What you'll learn
This revision guide covers cell division, chromosomes and genes as required for the CIE IGCSE Co-ordinated Science (Double Award) specification. You will understand how cells replicate through mitosis and meiosis, the structure and function of chromosomes, and how genetic information is organised and passed from parent to offspring. These concepts form the foundation for understanding inheritance, variation and reproduction.
Key terms and definitions
Chromosome — a thread-like structure of DNA carrying genetic information in the form of genes, found in the nucleus of cells
Gene — a section of DNA that codes for a specific protein, determining a particular characteristic
Mitosis — cell division producing two genetically identical daughter cells from one parent cell, used for growth and repair
Meiosis — cell division producing four genetically different daughter cells (gametes) with half the chromosome number of the parent cell
Diploid — a cell containing two complete sets of chromosomes (one from each parent), represented as 2n
Haploid — a cell containing one complete set of chromosomes, represented as n (found in gametes)
Gamete — a sex cell (sperm or egg) produced by meiosis with half the normal chromosome number
DNA — deoxyribonucleic acid; the molecule that carries genetic information in the form of genes
Core concepts
Chromosomes and DNA structure
Chromosomes are found in the nucleus of cells. In humans, body cells contain 46 chromosomes arranged in 23 pairs. Each chromosome consists of a long molecule of DNA coiled up with proteins.
DNA structure:
- DNA is a double helix made of two strands twisted together
- Each strand contains a sequence of bases
- Four bases exist: adenine (A), thymine (T), cytosine (C) and guanine (G)
- Bases pair specifically: A with T, C with G
- The sequence of bases determines the genetic code
Genes and proteins:
- Genes are specific sections of DNA located on chromosomes
- Each gene codes for a particular sequence of amino acids
- Amino acid sequences determine protein structure
- Different proteins result in different characteristics (e.g. eye colour, blood type)
- Humans have approximately 20,000-25,000 genes
Mitosis and cell division for growth
Mitosis produces two identical daughter cells from one parent cell. This type of cell division is essential for:
- Growth of multicellular organisms
- Repair of damaged tissues
- Replacement of worn-out cells
- Asexual reproduction in some organisms
The process of mitosis:
- Before division: The cell grows and replicates its DNA so that each chromosome becomes two identical structures (called chromatids) joined together
- Early mitosis: The chromosomes condense and become visible; the nuclear membrane breaks down
- Middle stage: Chromosomes line up along the centre of the cell
- Separation: The chromatids are pulled apart to opposite ends of the cell
- Completion: Two new nuclei form; the cytoplasm divides; two identical daughter cells result
Key features of mitosis:
- Maintains the diploid chromosome number (2n → 2n → 2n)
- In humans: 46 chromosomes → 46 chromosomes in each daughter cell
- Daughter cells are genetically identical to each other and to the parent cell
- Each daughter cell receives exactly the same genetic information
Meiosis and gamete formation
Meiosis is a special type of cell division that produces gametes (sex cells). It occurs only in the reproductive organs: testes in males and ovaries in females.
The process of meiosis:
- Before division: DNA replicates so each chromosome consists of two chromatids
- First division: Chromosome pairs separate, and the cell divides into two cells
- Second division: Chromatids separate, and both cells divide again
- Result: Four haploid gametes are produced from one diploid parent cell
Key features of meiosis:
- Reduces the chromosome number by half (2n → n)
- In humans: 46 chromosomes → 23 chromosomes in each gamete
- Produces four daughter cells from one parent cell
- Daughter cells are genetically different from each other and from the parent cell
- Creates genetic variation through:
- Random distribution of chromosomes into gametes
- Mixing of maternal and paternal chromosomes
Why meiosis is necessary:
- Gametes must have half the normal chromosome number
- At fertilisation, sperm (23 chromosomes) + egg (23 chromosomes) = zygote (46 chromosomes)
- Without meiosis, chromosome numbers would double each generation
- The diploid number is restored at fertilisation
Comparing mitosis and meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Gamete production |
| Location | Body cells (somatic cells) | Reproductive organs only |
| Number of divisions | One | Two |
| Daughter cells produced | Two | Four |
| Chromosome number | Diploid to diploid (2n → 2n) | Diploid to haploid (2n → n) |
| Genetic makeup | Identical to parent | Different from parent |
| Variation | No variation produced | Creates variation |
Stem cells and differentiation
Stem cells are unspecialised cells that can divide by mitosis to produce more cells. These cells can then differentiate to become specialised for particular functions.
Types of stem cells:
Embryonic stem cells:
- Found in early embryos
- Can differentiate into any type of cell
- Used in research for treating diseases like Parkinson's disease and diabetes
- Ethical concerns exist regarding their use
Adult stem cells:
- Found in some tissues (e.g. bone marrow, skin)
- Can only differentiate into certain cell types
- Fewer ethical concerns
- Used to treat some blood diseases
Plant stem cells:
- Found in meristems (root and shoot tips)
- Can differentiate into any plant cell type throughout the plant's life
- Used in horticulture for cloning plants
Genetic variation and its importance
Genetic variation refers to differences in DNA sequences between individuals. This variation is essential for evolution and species survival.
Sources of genetic variation:
Meiosis:
- Random distribution of chromosomes into gametes
- Each gamete receives a different combination of maternal and paternal chromosomes
- In humans, over 8 million possible chromosome combinations in gametes
Fertilisation:
- Random fusion of any sperm with any egg
- Creates unique combinations of genetic material
- In humans, over 70 trillion possible genetic combinations in offspring
Mutation:
- Random changes in DNA sequences
- Can occur during DNA replication
- Most mutations have no effect; some are harmful; rarely, some are beneficial
Importance of variation:
- Allows populations to adapt to environmental changes
- Provides raw material for natural selection
- Prevents populations from being wiped out by single diseases
- Essential for evolution and species survival
Worked examples
Example 1: Calculating chromosome numbers
Question: A horse has 64 chromosomes in its body cells.
(a) State how many chromosomes will be in a horse sperm cell. [1]
(b) Explain why gametes must have half the chromosome number of body cells. [2]
Mark scheme answers:
(a) 32 chromosomes [1 mark]
Explanation: Gametes are haploid (n), containing half the diploid (2n) number. 64 ÷ 2 = 32
(b)
- So that when fertilisation occurs / sperm and egg fuse [1]
- The diploid / normal / 64 chromosome number is restored / the chromosome number doesn't double each generation [1]
Common error: Students often forget to explain what happens at fertilisation or why maintaining chromosome number is important.
Example 2: Comparing cell division processes
Question: The diagram shows stages in cell division.
Distinguish between mitosis and meiosis by completing the table. [4]
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of daughter cells produced | ||
| Are daughter cells genetically identical? |
Mark scheme answers:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of daughter cells produced | Two [1] | Four [1] |
| Are daughter cells genetically identical? | Yes / identical [1] | No / different / variation [1] |
Key point: Be specific with your answers. "More" or "less" is not sufficient; exact numbers are required.
Example 3: Understanding stem cells
Question: Stem cells can be used in medicine to treat certain diseases.
(a) Define what is meant by a stem cell. [2]
(b) Suggest one reason why some people object to the use of embryonic stem cells. [1]
Mark scheme answers:
(a)
- An unspecialised / undifferentiated cell [1]
- That can divide / differentiate to form specialised cells / different cell types [1]
Alternative acceptable: a cell that can divide to form more stem cells
(b) Any one from:
- Embryos are destroyed / killed in the process [1]
- Religious / ethical / moral objections [1]
- Concerns about potential for human cloning [1]
Note: Be balanced in your response. This is a factual biology exam, not an ethics essay.
Common mistakes and how to avoid them
Confusing mitosis with meiosis: Remember that mitosis produces TWO identical cells for growth; meiosis produces FOUR different cells for reproduction. Create a comparison table and memorise the key differences.
Incorrectly stating chromosome numbers: Body cells are diploid (2n); gametes are haploid (n). In humans: body cells have 46 chromosomes, gametes have 23. Always halve the body cell number for gametes.
Stating genes are on DNA instead of DNA forming chromosomes: Genes are sections of DNA, and DNA molecules make up chromosomes. The correct hierarchy is: bases → DNA → genes → chromosomes → nucleus.
Writing that meiosis creates variation through "mixing genes": Be specific. Variation arises from random distribution of chromosomes into gametes and random fertilisation. Use precise terminology, not vague phrases.
Forgetting to explain the role of fertilisation when asked why meiosis is necessary: Meiosis halves the chromosome number so that at fertilisation, when gametes fuse, the diploid number is restored. Always link meiosis to fertilisation in your answers.
Confusing stem cell types: Embryonic stem cells can become any cell type; adult stem cells are more limited. Know the differences and be able to explain advantages and concerns for each type.
Exam technique for "Cell division, chromosomes and genes"
Command word precision: "State" requires a brief answer without explanation (1 mark). "Explain" requires reasons or mechanisms (2-3 marks). "Compare" or "distinguish" requires similarities AND differences between two things, with clear linking.
Use accurate scientific terminology: The examiner expects terms like diploid, haploid, chromatid, differentiation and chromosome. Vague language like "baby cells" or "splits" will not gain marks. Learn and use the correct vocabulary consistently.
Show calculations clearly: When working out chromosome numbers, show your division or multiplication. For example, "46 ÷ 2 = 23 chromosomes in the gamete." This can gain method marks even if your final answer is incorrect.
Answer length matches mark allocation: A 1-mark question needs one clear point. A 3-mark question typically needs three distinct points or one developed explanation with three elements. Count the marks and ensure sufficient detail in your response.
Quick revision summary
Chromosomes are made of DNA and carry genes in the cell nucleus. Mitosis produces two identical diploid daughter cells for growth and repair. Meiosis produces four genetically different haploid gametes for sexual reproduction. In humans, body cells contain 46 chromosomes (diploid); gametes contain 23 (haploid). At fertilisation, the diploid number is restored. Genetic variation arises from meiosis and random fertilisation. Stem cells are unspecialised cells that can differentiate into specialised cell types.