Cells with nuclei are known as eukaryotic cells, the cells that make up animals, plants and yeast. The nucleus is the organelle that contains the majority of the cell's genetic material, where it is organized into dense linear complexes called chromosomes. The number of these chromosomes varies from species to species. Each chromosome has a central, bulb-like thickening called the centromere which separates the chromosome into two arms: a long arm (q) and a short arm (p). Under normal conditions these arms are not clearly visible.
Eukaryotic cells also store genetic information in another organelle called the mitochondria, a small structure that supplies energy for the various processes that are carried out in the cells. Mitochondria are therefore often called the "power houses" of the cell. Mitochondria have their own independent genome, the mitochondrial DNA. Mitochondrial genes may be small and circular in shape or linear like nuclear genes.
The DNA within nuclear genes is organized into a coil with histone proteins at its centre. Histones are the structural proteins that compact the DNA and this combination of DNA and histones forms the ultimate, dense chromosomal package inside the nucleus called chromatin. Chromatin is only found in eukaryotic cells, with prokaryotic cells having a different arrangement of their genetic material called a genophore - a chromosome that doesn't contain chromatin.
The structure of chromatin varies according to the stage of the cell cycle. For example, when the cell is not dividing or is in interphase, the chromatin may be present in two forms - as euchromatin, the active part of the genome that is usually undergoing transcription, or heterochromatin, which contains mainly inactive DNA, but provides structural support to the chromosomes. Heterochromatin is further divided into constitutive heterochromatin, that which is present near the centromere and contains repetitive sequences that are never expressed, and facultative heterochromatin that may sometimes be expressed as proteins.
During interphase, the chromosomes are present as a tangle of both DNA and proteins and are not easily distinguishable.
As the cell prepares for cell division (meiosis or mitosis), the chromatin strands start to condense and form short, thick thread-like structures. These chromosomes do not express proteins. Each of the chromosomes takes on the classic four-armed structure with the centromere in the middle. The arms are called chromatids, the shorter of which is called the "p" arm and the longer arm "q".
During cell division, long microtubules form the mitotic spindle which pulls the chromatids apart dividing them into equal portions for incorporation into each of the daughter cells that arise from cell division. After cell division is complete, the chromatids uncoil to make chromatin available for transcription processes inside the cell to produce proteins.