What you need to know
- Describe the relationship between DNA, genes and chromosomes.
- Explain the main stages of the cell cycle.
- State that mitosis produces two genetically identical daughter cells.
- Explain roles of mitosis in growth, repair and asexual reproduction.
- Compare differentiation in animals and plants.
- Describe cancer as uncontrolled cell division.
Chromosomes and genes
The nucleus contains chromosomes made from long DNA molecules. A gene is a section of DNA that contains instructions for making a particular protein. Human body cells normally contain chromosomes in pairs, with one chromosome of each pair inherited from each parent.
Misconception AlertAn unreplicated chromosome is made from many separate DNA molecules.Select to reveal the correctionSelect to hide the correction
An unreplicated chromosome consists of one long DNA molecule associated with proteins.
The cell cycle
The cell cycle includes cell growth, DNA replication, mitosis and division of the cytoplasm. Mitosis is one part of the cycle, not the name for every stage.
The two new cells produced by division. After mitosis, they are genetically identical to each other and to the parent cell, apart from rare mutations.
Mitosis in more detail
All GCSE courses require the overall outcome of mitosis. Some specifications also require the named stages.
AQA requires the three overall cell-cycle stages but not the named phases of mitosis. Pearson Edexcel and some other specifications may require interphase, prophase, metaphase, anaphase, telophase and cytokinesis. Use your board filter when revising.
Mitosis maintains chromosome number. It produces diploid body cells from a diploid parent cell.
Growth and differentiation
Why mitosis matters
- Growth: repeated division increases cell number.
- Repair: damaged or worn-out cells are replaced.
- Asexual reproduction: one parent produces genetically identical offspring.
Growth in animals mainly occurs through cell division followed by differentiation. Plant growth involves cell division in meristems, cell elongation and differentiation.
In animals, most cells differentiate early in development. Mature animals retain some stem cells for repair and replacement, but many specialised cells cannot divide. In plants, meristem cells continue dividing and differentiating throughout life, so plants can grow new tissues and organs.
Growth is not simply cells becoming larger. Multicellular organisms grow mainly by increasing cell number through mitosis.
Cancer and loss of cell-cycle control
Cell division is normally controlled by genes. Mutations can cause cells to divide repeatedly when they should not. This uncontrolled growth produces a tumour.
Benign tumour
Abnormal cells remain in one location and do not invade other tissues. They can still cause harm by pressing on organs or producing hormones.
Malignant tumour
Cancer cells invade nearby tissues and can travel in blood or lymph to form secondary tumours elsewhere.
Cancer risk can be influenced by inherited alleles and environmental or lifestyle factors such as ionising radiation, smoking and some viral infections. A risk factor increases probability; it does not guarantee that a person will develop cancer.
Quick retrieval check
1. What happens to DNA before mitosis?
2. What is the outcome of mitosis?
3. Give three roles of mitosis.
4. How does plant differentiation differ from animal differentiation?
Exam connection
Explain why DNA must replicate before a cell divides by mitosis.
Show the mark points
- Two copies of each chromosome are needed.
- One complete chromosome set can then move to each end of the cell.
- Each daughter cell receives the same genetic information and chromosome number as the parent cell.