The Cell Cycle and Mitosis

What Is the Cell Cycle?

The cell cycle is the series of stages that a dividing cell undergoes, from one division to the next. It prepares cells to divide again and includes periods of growth, DNA replication, and division.

Interphase: The Longest Phase of the Cell Cycle

Interphase accounts for the majority of the cell cycle and includes three stages:

1.G₁ Phase (Gap 1)

• Cell grows in size

• Organelles replicate, ensuring enough materials for two new cells

2.S Phase (Synthesis)

• DNA replication occurs

• DNA is checked for errors

3.G₂ Phase (Gap 2)

• Proteins necessary for mitosis are synthesised

• ATP production increases to meet the energy demand for cell division

Mitosis vs Meiosis

It's essential to know the difference between these two types of cell division:

Feature	Mitosis	Meiosis
Daughter Cells	2 genetically identical cells	4 genetically different cells
Chromosome Number	Same as parent (diploid)	Half of parent (haploid)
Tip to remember	Mitosis has a “T” → T = Two	Meiosis has no T

Chromosomes and DNA Replication

Chromosomes become visible only during cell division.

Before S Phase:

• One chromosome = one chromatid, joined by a centromere

After S Phase:

• Each chromosome consists of two identical sister chromatids, still joined at the centromere → X-shaped structure

After Mitosis:

• Each new cell contains chromosomes with a single chromatid, restoring the original state

Stages of Mitosis

Prophase

Metaphase

Anaphase

Telophase

Cytokinesis

1. Prophase

• Chromosomes condense and become visible

• In animal cells: centrioles form at spindle poles

• Nuclear envelope breaks down

2. Metaphase

• Chromosomes align at the cell’s equator

• Spindle fibres attach to centromeres on either side of each chromosome

3. Anaphase

• Centromeres split, and chromatids are pulled to opposite poles

4. Telophase

• Chromosomes uncoil

• New nuclear envelopes form around each set of DNA

5. Cytokinesis

• Cytoplasm and organelles divide

• Two new daughter cells are formed, entering G₁ phase of the next cycle

how to calculate time spent in a stage of mitosis:

You need:

1. Number of cells in a specific mitotic phase (e.g. anaphase)

2. Total number of cells counted

3. Duration of the entire cell cycle

Formula to calculate the time spent in a stage of mitosis

Time in phase = (Cells in phase / Total cells) × Cell cycle duration