🌱 Growth
A seed grows into a plant because its cells keep dividing. The new cells make the shoots and roots longer and bigger.
Cell Division » Chromosomes, Mitosis and the Cell Cycle
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.1.2.1, 4.1.2.2
- What chromosomes are and how they relate to DNA and genes
- The three overall stages of the cell cycle
- What happens to the chromosomes in mitosis
- Why mitosis matters for growth and development
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Unlock This CourseChromosomes, DNA and genes
Every living thing grows, repairs itself and makes new cells. To do that, cells need instructions. In plant and animal cells these instructions are kept in the nucleus, in structures called chromosomes.
A chromosome is made of a long DNA molecule. Each chromosome carries a large number of genes. You can think of a chromosome as a long instruction book. The genes are the separate instructions written in it. Each one is a piece of information that helps to control how the cell works.
In body cells the chromosomes are normally found in pairs. The two chromosomes in a pair are a matching set, with the same genes in the same places.
Key terms:
- Chromosome: a structure in the nucleus made of a DNA molecule. It carries a large number of genes.
- Gene: a section of a chromosome that carries one set of instructions for the cell.
- Cell cycle: the series of stages a cell goes through so that it can divide into two identical cells.
- Mitosis: the stage of the cell cycle in which one set of chromosomes is pulled to each end of the cell and the nucleus divides.
Why do cells divide?
A single cell is far too small to make a whole plant or animal. Multicellular organisms grow by making more cells. They also need new cells to replace ones that are damaged or worn out.
🩹 Replacement
When you graze your knee, nearby skin cells divide to fill the gap. The new cells are copies of the old ones, so the skin works as it did before.
In both cases the new cells must be identical to the cell they came from. They need the same genetic information, or they could not do the same job. That is why cells copy their chromosomes before they divide.
The cell cycle
Cells divide in a series of stages called the cell cycle. During the cell cycle the genetic material is doubled and then divided into two identical cells. There are three overall stages.
Stage 1: Growth and copying
Before a cell can divide it needs to grow. It also increases the number of its sub-cellular structures, such as ribosomes and mitochondria. This means each new cell gets enough of them to work properly.
The DNA replicates to form two copies of each chromosome. The cell now holds double the genetic material, ready to be shared out equally.
Stage 2: Mitosis
In mitosis one set of chromosomes is pulled to each end of the cell. The nucleus then divides, so there is a nucleus at each end of the cell. Each nucleus holds a full set of chromosomes.
Stage 3: Division of the cytoplasm and cell membrane
Finally the cytoplasm and cell membranes divide. The result is two identical cells. Each one has the same genetic information as the cell it came from, and as each other.
Do not need to know
Mitosis itself has several phases, but you do not need to know their names for this course. You only need to know the three overall stages above and what happens to the chromosomes.
Doubling the number of cells
Each time the cell cycle is completed, one cell becomes two. After the next cycle, those two cells become four, and so on. The number doubles each time.
Worked example
A single cell goes through the cell cycle 5 times, and every cell divides each time. How many cells are there at the end?
After 1 cycle: 2. After 2 cycles: 4. After 3 cycles: 8. After 4 cycles: 16. After 5 cycles: 32.
The answer is 32 cells, all identical to the first cell.
Mitosis in growth and development
Cell division by mitosis is important in the growth and development of multicellular organisms. A fertilised egg is a single cell. Mitosis makes the huge number of cells needed to build the whole body. Because mitosis produces identical cells, every one of them carries the same set of genes.
Mitosis does not stop when you stop growing taller. It carries on to repair damage and to replace cells that wear out.
Recognising where mitosis is happening
In the exam you may be given a situation and asked where mitosis is happening. Look for any case where the number of cells is increasing and the new cells are identical to the old ones.
🌾 Root tip
The tip of a growing root gets longer because its cells divide by mitosis.
🩹 Healing
A cut on your hand closes up as skin cells divide to replace the lost ones.
👶 Embryo
A tiny embryo becomes a baby as its cells divide again and again.
Common mistakes
Do not say mitosis is the whole cell cycle. It is only one stage, when the nucleus divides. Do not forget that the DNA is copied before mitosis begins, so the cell is not simply split in half. Do not say the new cells are different from each other, because mitosis makes identical cells. Do not say a chromosome and a gene are the same thing. A chromosome carries a large number of genes.
Exam-style question
A gardener plants a seed. Over several weeks it grows into a seedling with a longer root and new leaves. The cells in the root tip divide by mitosis.
(a) What does a chromosome carry? [1 mark]
(b) Describe what happens to the chromosomes in mitosis. [2 marks]
(c) Explain why mitosis is important for the seedling. [2 marks]
Model answer
(a) A chromosome carries a large number of genes (1).
(b) One set of chromosomes is pulled to each end of the cell (1) and the nucleus divides (1).
(c) Mitosis makes new cells for growth, so the root and leaves can get bigger (1). It makes identical cells, so every new cell has the same genes (1).
Exam tip
In part (c), name the process (growth) and then say what mitosis does to the cells (makes identical ones). Saying only that mitosis makes new cells will not get both marks.