Cell Division and Growth

This lesson covers mitosis, the role of cell division in growth, and how growth is measured — all essential content for Edexcel GCSE Biology (1BI0) Topic 2: Cells and Control. Understanding cell division is fundamental to explaining how organisms grow, repair damaged tissue, and replace worn-out cells.

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Why Do Cells Divide?

Cells divide for three main reasons:

1. Growth — to increase the number of cells in an organism so it can grow larger.
2. Repair — to replace cells that have been damaged (e.g., healing a wound).
3. Replacement — to replace cells that have been worn out and died (e.g., red blood cells are replaced approximately every 120 days).

All new cells are produced by the division of existing cells. Before a cell can divide, it must first copy its DNA so that each new cell receives a complete set of genetic information.

Exam Tip: In exam questions, always specify why cells divide — growth, repair, or replacement. Simply stating "cells divide to make more cells" is not a sufficient answer.

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Mitosis

Mitosis is a type of cell division that produces two genetically identical diploid daughter cells from one parent cell. The daughter cells have the same number of chromosomes as the parent cell.

In human body cells, the diploid number is 46 chromosomes (23 pairs). After mitosis, each daughter cell also has 46 chromosomes.

The Cell Cycle

Before mitosis occurs, the cell goes through a period called interphase. During interphase:

• The cell grows and increases in size.
• The number of organelles (mitochondria, ribosomes, etc.) increases.
• The DNA replicates — each chromosome is copied to form two identical copies joined at the centromere (these are called chromatids).

After interphase, the cell enters mitosis.

Stages of Mitosis

Mitosis can be divided into four stages:

Stage	What Happens
Prophase	Chromosomes condense and become visible. Each chromosome consists of two identical chromatids joined at the centromere. The nuclear membrane begins to break down. Spindle fibres begin to form.
Metaphase	Chromosomes line up along the middle (equator) of the cell. Spindle fibres attach to the centromere of each chromosome.
Anaphase	The centromeres split. Spindle fibres contract and pull the chromatids apart to opposite poles of the cell. Each chromatid is now an individual chromosome.
Telophase	A nuclear membrane forms around each set of chromosomes at each pole. The chromosomes begin to de-condense (uncoil). The cell now contains two nuclei.

After telophase, the cytoplasm divides in a process called cytokinesis, producing two separate daughter cells.

Exam Tip: A useful mnemonic for the stages of mitosis is PMAT — Prophase, Metaphase, Anaphase, Telophase. Remember that cytokinesis follows telophase to complete cell division.

Key Features of Mitosis

• Produces 2 daughter cells.
• Daughter cells are genetically identical to each other and to the parent cell (they are clones).
• Daughter cells are diploid (contain the full number of chromosomes).
• Used for growth, repair, and asexual reproduction.

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Growth

Growth is a permanent increase in size or mass. In multicellular organisms, growth involves three processes:

1. Cell division (mitosis) — increases the number of cells.
2. Cell elongation — cells increase in length or size.
3. Cell differentiation — cells become specialised for particular functions.

Growth in Animals

• Growth occurs by cell division throughout the body.
• Most animal cells differentiate early in development and then lose the ability to differentiate.
• Animals generally stop growing once they reach maturity (adulthood).
• Growth occurs relatively evenly across the whole organism during development.

Growth in Plants

• Growth mainly occurs at specific regions called meristems.
• Meristems are found at the tips of roots and tips of shoots (apical meristems).
• Plant cells can elongate significantly after division, which is a major contributor to plant growth.
• Plants continue to grow throughout their lives (indeterminate growth), unlike most animals.
• Plant cells can differentiate, de-differentiate, and re-differentiate — they retain the ability to become different cell types throughout the plant's life.

Feature	Animal Growth	Plant Growth
Where it occurs	Throughout the body	Mainly at meristems (root and shoot tips)
Duration	Stops at maturity	Continues throughout life
Cell differentiation	Usually permanent	Can be reversed (de-differentiation)
Main mechanism	Cell division	Cell division and elongation

Exam Tip: The Edexcel specification specifically requires you to know the difference between growth in animals and growth in plants. Be prepared to compare them in a table or extended-answer question.

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Measuring Growth

Growth can be measured in several ways:

Method	Description	Advantages	Disadvantages
Length	Measure the height or length of the organism over time.	Quick, easy, and non-destructive. Can be repeated on the same organism.	Does not account for growth in width or mass. Not always accurate (e.g., plants may wilt).
Wet mass	Weigh the organism as it is (including all water content).	Easy to measure. Non-destructive.	Varies with water content — an organism may weigh more after drinking water, which is not true growth.
Dry mass	Dry the organism in an oven at low temperature until constant mass, then weigh.	Most accurate and reliable measure of growth because it removes variable water content.	Destructive — the organism must be killed. Cannot be repeated on the same organism.

Why Dry Mass Is the Most Reliable

Water content fluctuates throughout the day depending on how much an organism has consumed. Wet mass therefore varies and is not a reliable indicator of actual growth. By removing all water, dry mass gives a true measure of the biological material (biomass) that has been produced. However, the major drawback is that it is a destructive method — the organism must be killed and dried.

Exam Tip: If asked "Which is the most accurate way to measure growth?", the answer is always dry mass. But make sure you explain why (removes variable water content) and acknowledge the disadvantage (organism must be killed).

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Percentile Growth Charts

Percentile growth charts are used to monitor the growth of children. They show the range of normal growth for a given age.

How to Read a Percentile Chart

• The x-axis shows the age of the child.
• The y-axis shows the measurement (e.g., height, mass, head circumference).
• Curved lines on the chart represent percentiles (e.g., 25th, 50th, 75th percentile).
• The 50th percentile represents the median — half of children are above this line and half are below.
• A child on the 75th percentile for height is taller than 75% of children of the same age.

Why Percentile Charts Are Used

• To monitor whether a child is growing at a normal rate.
• To identify potential growth problems — if a child suddenly drops across percentile lines, it may indicate a health issue.
• Children are expected to follow roughly the same percentile line as they grow. A sudden change may require medical investigation.

Interpreting Results

• Being on a high or low percentile is not necessarily a problem — children vary naturally in size.
• What matters is that growth follows a consistent pattern along a percentile line.
• A child moving from the 75th to the 25th percentile over a short period would be a concern.

Exam Tip: You may be given a percentile chart in the exam and asked to read off values or explain what a particular percentile means. Practise reading values from curves accurately.

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Worked Example

Question: A cell with 46 chromosomes undergoes mitosis. How many chromosomes will each daughter cell have?

Answer: Each daughter cell will have 46 chromosomes. Mitosis produces genetically identical diploid cells, so the chromosome number stays the same.

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Question: Explain why dry mass is considered a more reliable measure of growth than wet mass.

Answer: Dry mass is more reliable because it removes the water content, which is variable and fluctuates depending on how much the organism has consumed. This means dry mass measures only the actual biological material (biomass) produced by the organism, giving a more accurate measure of true growth.

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Summary

• Mitosis produces two genetically identical diploid daughter cells.
• The stages of mitosis are: prophase, metaphase, anaphase, telophase, followed by cytokinesis.
• Growth involves cell division, elongation, and differentiation.
• Animals grow throughout their body; plants grow mainly at meristems.
• Growth can be measured by length, wet mass, or dry mass.
• Dry mass is the most reliable but is destructive.
• Percentile charts are used to monitor children's growth patterns.