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2.1 Cell Division

GCSE Biology · AQA · Chapter 2: Cell Division

A free GCSE Biology lesson for AQA (8461) from OBA School: chromosomes, the three stages of the cell cycle, and what mitosis is used for. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how genetic material is arranged in the nucleus as chromosomes and genes
  • Describe the three stages of the cell cycle, including mitosis
  • Recognise the stages of the cell cycle in diagrams and photographs
  • Explain why mitosis is needed for growth, repair and asexual reproduction
  • Use cell counts to calculate how long a stage of the cell cycle lasts

Key words

  • Chromosome: A long, coiled molecule of DNA (with proteins) found in the nucleus. It carries a large number of genes.
  • Gene: A small section of DNA on a chromosome that codes for a particular protein (and so a characteristic).
  • Cell cycle: The series of stages a body cell goes through as it grows and divides into two.
  • Mitosis: The stage of the cell cycle when one set of chromosomes is pulled to each end of the cell and the nucleus divides.
  • Genetically identical: Having exactly the same genes. The two cells made by the cell cycle are identical to each other and to the parent cell.

Chromosomes and genes

The nucleus of a cell contains chromosomes made of DNA. Each chromosome carries hundreds or thousands of genes. In body cells, chromosomes are found in pairs: one of each pair came from each parent.

  • Human; Chromosomes in a body cell 46; Pairs 23
  • Dog; Chromosomes in a body cell 78; Pairs 39
  • Pea plant; Chromosomes in a body cell 14; Pairs 7

Key fact: Gametes (sex cells: sperm and egg cells) are different. They have only one chromosome from each pair, so a human gamete has 23 chromosomes.

The cell cycle

Body cells divide in a series of stages called the cell cycle. The result is two new cells that are genetically identical to each other and to the parent cell.

Exam tip: In Stage 1 the DNA replicates (copies itself), so each chromosome becomes two identical strands joined in the middle: the X shape. In Stage 2 these copies are pulled apart, so each new cell gets a full, identical set.

Why cells divide by mitosis

  • Growth: multicellular organisms grow by making more cells, e.g. a baby growing into an adult, or the tip of a plant root.
  • Repair and replacement: replacing damaged or worn-out cells, e.g. healing a cut, or replacing skin cells and red blood cells.
  • Asexual reproduction: some organisms reproduce using mitosis, e.g. strawberry runners, potato tubers, and single-celled organisms such as yeast.
  • Used for; Mitosis (this topic) growth, repair, asexual reproduction; Meiosis (later in the course) making gametes (sperm and egg cells)
  • Cells made; Mitosis (this topic) 2 identical cells, each with 46 chromosomes (human); Meiosis (later in the course) 4 different cells, each with 23 chromosomes (human)

How long does each stage last?

Cells that divide quickly, such as those in a root tip or an early embryo, have a short cell cycle. Most adult cells divide only rarely. In a slide of a root tip, each cell is caught at a random moment. So the fraction of cells in a stage equals the fraction of the cycle time spent in that stage.

time in a stage = (number of cells in that stage ÷ total cells counted) × length of the whole cycle

Worked example: time spent in mitosis

A student counts 250 cells in a root tip. 30 of them are in mitosis. The whole cell cycle takes 20 hours.
Fraction in mitosis = 30 ÷ 250 = 0.12 (12%)
Time in mitosis = 0.12 × 20 = 2.4 hours
0.4 hours = 0.4 × 60 = 24 minutes

Answer: 2.4 hours (2 hours 24 minutes)

Common mistakes

  • Saying mitosis produces “two different cells” or “four cells”. Mitosis gives two genetically identical cells.
  • Calling the whole cell cycle “mitosis”. Mitosis is only Stage 2, when the nucleus divides.
  • Forgetting that DNA replicates in Stage 1, before mitosis.
  • Saying gametes are made by mitosis. Gametes are made by meiosis.
  • Writing 2.4 hours as “2 hours 40 minutes”. Multiply the decimal part by 60.

Revision questions

Try these, then check your answers below each one.

Question 1. How many chromosomes are there in a normal human body cell? (Choose from: 23, 46, 92, 22.)

Answer: 46

Working: Human body cells have 23 pairs, so 46 chromosomes. Gametes have 23.

Question 2. True or false? The two cells made by the cell cycle are genetically identical to the parent cell.

Answer: True

Working: True. Each new cell gets an identical, full set of chromosomes.

Question 3. In a root tip, 10% of cells are in mitosis. The cell cycle lasts 20 hours. How many hours does mitosis last?

Answer: 2 hours

Working: 0.10 × 20 = 2 hours.

Related lessons

Continue in the full course

This lesson is section 2.1 (AGCBH0201) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.

 

 

 

 

 

 

 

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