You began life as a single cell, a fertilized egg. Today your body holds tens of trillions of cells, and almost every one of them carries the same set of instructions as that first cell. A skin cell that heals a cut, a bone cell that lengthens your leg during a growth spurt, and a cell lining your gut that is replaced every few days were all made by the same process: mitosis, cell division that produces two genetically identical cells. Before you can see why mitosis copies information so faithfully, you need to know what that information is and how it is packaged. This lesson starts with DNA and builds up to whole chromosomes, then follows a cell through its life cycle.
From nucleotide to genome
DNA (deoxyribonucleic acid) is a long chain built from units called nucleotides. Each nucleotide has three parts: a phosphate group, a sugar called deoxyribose, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) or cytosine (C). Two chains wind around each other to form a double helix, held together by pairs of bases: A always pairs with T, and G always pairs with C. This complementary base pairing is the key to copying: if you know one strand, you can rebuild the other. The order of bases is the code. A gene is a section of DNA whose base sequence carries the instructions for one protein (or a functional RNA), and proteins do most of the cell's work. DNA is wrapped around proteins called histones and coiled into chromosomes; each chromosome holds hundreds to thousands of genes. The complete set of DNA in an organism is its genome.
| Level | What it is | In humans |
|---|---|---|
| Nucleotide | Phosphate + deoxyribose sugar + one base (A, T, G or C) | About 3 billion base pairs in one set of chromosomes |
| Gene | A segment of DNA that codes for a protein or functional RNA | About 20 000 protein-coding genes |
| Chromosome | One very long DNA molecule packaged with histone proteins | 46 in somatic cells (23 pairs); 23 in gametes |
| Genome | All the DNA in one set of an organism's chromosomes | Stretched out, the DNA in one cell would be about 2 m long |
The cell cycle
The cell cycle is the sequence of events from one division to the next. Most of it is interphase, which is not a resting stage at all. In G1 (gap 1) the cell grows, makes proteins and does its normal job. In the S phase (synthesis) it replicates its DNA, so every chromosome becomes two identical sister chromatids joined at a centromere. In G2 it grows further, makes extra organelles and checks that replication went correctly. Only then does it enter the M phase: mitosis (division of the nucleus) followed by cytokinesis (division of the cytoplasm). Checkpoints between stages stop the cycle if DNA is damaged. Some cells, such as most mature neurons, leave the cycle and stop dividing (a state called G0). When checkpoints fail and cells divide without control, the result can be cancer.
The phases of mitosis
| Phase | What happens | How to recognize it under a microscope |
|---|---|---|
| Prophase | Chromatin coils into visible chromosomes (two sister chromatids each); the nuclear membrane breaks down; spindle fibres start to form | Thick, thread-like chromosomes inside a fading nucleus |
| Metaphase | Spindle fibres attach to each centromere and line the chromosomes up across the middle (equator) of the cell | Chromosomes in a single line across the centre |
| Anaphase | Centromeres split and sister chromatids are pulled to opposite poles; each is now a separate chromosome | Two V-shaped groups moving apart |
| Telophase | Chromosomes reach the poles and uncoil; a nuclear membrane forms around each set | Two nuclei at opposite ends of one elongated cell |
| Cytokinesis | The cytoplasm divides: animal cells pinch in with a furrow; plant cells build a cell plate that becomes a new wall | Two separate daughter cells |
Why are the two daughter cells identical? Because the S phase makes an exact copy of every chromosome, and anaphase hands one chromatid of each chromosome to each new cell. A human cell entering mitosis has 46 chromosomes made of 92 chromatids; each daughter receives 46 chromosomes, the same set the parent started with. Mitosis happens in somatic cells (body cells), where it is used for growth, repairing tissue and replacing worn-out cells; some organisms also use it for asexual reproduction. Germ cells, the cells in the ovaries and testes that give rise to eggs and sperm, use a different division, meiosis, which halves the chromosome number and mixes alleles. That is the subject of the next lesson.
Observing mitosis: the onion root tip
Most cells in a mature plant are not dividing, but the tip of a growing root contains a meristem, a zone of rapid division. Onion root tips are a classic specimen: the tip is softened in acid, squashed flat on a slide and stained so that chromosomes show up dark. Every cell is frozen at whatever stage it was in at that moment. That is useful: if a cell spends a long time in one phase, many cells will be caught in that phase. So the percentage of cells in a phase estimates the percentage of the cycle's time spent in it. Biologists count at least 100 to 200 cells, ideally from several fields of view, to get a reliable estimate. Interphase cells always dominate the count, because interphase takes most of the cycle.