage of cells are currently dividing?"
\( \text{Mitotic Index} = \frac{\text{Number of cells with visible chromosomes (in mitosis)}}{\text{Total number of cells observed}} \)
Measuring Cell Size
When you look through a microscope, the cell looks much bigger than it actually is. You can calculate the actual size using this formula:
\( \text{Actual Size} = \frac{\text{Size of Image}}{\text{Magnification}} \)
Analogy: If you see a giant 10cm ant in a photo, and the caption says "10x magnification," you know the real ant is only 1cm long. It's the same for cells!
Key Takeaway for Practicals: Always ensure you are counting all cells in the field of view for the "total" number, not just the ones that look "interesting."
Summary: The Big Picture
- Eukaryotic cells use the Cell Cycle (Interphase + PMAT + Cytokinesis) to make identical clones for growth and repair.
- Uncontrolled division leads to cancer.
- Prokaryotic cells use Binary Fission to replicate their circular DNA and plasmids.
- Viruses are non-living and must infect host cells to replicate.
- Microscopy allows us to see these stages and calculate the Mitotic Index to see how fast a tissue is growing.