3.2.6Unit 3: Practical Skills in Biology I
The Cell Cycle and Mitosis — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 20 of 27 Unit quiz
Section 1 of 4The Cell Cycle
The cell cycle is the regulated sequence of events between one cell division and the next, and cyclins trigger the move from one phase to the next.
G₁ phase
- The cell grows, makes RNA, enzymes and other proteins and copies its organelles;
- a signal to divide is received during this gap.
S phase
- DNA is replicated so each chromosome consists of two identical sister chromatids;
- S stands for synthesis of DNA.
G₂ phase
- Growth continues, newly made DNA is checked and errors are usually repaired, and tubulin is produced for the spindle.
M phase
- Nuclear division by mitosis produces two genetically identical nuclei, and cell growth stops while it happens.
Cytokinesis
- The whole cell divides to give two daughter cells, by pinching of the cytoplasm in animals and a new cell wall in plants.
| Phase | Main event |
|---|---|
| G₁ | Cell growth and protein synthesis |
| S | DNA replication |
| G₂ | Checking DNA, preparing spindle |
| M | Mitosis (nuclear division) |
| Cytokinesis | Cytoplasm divides |
Section 2 of 4Stages of Mitosis
Mitosis is one continuous process that is divided into four stages for study (PMAT).
1
Prophase: chromosomes condense and become visible, the nuclear envelope breaks down, and centrosomes move to opposite poles as spindle fibres form.
2
Metaphase: spindle fibres attach to the centromeres and the chromosomes line up on the equator, with sister chromatids facing opposite poles.
3
Anaphase: the centromeres divide and the shortening spindle fibres pull the separated sister chromatids, now chromosomes, to opposite poles.
4
Telophase: chromosomes decondense, nuclear envelopes reform around each set, and the spindle breaks down before cytokinesis follows.
Section 3 of 4Significance of Mitosis
Significance of Mitosis
Growth
- Daughter cells are genetically identical clones, so a zygote can grow into a multicellular organism, for example at plant meristems.
Repair and replacement
- Dead or damaged cells are replaced by identical new ones, which happens quickly in skin and the lining of the gut.
Asexual reproduction
- A single parent produces genetically identical offspring, as in budding of yeast or Hydra and runners of strawberry plants.
Genetic stability
- Each daughter nucleus receives the same number and type of chromosomes as the parent, preserving the genetic information.
Section 4 of 4Exam Checklist & Tips
3.2.6 – Key Points for the Exam
- Cell cycle The regulated sequence of events between the end of one cell division and the end of the next.
- Interphase The period of G₁, S and G₂ when the cell grows and replicates its DNA.
- Sister chromatids Two identical copies of a chromosome joined at the centromere after DNA replication.
- Spindle Microtubule fibres that move chromosomes during nuclear division.
- Cytokinesis Division of the cytoplasm that follows mitosis and gives two cells.
Common Mistakes
- Counting cytokinesis as a stage of mitosis, when it is a separate part of the cell cycle that follows telophase.
- Writing that DNA replicates in prophase; replication takes place earlier, in the S phase of interphase.
- Stating that chromosomes line up at the poles in metaphase, when they line up along the equator.
- Forgetting that a human cell holds 92 DNA molecules after S phase even though it has 46 chromosomes.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.