Ch 1 · 5.1 Replication and division of nuclei and cells · AS & A Level Biology 9700
Topic 1 of 2 in The mitotic cell cycle — AS & A Level Biology 9700 — 68 lessons.
Introduction
A human cell packs about two metres of DNA into a nucleus a few micrometres across, and must copy every base before it divides. This lesson covers how a chromosome is built, the cell cycle that copies and shares it, why the ends of chromosomes need protecting, and what goes wrong when division is no longer controlled. (Syllabus section 5.1.)
Key ideas — The structure of a chromosome
A chromosome is one long molecule of DNA wound around histone proteins. Histones are positively charged and bind the negatively charged phosphate backbone of DNA; DNA wraps around groups of histones and is coiled again and again (supercoiled), so it fits in the nucleus.
🧑🏫 Teacher's explanation
Think of a chromosome as a giant, tangled ball of yarn. If you just threw a 2-meter long string into a tiny box, it would be a mess. To make it fit, you wrap the string around small wooden spools. In your cells, these spools are histone proteins. The string is your DNA. By wrapping the DNA around histones and then twisting that whole structure, the cell 'supercoils' it into a compact chromosome that fits inside the nucleus.
📖 Definitions
🔍 Worked example — Easy
A student observes a cell under a light microscope during metaphase. They identify a structure consisting of two identical strands joined at a central point. Identify the term for the two strands and the point of attachment.
🔍 Worked example — Standard
Explain why telomeres are essential during the process of DNA replication.
🔍 Worked example — Exam-style
A researcher observes a cell line that has lost the ability to regulate the cell cycle, leading to rapid, uncontrolled division. What is the biological term for the mass of cells formed by this process, and what is the underlying cause?
⚠️ Common errors
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🎯 Exam question
Key ideas — The mitotic cell cycle
The cell cycle is the sequence of events from one division to the next:
🧑🏫 Teacher's explanation
Think of the cell cycle like a high-stakes construction project. Before you can build a second identical building (the daughter cell), you first need to make a perfect photocopy of the blueprints (DNA replication) and gather all the bricks and mortar (organelles and proteins). If you try to divide without copying the blueprints first, one building will end up with half the instructions, which is a disaster.
📖 Definitions
🔍 Worked example — Easy
A cell contains 2n = 46 chromosomes. After the S phase of interphase, how many sister chromatids are present in the cell?
🔍 Worked example — Standard
Compare the functions of the G1 phase and the S phase within interphase.
🔍 Worked example — Exam-style
Explain how uncontrolled cell division leads to the formation of a tumour.
⚠️ Common errors
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🎯 Exam question
(a) State the term used to describe the mass of cells formed by this uncontrolled division. [1] (b) Explain why mitosis is essential for the repair of damaged tissues. [2] [3 marks]
Key ideas — Telomeres
DNA polymerase cannot copy the very end of a DNA strand, so each time DNA replicates a short piece is lost from the end of the chromosome. Telomeres are the buffer: it is the non-coding telomere repeats that are lost, so genes near the ends of chromosomes are not lost. Without telomeres, important genes would be cut off at every division. In most body cells telomeres shorten with each cycle and the cell eventually stops dividing. Stem cells and gametes make the enzyme telomerase, which rebuilds telomeres, so they can go on dividing.
🧑🏫 Teacher's explanation
Imagine you are reading a book, but every time you photocopy a page, the printer accidentally clips off the last few letters of the last sentence. If you keep photocopying that copy, eventually, the sentences will become nonsense because the letters are gone. This is the problem DNA replication faces.
📖 Definitions
🔍 Worked example — Easy
A chromosome has a total length of 100 mum. The telomeres at both ends account for 5 mum each. If the cell undergoes 5 rounds of mitosis and loses 0.5 mum of telomere length per division, calculate the total length of the telomeres remaining after 5 divisions.
🔍 Worked example — Standard
Explain why the loss of telomeres does not result in the loss of essential genetic information during the first few rounds of the cell cycle.
🔍 Worked example — Exam-style
Compare the ability of a somatic skin cell and a stem cell to undergo continuous cell division in terms of telomere maintenance.
⚠️ Common errors
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🎯 Exam question
(a) Explain why the length of the telomeres decreases with each mitotic division. [2] (b) Describe how stem cells avoid this limitation. [2] [4 marks]
Key ideas — Why mitosis matters
Mitosis produces two daughter cells that are genetically identical to the parent cell. That is exactly what is needed for:
🧑🏫 Teacher's explanation
Think of mitosis as a high-fidelity photocopier. If you want to build a massive skyscraper (a multicellular organism), you don't want every new brick to be a slightly different shape or color; you need exact replicas of the original blueprint to ensure the structure is stable and functional.
📖 Definitions
🔍 Worked example — Easy
A multicellular organism starts with 2 × 10^6 cells. After several rounds of mitosis, the cell count increases to 8 × 10^6 cells. Calculate the percentage increase in the number of cells.
🔍 Worked example — Standard
A student observes a cell under a light microscope. The cell is in the S phase of interphase. Describe the state of the DNA in this cell compared to a cell in the G1 phase.
🔍 Worked example — Exam-style
Explain why uncontrolled cell division is significant in the context of human health.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Name the mass of cells that forms as a result of this uncontrolled division. [1] (b) Explain why mitosis is essential for the repair of damaged skin tissue. [2] [3 marks]
Key ideas — Stem cells
A stem cell is an unspecialised cell that can divide by mitosis again and again, and whose daughter cells can differentiate into specialised cell types. Stem cells replace and repair tissues: stem cells in the bone marrow divide to replace blood cells (around two million red blood cells every second); stem cells in the skin and gut lining replace cells that are worn away. One daughter cell often stays a stem cell while the other differentiates, so the supply never runs out.
🧑🏫 Teacher's explanation
Think of a stem cell like a master template or a 'blank' piece of paper in a factory. While most cells are like finished products (a car, a chair, a phone) that can only do one job, a stem cell is the raw material that can be turned into anything.
📖 Definitions
🔍 Worked example — Easy
A stem cell divides to produce two daughter cells. One cell remains a stem cell, while the other differentiates into a specialized skin cell. If this process occurs n times, how many stem cells will exist if we start with 1 stem cell?
🔍 Worked example — Standard
Explain why the differentiation of a stem cell is essential for the survival of a multicellular organism during tissue repair.
🔍 Worked example — Exam-style
Compare the outcome of mitosis in a normal somatic cell versus the outcome of uncontrolled cell division in a tumour.
⚠️ Common errors
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🎯 Exam question
(a) Describe the process by which stem cells contribute to the repair of damaged tissue. [2] (b) Explain why uncontrolled cell division can lead to the formation of a tumour. [2] [4 marks]
Key ideas — Uncontrolled cell division and tumours
The cell cycle is normally controlled by genes that start and stop division and that check for DNA damage. A mutation in these control genes can mean that a cell divides repeatedly and without control, ignoring signals to stop. Its daughter cells inherit the mutation and also divide uncontrollably, producing a mass of cells called a tumour. A benign tumour stays in one place; a malignant tumour (cancer) invades nearby tissue, and its cells can spread in the blood or lymph to form secondary tumours (metastasis).
🧑🏫 Teacher's explanation
Think of the cell cycle like a highly regulated factory assembly line. In a normal factory, there are quality control inspectors (control genes) that check if the machines are working correctly before the next batch starts. If the machines are broken or the inspectors fall asleep, the factory might start pumping out defective products non-stop, cluttering the floor. This pile-up of unwanted products is like a tumour.
📖 Definitions
🔍 Worked example — Easy
A cell undergoes mitosis to produce two daughter cells. Describe the role of the S phase of interphase in this process.
🔍 Worked example — Standard
Compare the functions of benign and malignant tumours.
🔍 Worked example — Exam-style
Explain how a mutation in genes that control the cell cycle can lead to the formation of a tumour.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify whether this mass is likely a benign or malignant tumour. [1] (b) Explain the role of stem cells in the repair of the damaged tissue observed. [2] [3 marks]
Worked example
Question. A cell in G1 contains 4.0 arbitrary units of DNA. (a) State the mass of DNA in the cell at the end of G2, and explain your answer. [2] (b) State the mass of DNA in each daughter cell after cytokinesis. [1]
🔍 Worked example — Easy
A cell enters the S phase of the cell cycle with 2n = 4 chromosomes. How many chromatids are present in the cell at the end of the G2 phase? State the number of chromosomes present.
🔍 Worked example — Standard
A researcher observes a cell under a microscope. The cell has twice the amount of DNA found in a normal G1 cell. Which phase of interphase is the cell likely in, and what specific process has occurred?
🔍 Worked example — Exam-style
A cell in G1 contains 4.0 arbitrary units of DNA. (a) State the mass of DNA in the cell at the end of G2, and explain your answer. (b) State the mass of DNA in each daughter cell after cytokinesis. [3]
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