Preview
IB DP Biology — D: Continuity and Change | SL | D2.1 Cell and Nuclear Division
📋 What You Will Learn
- Identify the stages of the cell cycle (G1, S, G2, mitosis, cytokinesis) and describe what occurs at each stage
- Explain the events of mitosis — prophase, metaphase, anaphase, telophase — and how they produce two genetically identical daughter cells
- Explain the events of meiosis I and meiosis II, including crossing over and independent assortment, and how they generate genetic variation
- Analyse micrograph data to identify and calculate the mitotic index and infer the relative duration of cell cycle stages
- Evaluate how cell cycle checkpoints regulate division and how failures in regulation lead to cancer
- Compare mitosis and meiosis in terms of purpose, chromosome number, and genetic outcome
🔍 Guiding Question How do cells divide to produce daughter cells that are either genetically identical to or genetically different from the parent cell, and what mechanisms ensure that division is controlled?
🌍 Why This Matters Every second, your bone marrow produces roughly two million new red blood cells — each one a perfect genetic copy of the stem cell that made it. Meanwhile, in your gonads, a completely different division process is shuffling your DNA into combinations that have never existed before. Two types of division, two entirely different outcomes, and the difference between them shapes everything from wound healing to the inheritance of genetic disease.
The Problem the Cell Had to Solve
Growth, repair, and reproduction all require one fundamental act: copying yourself. But copying a cell is not like photocopying a document. The cell must first duplicate roughly two metres of DNA, then physically separate two complete sets of genetic information into two new compartments — all without tangling, tearing, or losing a single gene. The cell cycle is the solution evolution arrived at.
Before exploring the stages, we need precise vocabulary. These terms are not interchangeable, and confusing them is one of the most common sources of lost marks in IB exams.
📖 Key Definition — Cell Cycle The cell cycle is the ordered sequence of events a eukaryotic cell passes through from one division to the next, consisting of interphase (G1, S, G2) and the mitotic phase (mitosis and cytokinesis).
📖 Key Definition — Chromosome A chromosome is a single, linear DNA molecule associated with histone proteins, forming a compact structure visible under a light microscope during cell division. In humans, somatic cells contain 46 chromosomes arranged in 23 homologous pairs.
📖 Key Definition — Sister Chromatids Sister chromatids are the two identical copies of a chromosome produced by DNA replication during S phase, joined at a region called the centromere. They are genetically identical and remain attached until separated during anaphase.
📖 Key Definition — Homologous Chromosomes Homologous chromosomes are pairs of chromosomes — one inherited from each parent — that carry genes for the same traits at the same loci but may carry different alleles. They are the same length and have the same centromere position.
The Cell Cycle: Interphase and the Mitotic Phase
Think of the cell cycle as a project with a strict preparation phase before the main event. The cell spends the vast