Ch 2 · Mitosis · IGCSE Biology 0610
Topic 2 of 4 in Inheritance — IGCSE Biology 0610 — 48 lessons.
Introduction
You began as one cell and now consist of trillions, almost all with the same 46 chromosomes. The division responsible is mitosis. It also heals a cut, replaces the gut lining every few days and produces the clones of asexual reproduction. This lesson gives the definition and roles of mitosis, explains (Supplement) how the chromosome number is kept constant and what stem cells are, and shows how to recognise the stages in diagrams.
Key ideas — What mitosis is and what it is for
Mitosis is nuclear division giving rise to genetically identical cells. One diploid parent cell produces two diploid daughter cells whose nuclei carry the same chromosomes and the same alleles as the parent. Nuclear division is followed by division of the cytoplasm (cytokinesis) to give two separate cells.
🧑🏫 Teacher's explanation
Imagine you have a master blueprint for a house. If you want to build an exact replica of that house, you first need to make a perfect photocopy of that blueprint so that both houses have the same instructions. Mitosis is that photocopying process for your DNA. Before the cell divides, it must replicate its chromosomes so that each new cell gets a full set of instructions.
📖 Definitions
🔍 Worked example — Easy
A multicellular organism increases in size through the process of mitosis. State the two main outcomes of mitosis regarding the genetic material and the chromosome number.
🔍 Worked example — Standard
Compare the role of mitosis in 'growth' versus 'repair of damaged tissues'.
🔍 Worked example — Exam-style
A researcher observes a population of unspecialised stem cells under a microscope. Explain why these cells are essential for the development of a complex organism.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the role of mitosis in the replacement of cells. [1] (b) Describe how mitosis contributes to the growth of the organism. [2] [3 marks]
Key ideas — Keeping the chromosome number constant
🧑🏫 Teacher's explanation
Imagine you have a master blueprint for building a house. If you want to build an identical second house next door, you wouldn't just tear the first blueprint in half; you would first photocopy it so you have two identical sets. Only then do you give one copy to the first builder and one copy to the second builder. This ensures both houses are built exactly the same way.
📖 Definitions
🔍 Worked example — Easy
A parent cell contains 2n = 46 chromosomes. If the DNA replicates exactly once before mitosis, how many chromatids are present in the cell during the middle of mitosis?
🔍 Worked example — Standard
A cell undergoes mitosis to produce two daughter cells. If the parent cell had 20 chromosomes, describe the chromosomal composition of the daughter cells and explain why they are genetically identical.
🔍 Worked example — Exam-style
A researcher observes a cell under a microscope during mitosis. The chromosomes are lined up along the equator of the cell. Explain the importance of the replication step that occurred prior to this stage.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the process of mitosis in terms of chromosome number and genetic content. [3] (b) State the role of stem cells in this process. [1] [4 marks]
Key ideas — Recognising the stages
You are not required to name the stages, but you must identify what is happening from a diagram or photograph. Look for these features in order.
🧑🏫 Teacher's explanation
Think of mitosis like a high-precision photocopier. Before you can make a copy of a document, you first have to make a duplicate of every single page. If you just tried to split the original book in half, you'd end up with two incomplete books. In biology, the cell first replicates its DNA so that when it splits, each new cell gets a full, perfect set of instructions.
📖 Definitions
🔍 Worked example — Easy
A cell has a diploid number of 2n = 46. How many chromosomes will be present in each daughter cell after mitosis is complete?
🔍 Worked example — Standard
A student observes a root tip under a microscope. They count 50 cells in interphase, 10 cells in prophase, 20 cells in metaphase, 15 cells in anaphase, and 5 cells in telophase. Calculate the percentage of cells currently undergoing mitosis.
🔍 Worked example — Exam-style
Describe the appearance of chromosomes during the stage where they line up at the equator of the cell.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the purpose of mitosis in the plant root tip. [1] (b) A student observes a cell where the chromosomes are clearly visible and are arranged in a single line along the center of the cell. (i) Identify the stage of mitosis shown. [1] (ii) Describe what happens to the chromosomes in the next stage of mitosis. [2] [4 marks]
Key ideas — Stem cells
🧑🏫 Teacher's explanation
Think of mitosis as a high-precision photocopier. Before you can copy a document, you must first make an exact duplicate of the original pages so that when you split the stack in half, both sets have everything they need. In a cell, this means the chromosomes must replicate exactly before the cell divides.
📖 Definitions
🔍 Worked example — Easy
A single stem cell undergoes mitosis to produce two daughter cells. If the original cell had 2n = 46 chromosomes, how many chromosomes will each daughter cell have after mitosis is complete?
🔍 Worked example — Standard
Compare the potential of embryonic stem cells and adult stem cells (e.g., bone marrow) in terms of their ability to differentiate.
🔍 Worked example — Exam-style
A diagram shows a cell where chromosomes are being pulled to opposite poles of the cell. Describe the role of mitosis in this context and state what must happen to the chromosomes before this process begins.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the purpose of mitosis in a multicellular organism. [1] (b) Explain how the daughter cells produced by a stem cell can become specialized for different functions. [2] [3 marks]
Worked example
(a) R, S, P, Q [B1].
🔍 Worked example — Easy
A plant cell has 2n = 46 chromosomes. After one round of mitosis, how many chromosomes will each daughter cell have?
🔍 Worked example — Standard
A student observes a root tip under a microscope. They see a cell where the chromosomes are lined up along the equator (middle) of the cell. Identify this stage and state what happens immediately after.
🔍 Worked example — Exam-style
An onion root tip cell contains 16 chromosomes. During the process of mitosis, the DNA is replicated. How many chromatids are present in a single chromosome during the stage where chromosomes are visible as double threads?
⚠️ Common errors
Common mistakes
Practice
Questions