Mitosis (Cell Division)

Mitosis is the process by which a cell splits in half to make two identical copies of itself. It is how you grew from a single fertilised egg cell into the trillions of cells that make up your body today. It is also how you heal a cut, replace dead skin and grow longer hair. Mitosis happens constantly in almost every multi-cellular living thing, around the clock. In your body, roughly 2 trillion cells divide by mitosis every day, replacing cells that have worn out or died.

  • Result2 identical cellsFrom one parent cell
  • Cell divisions per dayapprox. 2 trillionIn an adult human body
  • Time for one mitosisapprox. 1 to 24 hoursDepends on cell type
  • Main stages4Prophase, metaphase, anaphase, telophase
  • DNA before splittingDoubledIn a process called S phase
  • Different fromMeiosisWhich makes egg and sperm cells, not identical copies

Why cells need to divide

Living things need cell division for two big reasons.

  • To grow. You started as a single fertilised egg cell. By the time you were born you had over a trillion cells. By the time you are an adult you have around 37 trillion. Every one of those cells came from earlier cells dividing.
  • To replace worn-out cells. Many cells in your body do not last long. Skin cells live for a few weeks. Red blood cells about 4 months. The cells lining your gut last only a few days. Without constant cell division to replace them, your body would fall apart in weeks.

The cell cycle

Every time a cell divides, it goes through a sequence of stages called the cell cycle. It has two main phases:

  • Interphase: the cell grows, does its normal job, and copies its DNA. Most of the cycle is spent here.
  • Mitotic phase: the cell actually divides. This part is fast (usually just an hour or two) but it is when all the dramatic action happens.

The four stages of mitosis

Mitosis itself has four main stages, traditionally remembered with the mnemonic PMAT:

  1. Prophase: the DNA condenses into visible X-shaped chromosomes (each containing the original DNA plus the new copy stuck together). The nucleus starts to break down.
  2. Metaphase: the chromosomes line up neatly in the middle of the cell, attached to a spindle of fibres anchored at the cell's two poles.
  3. Anaphase: the two halves of each chromosome are pulled apart, one to each end of the cell.
  4. Telophase: a new nucleus forms around each set of chromosomes. The cell then pinches in the middle and splits in two (a step sometimes called cytokinesis).

The result is two daughter cells, each genetically identical to the original parent cell and to each other.

How fast does it happen?

Different cells divide at very different rates. Some examples:

  • Embryo cells in a brand new fertilised egg can divide every 20 to 30 minutes.
  • Gut lining cells divide roughly every 3 to 4 days, because they get worn out so quickly.
  • Skin cells divide about once a month.
  • Liver cells divide rarely (every year or two) but can ramp up dramatically if the liver is injured.
  • Brain and heart cells mostly do not divide at all once you are an adult, which is why brain and heart damage is so hard to recover from.
Fact Mitosis happens so often in your body that, over an average lifetime, around 10 thousand trillion cell divisions take place. The vast majority go perfectly. But occasionally one goes wrong, and a cell gets the wrong number of chromosomes, or a damaged copy of its DNA. Most of these bad cells are spotted by the body's quality-control systems and destroyed. The ones that escape are the seeds of cancer.

Mitosis vs meiosis

Mitosis is the everyday cell division that makes identical copies. Almost every cell in your body uses mitosis. But there is a second, special kind of cell division called meiosis, used only for making sex cells (eggs and sperm). Meiosis is different in two big ways:

  • It produces 4 daughter cells, not 2.
  • Each daughter cell ends up with half the normal number of chromosomes.

The reason for the half-number is that when an egg and a sperm join up to create a new baby, the two halves combine to make a full set again. This is also how each person gets a unique mix of features from both parents.

Did you know? A few special cells in your body almost never divide and have to last your entire life. Most of the neurons in your brain were made before you were a few months old, and you keep most of them for life (though you do grow a few new ones in some brain regions). Almost all of your egg cells (if you have ovaries) were made before you were born. The lenses in your eyes never replace their cells; if they did, you would briefly go blind every time it happened.
Deeper dive: when mitosis goes wrong, cancer

The body has incredibly tight control over when cells are allowed to divide. Most cells are not allowed to divide unless they receive specific chemical signals telling them it is time, and most cells also have a built-in counting system (linked to small caps on the ends of chromosomes called telomeres) that limits the total number of times they can divide. These controls keep the body in shape and stop cells from going rogue.

Cancer is what happens when a cell's controls break down. Random damage to the DNA, often caused by ultraviolet radiation, smoke or other chemicals, can mutate the genes that control cell division. The damaged cell starts dividing when it should not, ignoring all the normal stop signals. Its descendant cells inherit the same broken controls and keep dividing too. Eventually, after enough mutations have built up, a single bad cell can produce a growing lump of millions or billions of cells: a tumour.

Modern medicine treats cancer by trying to kill the rogue cells using surgery (cut them out), radiation (damage their DNA so they cannot divide), chemotherapy (poison fast-dividing cells) or newer techniques like immunotherapy (train the immune system to attack tumours). The fact that we now have all these tools comes directly from learning how mitosis works at the molecular level. Modern biology has saved millions of lives by understanding this one process.

For more on cells, see what is a cell and parts of a cell. For the cells used to grow whole new organs, see stem cells.