Introduction to the Cell Cycle and Mitosis
Have you ever wondered how a tiny embryo grows into a complex human being, or how your skin heals after a scrape? The answer lies in the cell cycle and mitosis. In this chapter, we will explore how cells prepare for division, how they ensure every new cell gets a perfect copy of the DNA, and how we can calculate the rate of this growth using the mitotic index.
Don’t worry if this seems like a lot of steps at first! We’ll break it down into simple stages and use a few tricks to help you remember the order of events.
1. The Cell Cycle: More Than Just Dividing
Many students think that cells are always dividing, but mitosis is actually just a small part of a cell's life. The cell cycle is an ordered sequence of events that a cell goes through as it grows and divides. It is divided into two main phases: Interphase and the Mitotic phase (M-phase).
Interphase: The Preparation Phase
Think of Interphase as the "behind-the-scenes" preparation. The cell spends about \(90\%\) of its time here. It isn't "resting"; it is incredibly busy performing its normal functions and getting ready for the big split. Interphase is divided into three parts:
1. G1 Phase (First Gap): The cell grows larger, makes new organelles (like mitochondria and ribosomes), and carries out normal functions.
2. S Phase (Synthesis): This is the most critical part! The cell replicates its DNA. By the end of this phase, every chromosome consists of two identical sister chromatids joined at a centromere.
3. G2 Phase (Second Gap): The cell continues to grow and synthesizes the proteins needed for division (like the spindle fibers).
Quick Review: If a cell starts with \(46\) chromosomes, after the S phase, it still has \(46\) chromosomes, but each one is "double-armed" and contains twice the amount of DNA!
2. The Stages of Mitosis (PMAT)
Mitosis is the process where the nucleus divides to create two genetically identical daughter nuclei. To remember the order of the stages, just remember the name PMAT: Prophase, Metaphase, Anaphase, and Telophase.
Prophase: The Preparation
• The chromosomes condense (coil up tightly), becoming visible under a light microscope.
• The nucleolus disappears and the nuclear envelope begins to break down.
• Centrioles (which you learned about in the ultrastructure chapter) move to opposite poles of the cell.
• Spindle fibers begin to form from the centrioles.
Metaphase: The Middle
• The nuclear envelope is completely gone.
• The spindle fibers attach to the centromeres of the chromosomes.
• The chromosomes line up along the equator (the middle) of the cell.
Anaphase: The Action
• The centromeres split.
• The spindle fibers shorten, pulling the sister chromatids apart.
• The chromatids (now called individual chromosomes) move toward opposite poles of the cell.
Telophase: The Two Nuclei
• The chromosomes reach the poles and begin to de-condense (become long and thin again).
• Two new nuclear envelopes form around each set of chromosomes.
• The nucleolus reforms in each new nucleus.
Memory Aid:
Prophase = Prepare
Metaphase = Middle
Anaphase = Apart
Telophase = Two nuclei
3. Cytokinesis: The Final Split
Mitosis is the division of the nucleus, but cytokinesis is the division of the cytoplasm. This usually happens during or immediately after telophase. The cell membrane pinches in, dividing the cell into two separate, genetically identical daughter cells.
Note: For more on how variation is created in other types of division, see the chapter on "Meiosis, Linkage and Genetic Variation".
4. The Mitotic Index
In a tissue where growth is happening (like a root tip or a healing wound), many cells will be in various stages of mitosis at any given time. We can calculate the Mitotic Index to see how quickly the tissue is growing.
The Formula
The Mitotic Index is the ratio of cells undergoing mitosis to the total number of cells observed:
\( \text{Mitotic Index} = \frac{\text{Number of cells with visible chromosomes (in mitosis)}}{\text{Total number of cells observed}} \)
Example Calculation:
If you look through a microscope and see \(20\) cells in various stages of mitosis (prophase, metaphase, etc.) and \(80\) cells in interphase, the total number of cells is \(20 + 80 = 100\).
\( \text{Mitotic Index} = \frac{20}{100} = 0.2 \)
(You can also express this as a percentage: \(20\%\)).
Common Mistake: Forgetting to include the mitotic cells in your "total count." The denominator must be every cell in the field of view, including those in interphase!
5. Core Practical 6: The Root Tip Squash
In your exams, you may be asked about the procedure for Core Practical 6. We use the growing tips of plant roots (usually onions or garlic) because this is where mitosis is most active.
Step-by-Step Procedure:
1. Preparation: Cut a small piece (about \(5\text{mm}\)) from the very tip of a growing root.
2. Acid Hydrolysis: Place the root tip in hydrochloric acid. This breaks down the middle lamella (the "glue" holding plant cells together) so the tissue can be squashed easily.
3. Staining: Rinse the root and add a stain (like ethano-orcein or toluidine blue). This is vital because DNA and chromosomes are normally transparent; the stain makes them visible.
4. Squashing: Place the tip on a slide, add a coverslip, and press down firmly with your thumb. Crucial Tip: Do not smear the coverslip sideways; pressing straight down creates a single thin layer of cells without breaking the chromosomes.
5. Observation: Use a light microscope to identify the stages of mitosis and calculate the mitotic index.
Did you know? We use the tip because that is the site of the apical meristem—the region of the plant where cells are constantly dividing to make the root grow longer!
Key Takeaways Summary
• Interphase (G1, S, G2) is for growth and DNA replication.
• Mitosis produces two genetically identical daughter nuclei through four stages: Prophase, Metaphase, Anaphase, and Telophase.
• Cytokinesis is the physical splitting of the cell body.
• The Mitotic Index is a mathematical way to measure how many cells in a sample are currently dividing.
• Core Practical 6 involves staining and squashing root tips to see these stages in action using the formula \( \text{magnification} = \frac{\text{image size}}{\text{real size}} \) to measure what you see!