Each chromosome is comprised of one very long DNA molecule. Chromosomes are made up of a material called chromatin, which is made up of DNA and protein molecules. The protein molecules help organize the chromatin and help control the activity of its genes.
Most of the time, chromosomes aren't compact. In fact, they exist as a diffuse mass of fibers that could stretch for over 2 meters. However, once the cell prepares to divide, its chromatin fibers coil up, forming compact chromosomes, which are the ones we usually see images of (it looks like an X). When a cell isn't divided, the chromosomes are too loose and thin to be seen in a light microscope.
The reason the DNA is able to be packed so compactly is due to proteins called histones. They coil and fold the proteins, making them fit into a very small chromosome. How does it do this?
First, the histones attach to the DNA.
Next, the histones and DNA form a "bead", called a nucleosome, which is just DNA wrapped around 8 histones.
Next, that fiber is coiled AGAIN to make a supercoil.
Before a cell divides, it has to duplicate all of its chromosomes, so each DNA molecule is copied through DNA replication. That means each chromosome has two copies called sister chromatids, which contain identical genes. So, if a chromosome was an "X", the bottom "^" would be the sister chromatids. In addition the point at which the chromatids intersect is called the centromere.
When the cell divides, the sister chromatids separate, and then each chromatid is identical to the original chromosome.
The Cell Cycle
Eukaryotic cells that divide undergo a cell cycle, a cycle that continues from cell birth to division. The cell cycle can be split into two phases: Mitosis (Mitotic phase), which takes 10% of the time, and Interphase, which takes 90% of the time.
Interphase is when a cell performs its designed function. For example, a stomach cell may release digestive enzymes. During interphase, a cell doubles everything in its cytoplasm, including organelles, and it grows in size.
During G1 or Gap1, the cell makes a variety of proteins that are needed for DNA replication. This is the longest part.
During this phase, chromosome duplication occurs.
During this phase, each chromosome consists of two identical sister chromatids, and the cell is ready to divide.
Mitotic phase consists of mitosis and cytokinesis.
In mitosis, the nucleus and its contents, divid and are evenly distributed, forming two daughter nuclei.
In cytokinesis, the cytoplasm actually divides, and the cell is split into two.