Unit 1.6: Cell Division
Part 1: Chromosomes, the Cell Cycle
and Mitosis
1. Chromosomes, DNA and genes
Term Meaning
DNA The molecule that stores genetic information
Gene A section of DNA containing the code for a polypeptide or functional RNA
Chromosome A long DNA molecule wrapped around proteins, containing many genes
Chromatid One of two identical copies of a replicated chromosome
Centromere The region joining two sister chromatids together
2. Chromosomes and chromatids
• Most of the time DNA is loose chromatin; chromosomes only become visible when
DNA condenses before/during division.
• After DNA replication, each chromosome = two identical sister chromatids joined
at one centromere.
Key definition
Chromosome number does NOT double after replication. DNA amount doubles, but sister
chromatids joined at one centromere still count as one chromosome. A human cell after replication still
has 46 chromosomes, but now 92 chromatids.
3. Homologous chromosomes
, • A homologous pair = one chromosome from each parent, with the same genes in
the same positions, similar length/shape and the same centromere position, but
possibly different alleles.
Humans: 46 chromosomes in body cells = 23 homologous pairs (22 pairs of autosomes +
1 pair of sex chromosomes).
4. Diploid and haploid numbers
Term Symbol Meaning Human example
Diploid 2n Two sets of chromosomes Body cells: 2n = 46 (23 pairs)
Haploid n One set of chromosomes Gametes (sperm/egg): n = 23
Fertilisation restores the diploid number: n + n → 2n.
5. The cell cycle
• The regular sequence of growth and division: Interphase → Mitosis →
Cytokinesis. Most of a cell's life is spent in interphase, which is part of the cell
cycle but not a stage of mitosis.
Interphase
• The cell prepares for division: DNA replicates, mitochondria/chloroplasts replicate,
new organelles form, ribosomal material and proteins are synthesised, ATP is made,
and the cell increases in size.
Watch out! Marks get lost here
• Don't say 'the cell grows' during interphase. Say it increases in size — 'growth' usually implies
a permanent increase in dry mass.
Stage Main events DNA content
G1 Cell increases in size, makes proteins and organelles Normal amount
S DNA replication occurs Increases as replication
happens
G2 Further growth, protein synthesis, preparing for mitosis Double the original amount
The doubled DNA amount stays through G2 and mitosis, then splits between daughter cells
at cytokinesis, returning each cell to the original amount.
6. Mitosis
Part 1: Chromosomes, the Cell Cycle
and Mitosis
1. Chromosomes, DNA and genes
Term Meaning
DNA The molecule that stores genetic information
Gene A section of DNA containing the code for a polypeptide or functional RNA
Chromosome A long DNA molecule wrapped around proteins, containing many genes
Chromatid One of two identical copies of a replicated chromosome
Centromere The region joining two sister chromatids together
2. Chromosomes and chromatids
• Most of the time DNA is loose chromatin; chromosomes only become visible when
DNA condenses before/during division.
• After DNA replication, each chromosome = two identical sister chromatids joined
at one centromere.
Key definition
Chromosome number does NOT double after replication. DNA amount doubles, but sister
chromatids joined at one centromere still count as one chromosome. A human cell after replication still
has 46 chromosomes, but now 92 chromatids.
3. Homologous chromosomes
, • A homologous pair = one chromosome from each parent, with the same genes in
the same positions, similar length/shape and the same centromere position, but
possibly different alleles.
Humans: 46 chromosomes in body cells = 23 homologous pairs (22 pairs of autosomes +
1 pair of sex chromosomes).
4. Diploid and haploid numbers
Term Symbol Meaning Human example
Diploid 2n Two sets of chromosomes Body cells: 2n = 46 (23 pairs)
Haploid n One set of chromosomes Gametes (sperm/egg): n = 23
Fertilisation restores the diploid number: n + n → 2n.
5. The cell cycle
• The regular sequence of growth and division: Interphase → Mitosis →
Cytokinesis. Most of a cell's life is spent in interphase, which is part of the cell
cycle but not a stage of mitosis.
Interphase
• The cell prepares for division: DNA replicates, mitochondria/chloroplasts replicate,
new organelles form, ribosomal material and proteins are synthesised, ATP is made,
and the cell increases in size.
Watch out! Marks get lost here
• Don't say 'the cell grows' during interphase. Say it increases in size — 'growth' usually implies
a permanent increase in dry mass.
Stage Main events DNA content
G1 Cell increases in size, makes proteins and organelles Normal amount
S DNA replication occurs Increases as replication
happens
G2 Further growth, protein synthesis, preparing for mitosis Double the original amount
The doubled DNA amount stays through G2 and mitosis, then splits between daughter cells
at cytokinesis, returning each cell to the original amount.
6. Mitosis
