1. The cell cycle
Cell cycle. The sequence of events by which a cell grows, copies its DNA and divides into two daughter cells. It has two parts:
- Interphase — the longest part; the cell is very active (not "resting").
- G1 (first gap): the cell grows, makes proteins and new organelles.
- S (synthesis): the DNA is replicated; each chromosome now consists of two sister chromatids.
- G2 (second gap): the cell makes proteins for the spindle and prepares to divide.
- M phase — karyokinesis (division of the nucleus by mitosis) followed by cytokinesis (division of the cytoplasm).
DNA content (taking a gamete as 1C): G1 2C → after S 4C → each daughter cell 2C.
Cell division is needed for growth, repair, replacement of cells and reproduction.
Key terms
- Cell cycle
The sequence of growth, DNA replication and division of a cell.
- Interphase
The stage between two divisions, made of G1, S and G2.
- S phase
The phase in which DNA is replicated.
2. Mitosis and cytokinesis
Mitosis is the division of the nucleus that gives two daughter nuclei identical to the parent nucleus, with the same chromosome number. It occurs in somatic (body) cells, especially in meristems (root and shoot tips), skin and bone marrow.
| Stage | Events |
|---|---|
| Prophase | Chromatin condenses into chromosomes, each of two chromatids joined at a centromere; nucleolus and nuclear membrane disappear; in animal cells the centrioles move to opposite poles and the spindle forms |
| Metaphase | Chromosomes line up on the equator (metaphase plate); each centromere is attached to spindle fibres from both poles; chromosomes are most distinct |
| Anaphase | Centromeres split; sister chromatids separate and are pulled to opposite poles (V-shaped, centromere leading) |
| Telophase | Chromosomes reach the poles and uncoil; nuclear membrane and nucleolus re-form; two nuclei |
Cytokinesis
- Animal cell: a furrow (cleavage) in the cell membrane deepens from outside inwards and divides the cell.
- Plant cell: vesicles join at the centre to form a cell plate, which grows outwards; a new cell wall forms. Higher plant cells have no centrioles; the spindle forms from the polar cytoplasm.
Key terms
- Mitosis
Nuclear division giving two identical nuclei with the same chromosome number.
- Karyokinesis
Division of the nucleus.
- Cytokinesis
Division of the cytoplasm.
- Cell plate
The partition formed at the centre of a dividing plant cell that becomes the new wall.
3. Meiosis, homologous chromosomes and crossing over
Meiosis is the division that forms gametes (sperms, eggs, pollen, ova). One diploid (2n) cell divides to give four haploid (n) cells, so it is called a reduction division. (The stages of meiosis are not in the syllabus.)
Why halve the number? At fertilisation two gametes fuse. Haploid gametes give a diploid zygote, so the chromosome number stays constant from generation to generation (human: 46 → 23 in gametes → 46 in zygote).
Homologous chromosomes are a pair of chromosomes alike in size and shape, carrying genes for the same characters at the same positions — one from the father, one from the mother.
Crossing over. During meiosis homologous chromosomes pair up; non-sister chromatids cross at points called chiasmata and exchange segments. The chromatids then carry new combinations of genes. Together with the random sorting of chromosomes into gametes, this produces variation.
Key terms
- Meiosis
Reduction division forming four haploid gametes from a diploid cell.
- Homologous chromosomes
A matching pair of chromosomes, one from each parent.
- Crossing over
Exchange of segments between non-sister chromatids of homologous chromosomes.
- Chiasma
The point where chromatids cross over.
4. Significance of mitosis and meiosis; differences
Significance of mitosis: growth of a multicellular organism from a zygote; repair of wounds and regeneration; replacement of worn-out cells (red blood cells, skin, gut lining); asexual reproduction (amoeba, vegetative propagation); keeps the genetic material unchanged in body cells.
Significance of meiosis: forms gametes; keeps the chromosome number constant across generations; produces variation, the raw material of evolution.
| Mitosis | Meiosis |
|---|---|
| Somatic cells | Reproductive cells (gamete formation) |
| Two daughter cells | Four daughter cells |
| Chromosome number unchanged (2n → 2n) | Chromosome number halved (2n → n) |
| No crossing over | Crossing over occurs |
| Daughter cells identical | Daughter cells genetically different |
| Mitosis | Meiosis |
|---|---|
| Two identical cells | Four different cells |
| 2n to 2n | 2n to n |
Key terms
- Diploid (2n)
Having two sets of chromosomes, as in body cells.
- Haploid (n)
Having one set of chromosomes, as in gametes.
5. Structure of a chromosome
Chromosomes are thread-like structures in the nucleus that carry hereditary information. Humans have 46 (23 pairs) in each body cell.
- Chromatin: in a non-dividing nucleus the genetic material is a fine network of DNA wound around proteins. During division it coils into compact chromosomes.
- Chromatids: after the S phase each chromosome has two identical sister chromatids.
- Centromere: the narrow region that holds the chromatids together and to which spindle fibres attach.
- DNA (deoxyribonucleic acid): a long double helix that carries the coded genetic instructions.
- Gene: a segment of DNA at a particular position on a chromosome that controls a particular character; genes are arranged in a linear order.
Relationship: genes are parts of DNA; DNA with proteins forms chromatin; chromatin coils to form a chromosome.
Key terms
- Chromatin
DNA with proteins as a fine network in a non-dividing nucleus.
- Chromatid
One of the two identical copies of a duplicated chromosome.
- Centromere
The region joining sister chromatids and attaching to the spindle.
- Gene
A segment of DNA controlling a particular character.
Exam tips
Common mistakes
- Calling interphase a "resting stage".
- Doubling the chromosome number at metaphase: it is the chromatids that double (46 chromosomes, 92 chromatids in a human cell).
- Drawing centrioles in plant cells, or a cell plate in animal cells.
- Writing stages of meiosis (not in the syllabus) instead of its purpose.
- Saying meiosis forms two cells, or that its products are identical.
How marks are usually awarded
- 1 mark: a term, a stage, an odd one out with its category, a chromosome number.
- 2 marks: two reasons or two differences, each matched point for point.
- 3 marks: a neat, labelled diagram (stage of mitosis, chromosome, crossing over) — one mark each for the drawing, its key features and correct labels.