Biology · Mitosis, Growth and Stem Cells
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The Cell Cycle and Mitosis
New cells are made when existing cells divide. This lesson follows a cell through the cell cycle, shows what happens to the chromosomes in mitosis, and explains why the two new cells are identical to the one they came from.
Learning Objectives
- 1Describe the three stages of the cell cycle, including mitosis.
- 2Explain what happens to the DNA and to the sub-cellular structures before a cell divides.
- 3Explain why mitosis produces two genetically identical cells.
- 4Explain why mitosis is important in growth, repair, replacement and asexual reproduction.
- 5Calculate how many cells there are after a number of divisions.
Retrieval practice
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1
Where are the chromosomes in a human body cell?
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In the nucleus.
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2
How many chromosomes are there in a human body cell?
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46, which is 23 pairs.
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3
What is a gene?
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A short section of DNA that contains the instructions for making one protein.
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4
Which sub-cellular structure releases energy by aerobic respiration?
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The mitochondrion.
New cells come from old cells
A new cell is never built from nothing. It is made when an existing cell divides in two. This is how a single fertilised egg becomes a baby, how a cut heals, and how your body replaces the cells that wear out every day. A cell does not simply split down the middle, because each new cell needs a full set of instructions and its own organelles. The series of stages a cell goes through to prepare for and carry out division is called the cell cycle.
The three stages of the cell cycle
The cell cycle is a loop. When it ends, each of the two new cells starts it again.
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Stage 1: growth and DNA replication
The cell grows, and the number of sub-cellular structures such as ribosomes and mitochondria increases. The DNA replicates, so there are two copies of each chromosome. This is usually the longest stage.
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Stage 2: mitosis
One set of chromosomes is pulled to each end of the cell, and the nucleus divides.
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Stage 3: the cytoplasm divides
The cytoplasm and the cell membrane divide, forming two identical cells. These are called daughter cells.
The cell cycle
The three stages of the cell cycle. The stages are not drawn to scale: stage 1 takes by far the longest.
Mitosis, step by step
Follow one cell that has two pairs of chromosomes. A human cell would have 23 pairs, but the steps are the same.
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1
The cell grows
Before dividing, the cell grows and makes more organelles, such as ribosomes and mitochondria, so each new cell will have enough.
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2
The DNA is copied
Each chromosome is replicated, so there are now two identical copies of every chromosome, joined together.
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3
The chromosomes line up
The chromosomes line up across the middle of the cell.
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4
The copies are pulled apart
One set of chromosomes is pulled to each end of the cell.
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5
New nuclei form
A nucleus forms around each set of chromosomes, and the cell starts to pinch in the middle.
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6
The cytoplasm divides
The cytoplasm and cell membrane divide, making two identical daughter cells.
Mitosis in six steps
You do not need the names of the phases inside mitosis, but you do need these steps in order.
Why the two new cells are identical
Mitosis copies a cell, it does not mix cells up.
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The DNA is copied exactly
Before mitosis the DNA replicates, so every chromosome has an identical copy.
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Each cell gets one of each
One copy of every chromosome is pulled to each end, so each new nucleus has a complete set.
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Same chromosomes, same genes
The daughter cells have the same number of chromosomes as the parent cell. A human cell with 46 chromosomes makes two cells with 46 chromosomes each.
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Genetically identical
Because the genes are the same, the new cells are genetically identical to the parent cell and to each other.
Chromosome numbers stay the same
Mitosis makes two cells with the same chromosomes as the parent cell.
Why mitosis matters
Cell division by mitosis is important in the growth and development of multicellular organisms, and in more than that.
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Growth and development
A fertilised egg becomes an adult because its cells divide again and again. More cells make a bigger organism.
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Repair
When tissue is damaged, such as a cut in the skin, cells next to the damage divide by mitosis to make new cells that fill the gap.
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Replacement
Many cells wear out and have to be replaced all the time, for example skin cells and the cells lining the gut.
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Asexual reproduction
Some organisms make new individuals by mitosis alone. The offspring are genetically identical to the parent, so they are clones.
Three jobs for mitosis
Growth, repair and replacement all depend on mitosis.
How many cells after a number of divisions?
A cell divides by mitosis. Both new cells then divide, and so do all the cells made after that. How many cells are there after 4 divisions?
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- 1 Start with one cell After 0 divisions there is 1 cell.
- 2 Each division doubles the number 1 → 2 → 4 → 8 → 16, so after 4 divisions there are 16 cells.
- 3 Or use a power of 2 After n divisions there are \(2^{n}\) cells, and \(2^{4} = 2 \times 2 \times 2 \times 2 = 16\).
Answer16 cells.
Doubling
Each division doubles the number of cells.
The Stages of the Cell Cycle in Edexcel's Words
Edexcel names each stage (specification 2.1). Learn the order: interphase, prophase, metaphase, anaphase, telophase, then cytokinesis.
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1
Interphase
The cell grows, makes more ribosomes and mitochondria, and copies its DNA so that each chromosome has two identical copies.
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2
Prophase
The chromosomes condense and become visible, each made of two identical copies joined together. The nuclear membrane breaks down.
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3
Metaphase
The chromosomes line up across the middle of the cell, attached to spindle fibres.
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4
Anaphase
The spindle fibres pull each pair of copies apart, so one copy of every chromosome moves to each end of the cell.
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5
Telophase
A new nuclear membrane forms around each set of chromosomes at the two ends of the cell, and the chromosomes unwind.
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6
Cytokinesis
The cytoplasm and cell membrane divide, making two daughter cells. Each is a genetically identical diploid body cell, with the same chromosomes as the parent cell.
Growth in Animals and Plants
Edexcel asks you to describe growth as cell division followed by differentiation, with an extra step in plants (specification 2.2 and 2.5).
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Animal growth
Cells divide by mitosis to make more cells, and most of the new cells then differentiate into specialised types. In adults, mitosis mainly replaces and repairs cells.
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Plant growth
Cells divide by mitosis in the meristems at the tips of roots and shoots. The new cells then elongate, becoming longer as they take in water, before they differentiate.
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Asexual reproduction
Mitosis makes the new cells of organisms that reproduce asexually, such as a plant that grows new plants from runners. The offspring are genetically identical to the parent.
Percentile Charts and Growth
Doctors use percentile charts to monitor the growth of babies and children (specification 2.7).
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What a percentile means
It shows what percentage of children of the same age and sex are smaller. A child on the 25th percentile line for mass is heavier than 25% of children of that age and lighter than 75%.
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The 50th percentile
The middle line: half of children are above it and half are below it.
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Using the chart
Plot the child's measurement against their age and read off which line it lies on, or between. What matters is whether the child keeps following the same line as they grow.
Case study
Replacing cells in the human body
Mitosis is going on in your body all the time. A red blood cell lives for about four months before it is broken down, and the cells lining the small intestine last only a few days. Stem cells keep dividing by mitosis to make the replacements, so the number of cells stays about the same.
The big idea
Mitosis makes two cells that are genetically identical to the parent cell.
The DNA is copied first, so each new cell gets a full set of chromosomes.
The cell cycle and mitosis
The cell cycle and mitosis
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Stage 1
- the cell grows
- more ribosomes and mitochondria
- DNA is copied
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Stage 2: mitosis
- one set of chromosomes to each end
- the nucleus divides
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Stage 3
- the cytoplasm and cell membrane divide
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Result
- two genetically identical cells
- same chromosome number
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Uses
- growth
- repair
- replacement
- asexual reproduction
Summary and exam focus
- The cell cycle has three stages: growth and DNA replication, mitosis, and division of the cytoplasm.
- Before dividing, the cell grows and makes more ribosomes and mitochondria, and its DNA replicates.
- In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.
- Mitosis makes two genetically identical cells with the same number of chromosomes.
- Mitosis is needed for growth, repair, replacement and asexual reproduction.
Exam focus
Describe what happens to the chromosomes during the cell cycle. (4 marks) (4 marks)
Put the events in order and use the words copied, pulled and identical. Mark schemes reward "the DNA is replicated, giving two copies of each chromosome" and "one set is pulled to each end of the cell".
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Cell cycle
- The series of stages a cell goes through to grow, copy its DNA and divide into two 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.
- DNA replication
- The copying of DNA, so that there are two identical copies of each chromosome.
- Daughter cell
- One of the two cells formed when a cell divides.
- Genetically identical
- Having exactly the same genes.
- Clone
- An organism, or cell, that is genetically identical to its parent.
- Growth
- An increase in the number of cells (and so in the size of an organism).
- Repair
- Replacing damaged cells with new ones made by mitosis.
- Asexual reproduction
- Reproduction with one parent only, using mitosis, so the offspring are genetically identical to it.
- Sub-cellular structure
- A structure inside a cell, such as a nucleus, a mitochondrion or a ribosome.
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