Cell Cycle
The cell cycle is the process that cells go through to grow, copy their DNA, and split into two new cells. This cycle is how our bodies grow, heal, and stay healthy.
It happens in several steps:
- G₁ phase – The cell gets bigger and gets ready to copy its DNA.
- S phase – The cell copies all its DNA so each new cell will have a full set.
- G₂ phase – The cell checks everything and prepares to divide.
- M phase – The cell splits its nucleus (mitosis) and then divides into two new cells (cytokinesis).
The first three steps (G₁, S, and G₂) are called interphase. This is when the cell does most of its work, like growing and preparing for division.
In this article, we’ll walk through each part of the cell cycle in simple terms, so you can understand how cells work and why this process is so important.
Mitosis
This stage can be further subdivided into 4 stages: prophase, metaphase, anaphase, and telophase. Have a look at the diagrams so you are able to identify each stage in an exam situation.
The first subphase is prophase, during this the nuclear membrane (membrane surrounding the nucleus) begins to disappear, spindles begin to develop, and most importantly, the DNA condenses from a spaghetti-like mess to the distinct chromosomes you can see.
The next stage is metaphase where the nuclear membrane has disappeared and the chromosomes line up on the cell equator. It is closely followed by anaphase where the spindle fibres pull apart the chromatids to opposite poles of the cell.
The final stage is telophase, this is where a nuclear membrane forms around the sets of chromosome and the cell begins to seperate into two.
The Phases of Mitosis: How Cells Divide Step by Step
Mitosis is the process where a single cell divides to create two identical cells, each with the same genetic information. This is how our bodies grow, heal wounds, and replace old cells. Let’s walk through each phase of mitosis in a simple and easy-to-understand way:
Prometaphase – Getting Ready to Line Up
The nuclear envelope (the outer layer around the nucleus) breaks apart. This lets spindle fibers (tiny threads) reach the chromosomes. Each chromosome is now ready to be moved, and the spindle fibers attach to a special area on the chromosome called the centromere.
Metaphase – All Lined Up
Now, the chromosomes are moved to the middle of the cell. They line up in a straight row, which makes it easier to divide them equally. This stage is like the cell making sure everything is perfectly in place before moving forward.
Anaphase – Splitting Apart
The spindle fibers pull the chromosomes apart. Each chromosome splits into two identical halves, called sister chromatids, and these are pulled to opposite sides of the cell. This ensures that each new cell will get an exact copy of the DNA.
Telophase – Building New Nuclei
Once the chromatids reach the opposite ends, new nuclear membranes form around them. The chromosomes start to relax and turn back into their original string-like shape. At this point, you can already see two separate nuclei forming inside one cell.
Cytokinesis – The Final Split
This is the final stage. The cell’s cytoplasm divides, and the cell physically splits into two.
- In animal cells, the membrane pinches in the middle to create two separate cells.
- In plant cells, a new wall (called the cell plate) forms between the two new cells.
End result: Two completely new cells, each with the same DNA as the original cell.
What Is Meiosis
Meiosis is a special type of cell division that produces gametes — sperm and egg cells — in sexually reproducing organisms. Unlike regular cell division (mitosis), which creates identical cells, meiosis ensures that each gamete contains half the usual number of chromosomes. This is essential for keeping the correct number of chromosomes when sperm and egg come together during fertilization.
Key Purpose of Meiosis
- Starts with diploid cells (which have two sets of chromosomes — one from each parent).
- Ends with haploid cells (which have just one set of chromosomes).
- Creates genetic variation, which is important for evolution and diversity in offspring.
Meiosis Happens in Two Stages
Meiosis takes place in two main stages: Meiosis I and Meiosis II, each with multiple phases. But DNA is copied only once, before everything begins.
Meiosis I: Reducing the Chromosome Number
Prophase I – Crossing Over Begins
- Chromosomes condense and become visible.
- Homologous chromosomes (one from each parent) pair up closely in a process called synapsis.
- Sections of DNA are exchanged between pairs, creating new combinations (called crossing over or recombination).
This phase has 5 sub-stages:
- Leptotene: Chromosomes start to condense.
- Zygotene: Homologous chromosomes pair up tightly (form tetrads).
- Pachytene: Crossing over happens.
- Diplotene: Chromosomes start to separate, but stay connected at crossover points (chiasmata).
- Diakinesis: Chromosomes fully separate, and crossing over is finalized.
Prometaphase I
- The spindle fibers form and attach to chromosomes at a structure called the kinetochore.
Metaphase I – Chromosomes Line Up
- Homologous pairs line up in the center of the cell.
- The alignment is random, which creates genetic variety (called independent assortment).
- In humans, over 8 million combinations are possible just from this stage.
Anaphase I – Homologous Chromosomes Separate
- Each pair of chromosomes is pulled apart and moved to opposite ends of the cell.
- Unlike mitosis, sister chromatids stay together.
Telophase I & Cytokinesis – Two Cells Form
- The chromosomes arrive at opposite ends.
- New nuclei may form, and the cell splits in two.
- Each new cell is now haploid (has one chromosome from each pair).
Meiosis II: Separating the Chromatids
Meiosis II is similar to mitosis. Each haploid cell now divides again, this time splitting the sister chromatids apart.
Prophase II
- Chromosomes condense again.
- The nuclear membrane dissolves, and spindles form.
Metaphase II
- Chromosomes line up in the middle of the cell.
Anaphase II
- The centromeres split, and sister chromatids are pulled to opposite sides.
Telophase II & Cytokinesis
- New nuclei form, and the cells split again.
- Now, there are four unique haploid cells, each with a single set of chromosomes.
In males, these become four sperm cells. In females, typically one egg and a few smaller, non-functional cells are formed.
How Meiosis Creates Genetic Diversity
Meiosis plays a key role in producing variation in offspring through:
- Crossing Over – DNA is exchanged between chromosome pairs in Prophase I.
- Independent Assortment – Chromosomes line up randomly in Metaphase I and II.
- Mixing of Parental Genes – Every gamete is different, increasing diversity in the next generation.
Summary: Why Meiosis Matters
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Main Purpose | Reduce chromosome number | Separate sister chromatids |
| DNA Replication | Happens once before Meiosis I | Does not happen again |
| Final Outcome | 4 haploid cells (gametes), all genetically unique | |
Meiosis is essential for sexual reproduction, and it ensures that each generation starts with a fresh mix of genetic material, which drives evolution and biological diversity.
Updated: 23 April 2025