Structure of a Chromosome

Two chromosome arms, connected by a constriction (centromere) can be distinguished on each chromosome. During cell division, two spirally coiled chromatids can be seen in the chromosome arms. These uncoil between cell divisions (interphase) and so cannot be seen. Each chromatid consists of a single giant molecule, folded and wound in a complicated double strand in the shape of a double helix of deoxyribonucleic acid (DNA). It consists of two threads, only about two one-millionths of a millimeter (2 nm) thick, the length of which is determined by the amount of information stored in it. If, for instance, one were to place all the chromosomes of one cell end to end, they would extend 1 millimeter in a bacterium but more than 2 meters in a human. The two threads run in parallel but counter to each other (in opposite directions) and correspond to each other like a photographic negative to its print. They wind around an imaginary axis and can be compared to a twisted rope ladder (a double helix). The DNA forms complexes with basic proteins (histones) to form chromatin. Chromatin is coiled (condensed) into chromosomes, which are visible in optical microscopes only during cell division. During the interphase, it mostly becomes amorphous (euchromatin) apart from a few regions that do not uncoil (heterochromatin). Euchromatin is the genetically active chromatin (see Protein Synthesis), while heterochromatin is genetically inactive.

a, b Schematic representation of a chromosome in metaphase. (After Koolman and Röhm)

a The centromere (primary constriction) is located between the two arms of the chromosome, which are uneven in length and each of which consist of two chromatids 

b Section from a: DNA, together with basic histone proteins, forms tightly coiled complexes arranged like strings of pearls—the nucleosomes

Those histones that are intimately associated with DNA form about one half of the chromatin. The DNA is curled around the histone particles, so that a chromatin fiber is structured like a string of pearls. A histone particle with a DNA segment curled around it (~180 base pairs, see below) is called a nucleosome. Each histone particle consists of eight histone molecules (an octamere).
At the ends of the chromosome arms there are heterochromatin segments (telomeres, satellites) that determine the lifespan of the cell. During each cell division a small segment of chromatin separates until the satellite is used up. At that point the cell dies.
The building blocks of DNA are the nucleotides. They each consist of a base (adenine, cytosine, guanine, or thymine), a sugar (deoxyribose), and an acid phosphate radical. The phosphate radicals of two successive nucleotides form phosphate bridges that connect the nucleotides. Two opposing nucleotides are connected by hydrogen bonds between their bases. When viewed as a rope ladder, the sugar and phosphate units form the sides (the ropes) and the base pairs the rungs of the ladder. The opposing bases are joined in a tongue-and-groove fashion. Because of chemical affinity, adenine always forms a base pair with thymine, and guanine forms a base pair with cytosine.
The total human hereditary material is contained in 23 chromosome pairs in the form of deoxyribonucleic acid. The DNA can be subdivided into three separate segments, genes or hereditary factors, and has three important functions:
The storing of genetic information (the genetic code)
The transmission of information for protein biosynthesis
The identical duplication (replication) of genetic information during cell division