Cell division: mitosis and the cell cycle

What you'll learn

This revision guide covers everything you need to know about mitosis and the cell cycle for AQA GCSE Biology. You'll understand why cells divide, what happens during each stage of the cell cycle, and how mitosis produces two identical daughter cells. These processes are fundamental to growth, repair, and asexual reproduction in organisms.

Key terms and definitions

Mitosis — A type of cell division that produces two genetically identical daughter cells from one parent cell, used for growth and repair

Cell cycle — The series of events that take place in a cell leading to its division and duplication, consisting of interphase and mitosis

Chromosomes — Thread-like structures made of DNA molecules that contain genetic information, found in the nucleus of cells

Diploid — A cell containing two complete sets of chromosomes (one from each parent), represented as 2n

Cytokinesis — The physical splitting of the cytoplasm to form two separate daughter cells at the end of cell division

Stem cells — Undifferentiated cells capable of dividing to produce many more cells of the same type, and able to differentiate into other cell types

DNA replication — The process by which a DNA molecule makes an exact copy of itself, occurring during interphase before mitosis

Differentiation — The process by which cells become specialized to carry out particular functions

Core concepts

Why cells divide

Cells divide for three main reasons that you must understand for your exam:

Growth — Multicellular organisms grow by increasing the number of cells through mitosis. A single fertilized egg divides repeatedly to form all the cells in your body (approximately 37 trillion cells in an adult human).

Repair — When tissues are damaged, mitosis produces new cells to replace dead or damaged ones. For example, skin cells constantly divide to replace those lost from the surface, and bone cells divide to repair fractures.

Asexual reproduction — Some organisms reproduce asexually using mitosis to produce genetically identical offspring (clones). Examples include strawberry plants producing runners, bacteria dividing by binary fission, and yeast budding.

The stages of the cell cycle

The cell cycle has two main parts: interphase and mitosis. Understanding what happens in each stage is essential.

Interphase

This is the longest part of the cell cycle, typically taking 90% of the total time. During interphase, the cell is not dividing but preparing to divide. Three important processes occur:

Cell growth — The cell increases in size, produces more organelles (mitochondria, ribosomes, etc.), and synthesizes more proteins. The cell must grow large enough to divide into two viable daughter cells.

DNA replication — The cell makes exact copies of all its chromosomes. Each chromosome becomes two identical copies called chromatids, joined at a point called the centromere. This ensures each daughter cell receives a complete set of genetic information.

Energy stores increase — The cell produces more ATP (energy) to power the division process.

Mitosis

Mitosis is the division of the nucleus. Although it's a continuous process, we divide it into four stages for clarity. You need to know what happens in each stage:

Prophase — The chromosomes condense and become visible as pairs of chromatids. The membrane around the nucleus breaks down, and spindle fibres begin to form from structures called centrioles.

Metaphase — The chromosomes line up along the centre (equator) of the cell. Spindle fibres attach to each chromosome at the centromere.

Anaphase — The spindle fibres contract and pull the two chromatids of each chromosome apart to opposite poles (ends) of the cell. Each chromatid is now considered a separate chromosome.

Telophase — A nuclear membrane forms around each set of chromosomes at opposite ends of the cell, creating two nuclei. The chromosomes begin to uncoil and become less visible.

Cytokinesis

After telophase, the cytoplasm and cell membrane divide to form two separate, genetically identical daughter cells. In animal cells, the cell membrane pinches inward. In plant cells, a new cell wall forms down the middle.

Chromosome numbers in mitosis

This concept frequently appears in exam questions, so ensure you understand it clearly.

Human body cells are diploid and contain 46 chromosomes (23 pairs). After DNA replication in interphase, the cell still has 46 chromosomes, but each consists of two identical chromatids joined together.

During mitosis, these chromatids separate so that:

• Each daughter cell receives 46 chromosomes
• Each daughter cell is genetically identical to the parent cell
• Each daughter cell is diploid (2n)

The daughter cells have exactly the same genetic information as each other and as the original parent cell.

Stem cells and differentiation

Stem cells are unspecialised cells that can divide by mitosis and differentiate into specialized cell types. You need to know about different types of stem cells:

Embryonic stem cells — Found in early-stage embryos, these can differentiate into any type of cell. They're used to form all the specialized cells in a developing organism.

Adult stem cells — Found in specific locations in the body (like bone marrow), these can differentiate into a limited range of cell types. For example, bone marrow stem cells can produce different types of blood cells.

Meristems in plants — Plant stem cells found in growing regions (meristems) at root and shoot tips. These cells can differentiate into any plant cell type throughout the plant's life.

Cell specialization through differentiation

As cells divide by mitosis, many become differentiated to perform specific functions. Differentiation involves changes that allow cells to carry out particular jobs:

• Nerve cells develop long extensions (axons) to transmit electrical impulses
• Root hair cells develop extensions to increase surface area for water absorption
• Sperm cells develop tails for swimming and contain many mitochondria for energy
• Red blood cells lose their nucleus to make more space for haemoglobin

In animals, most cells differentiate early in development and lose the ability to divide or differentiate further. In plants, many cells retain the ability to differentiate throughout their lives, which is why you can grow a new plant from a cutting.

The importance of mitosis in organisms

Understanding the significance of mitosis helps answer evaluation and application questions:

Maintaining genetic consistency — Mitosis ensures all cells in an organism have identical genetic information, which is essential for coordinated functioning.

Asexual reproduction advantages — Organisms can reproduce quickly without finding a mate, useful in stable environments. However, this produces no genetic variation.

Medical applications — Understanding mitosis helps develop cancer treatments (as cancer involves uncontrolled cell division) and potential stem cell therapies for treating diseases.

Agricultural applications — Farmers use asexual reproduction (relying on mitosis) to clone plants with desirable characteristics, producing genetically identical crops.

Worked examples

Example 1: Describing the cell cycle

Question: A human skin cell goes through the cell cycle. The cell contains 46 chromosomes. Describe what happens to the chromosomes during the cell cycle. [4 marks]

Mark scheme answer:

1. During interphase, DNA replication occurs [1 mark]
2. Each chromosome becomes two chromatids joined at the centromere / number remains 46 chromosomes [1 mark]
3. During mitosis, the chromatids separate [1 mark]
4. Each daughter cell receives 46 chromosomes / same number as parent cell [1 mark]

Examiner tip: Notice the question asks specifically about chromosomes, so focus your answer on what happens to them. Each distinct point about chromosome behaviour earns a mark.

Example 2: Comparing cell types

Question: Explain why some cells in the human body can divide by mitosis throughout life, but other cells cannot. [3 marks]

Mark scheme answer:

1. Some cells remain undifferentiated / are stem cells [1 mark]
2. Most cells become specialized / differentiated [1 mark]
3. Differentiated cells often lose the ability to divide / stem cells retain the ability to divide [1 mark]

Examiner tip: "Explain" questions require you to give reasons. Use words like "because" or "therefore" to show you're explaining, not just describing.

Example 3: Application to cancer

Question: Cancer is caused by uncontrolled cell division. Suggest why cancer treatments that prevent DNA replication can stop cancer spreading. [2 marks]

Mark scheme answer:

1. DNA replication must occur before mitosis / before cells can divide [1 mark]
2. Preventing DNA replication stops cells dividing / stops cancer cells increasing in number [1 mark]

Examiner tip: "Suggest" means the answer may not be directly taught content, but you should apply your knowledge. Think about what you know about DNA replication in the cell cycle.

Example 4: Plant growth

Question: A gardener takes a cutting from a plant and grows a new plant. The new plant is genetically identical to the parent plant. Explain why. [3 marks]

Mark scheme answer:

1. The cutting uses mitosis to produce new cells / to grow [1 mark]
2. Mitosis produces genetically identical cells [1 mark]
3. All cells have the same genetic information as the parent plant / no variation introduced [1 mark]

Examiner tip: Link the process (mitosis) to the outcome (genetic identity). Show you understand why mitosis produces identical cells.

Common mistakes and how to avoid them

• Confusing mitosis and meiosis — Mitosis produces two identical diploid cells for growth and repair. Meiosis (covered separately) produces four non-identical haploid gametes for reproduction. If a question mentions growth, repair, or asexual reproduction, it's about mitosis.

• Saying chromosomes double during mitosis — The number of chromosomes doesn't change. DNA replicates during interphase (before mitosis), creating chromatids. These separate during mitosis so each daughter cell gets the same number of chromosomes as the parent.

• Mixing up the stages of mitosis — Learn the order: Prophase, Metaphase, Anaphase, Telophase (remember: PMAT). Don't include interphase or cytokinesis as stages of mitosis itself.

• Forgetting what happens in interphase — Many students focus only on mitosis but interphase is equally important. Always mention DNA replication and cell growth when describing the complete cell cycle.

• Thinking all cells can divide indefinitely — Only stem cells and some specialized cells (like skin cells) can continue dividing. Most differentiated cells lose this ability. Nerve cells, for example, rarely divide after development.

• Unclear about daughter cell genetics — Daughter cells are genetically identical to each other AND to the parent cell. Make this explicit in exam answers rather than just saying "identical cells."

Exam technique for "Cell division: mitosis and the cell cycle"

• Command words matter — "Describe" means state what happens (no explanation needed). "Explain" requires reasons using words like "because" or "so that." "Suggest" means apply your knowledge to unfamiliar contexts. Read the command word before writing.

• Use the mark scheme structure — If a question is worth 4 marks, aim to make 4-5 distinct points. Each relevant point typically earns one mark. Don't write long sentences that make the same point repeatedly.

• Draw and label diagrams accurately — You may be asked to identify stages of mitosis from diagrams or draw cells. Practice sketching chromosomes at different stages. Ensure labels have straight lines pointing to specific structures.

• Link structure to function — When discussing stem cells or differentiation, explain how structural changes allow cells to perform their specialized functions. This demonstrates deeper understanding worth higher marks.

Quick revision summary

The cell cycle consists of interphase (cell growth, DNA replication, energy production) and mitosis (nuclear division). Mitosis has four stages: prophase (chromosomes condense), metaphase (chromosomes align), anaphase (chromatids separate), telophase (nuclei form). Cytokinesis then divides the cytoplasm. Mitosis produces two genetically identical diploid daughter cells for growth, repair, and asexual reproduction. Stem cells can divide and differentiate into specialized cells, with most animal cells losing this ability early in development while plant meristem cells retain it.