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You started off as a fertilized cell inside your mom, called a zygote. Now, you’re a thriving community of hundreds of millions of cells, all working together towards a common purpose: to keep you alive. How did so many cells come from just one?
Generally speaking, the answer is straightforward: many cells come from just one by repeated cell division. Your first form as a zygote split to make two cells. Then those cells split, making four...and so on and so forth, until you became the living, functioning organism you are today.
There are two ways cell division can happen in humans and most other animals, called mitosis and meiosis. When a cell divides by way of mitosis, it produces two clones of itself, each with the same number of chromosomes. When a cell divides by way of meiosis, it produces four cells, called gametes. Gametes are more commonly called sperm in males and eggs in females. Unlike in mitosis, the gametes produced by meiosis are not clones of the original cell, because each gamete has exactly half as many chromosomes as the original cell.

The concept of a chromosome

A chromosome is a thread-like object (scientists literally called them threads or loops when they were first discovered) made of a material called chromatin. Chromatin is made of DNA and special structural proteins called histones. One way to think of a chromosome is as one very long strand of DNA, with a bunch of histone proteins stuck to it like beads on a string.
Figure of a chomosome, chromatin fiber, histones, nucleosome, and DNA
Chromosomes are stored in the nuclei of cells. If you compare the diameter of a cell nucleus (between 2 and 10 microns) to the length of a chromosome (between 1 and 10 centimeters, when fully extended!), you can see that a chromosome must be scrunched up into a very small package in order to fit inside a nucleus. Actually, the average chromosome is about a thousand times longer than a cell nucleus is wide. The situation is a bit like how a very long snake can coil up into a tight ball.
Illustration of an uncoiled and coiled snake
The basic construction of chromosomes (made of chromatin) and structure (long but scrunched up) is the same in all animals. The difference is that each species has its own set number of chromosomes. For instance, all human cells (except gametes) have 46 chromosomes. Cells of nematodes (worms), other than gametes, have 4 chromosomes. The number of total chromosomes in the non-gamete cells of a particular species is called the diploid number for that species. The diploid number of humans is 46, and the diploid number of nematodes is 4.
Figure of human and nematode diploid and haploid counts
The total number of chromosomes in the gametes of a particular species is referred to as the haploid number of that species. This number is always half of the diploid number. For instance, the haploid number in humans is 23, and the haploid number in nematodes is 2.

The concept of mitosis

The purpose of mitosis is to make more diploid cells. It works by copying each chromosome, and then separating the copies to different sides of the cell. That way, when the cell divides down the middle, each new cell gets its own copy of each chromosome.

The phases of mitosis

Diagram of the five phases of mitosis
In the first step, called interphase, the DNA strand of a chromosome is copied (the DNA strand is replicated) and this copied strand is attached to the original strand at a spot called the centromere. This new structure is called a bivalent chromosome. A bivalent chromosome consists of two sister chromatids (DNA strands that are replicas of each other). When a chromosome exists as just one chromatid, just one DNA strand and its associated proteins, it is called a monovalent chromosome. Here is a drawing of what happens in a nematode nucleus (diploid number 4) during interphase, with individual chromatids represented as numbers, sister chromatids as the same number, and the centromere represented as a “-”.
The second and third steps of mitosis organize the newly created bivalent chromosomes so that they they can be split in an orderly fashion. A lot of care has to be taken with this process, because unequal splitting of chromosomes creates malfunctioning cells. Down syndrome is one disease that results from unequal splitting of chromosomes.
In the second step, prophase, the bivalent chromosomes condense into tight packages, the mitotic spindle forms, and the nuclear envelope dissolves. Imagine the difference between a slinky fully stretched out, and a slinky that has been pressed back together. That's what happens to chromosomes during prophase: they get pressed together into tight packages.
In the third step of mitosis, called metaphase, each chromosome lines up in a single file line at the center of the cell. By this point in time, the membrane enclosing the nucleus has dissolved, and mitotic spindles have attached themselves to each chromatid in all the chromosomes. Here is a diagram of what a nematode cell nucleus looks like after prophase and metaphase.
In the fourth step, anaphase, the mitotic spindles pry each chromatid apart from its copy, and drag them to the opposite side of the cell. Four bivalent chromosomes become two groups of 4 monovalent chromosomes.
Diagram of prometaphase
Once anaphase is over, the heavy lifting of mitosis is complete. In the final phase, telophase, membranes form around the two new groups of chromosomes, and the mitotic spindles that provided the power to create these groups are disassembled. Once mitosis is complete, the cell has two groups of 46 chromosomes, each enclosed with their own nuclear membrane. The cell then splits in two by a process called cytokinesis, creating two clones of the original cell, each with 46 monovalent chromosomes.
Diagram of metaphase
Diagram of anaphase
Diagram of telophase and cytokinesis

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