The Cell Cycle and Mitosis

Cell division is fundamental to growth, repair, and asexual reproduction in eukaryotic organisms. The cell cycle is an ordered sequence of events that leads to the division of a cell into two genetically identical daughter cells. Understanding the stages of the cell cycle, the process of mitosis, and the mechanisms that control cell division is essential for AQA A-Level Biology (specification 3.2.2).

Key Definition: The cell cycle is the series of events that take place in a cell between one cell division and the next. It consists of interphase (G₁, S, and G₂ phases) and the mitotic phase (mitosis and cytokinesis).

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Interphase

Interphase is the longest part of the cell cycle, typically occupying about 90% of the total cycle time. It is not a 'resting phase' — the cell is highly metabolically active.

G₁ Phase (Gap 1)

• The cell grows in size: it synthesises new proteins, produces organelles (e.g., mitochondria, ribosomes, ER), and increases its cytoplasmic volume.
• The cell carries out its normal metabolic functions.
• The cell monitors its environment and internal conditions. At the end of G₁, there is a critical checkpoint called the restriction point (R point) — if conditions are favourable (adequate nutrients, growth factors, appropriate cell size), the cell commits to DNA replication and division. If conditions are unfavourable, the cell may enter a quiescent state called G₀.

S Phase (Synthesis)

• DNA replication occurs: each chromosome is duplicated to produce two identical sister chromatids joined at the centromere by protein complexes called cohesins.
• At the end of S phase, each chromosome consists of two sister chromatids, so the cell has twice the normal amount of DNA.
• The centrioles also replicate during S phase (in animal cells).
• Histone proteins are synthesised to package the newly replicated DNA into chromatin.

G₂ Phase (Gap 2)

• The cell continues to grow and synthesise proteins needed for cell division.
• Tubulin is synthesised for the construction of the mitotic spindle.
• The cell checks that DNA replication has been completed accurately (the G₂/M checkpoint). If errors are detected, the cell cycle is paused to allow DNA repair before entering mitosis.
• The cell produces ATP stores that will provide energy for mitosis.

Exam Tip: If given a graph showing the amount of DNA per cell over time, you should be able to identify interphase (DNA doubles during S phase), mitosis (DNA halves as cells divide), and the G₁ and G₂ phases (constant DNA amounts before and after S phase).

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Mitosis

Mitosis is the division of the nucleus into two genetically identical daughter nuclei. It is a continuous process but is described in four stages for convenience.

Prophase

• Chromatin condenses (supercoils) into visible chromosomes. Each chromosome consists of two sister chromatids joined at the centromere.
• The nuclear envelope begins to break down (disassembles into fragments that are absorbed into the ER).
• The nucleolus disappears.
• In animal cells, the two pairs of centrioles move to opposite poles of the cell.
• The mitotic spindle begins to form — microtubules extend from each centrosome (the organising centre around the centrioles). In plant cells, spindle microtubules form from diffuse microtubule-organising centres without centrioles.

Metaphase

• Chromosomes align at the metaphase plate (equator) of the cell.
• Each chromosome is attached to spindle fibres via its kinetochore — a protein complex assembled at the centromere of each sister chromatid.
• The spindle assembly checkpoint (SAC) ensures that every kinetochore is properly attached to a spindle fibre before anaphase begins. This prevents unequal chromosome distribution.

Anaphase

• The enzyme separase cleaves the cohesin proteins holding the sister chromatids together at the centromere.
• The sister chromatids are pulled apart to opposite poles of the cell by the shortening of kinetochore microtubules (depolymerisation of tubulin at the kinetochore end).
• The cell elongates as polar microtubules (which overlap at the cell centre) slide past each other.
• Each separated chromatid is now considered an individual chromosome.
• Anaphase is the shortest phase of mitosis.