Chromosome

Several kinds of proteins are important for maintaining chromosomes in terms of its organization and gene expression. Some proteins initiate DNA replication when the cell prepares to divide. Other proteins control gene transcription in the preliminary stages of protein synthesis. Structural proteins help the DNA fold into the intricate configurations within the packaged chromosome.

DNA in chromosomes is associated with proteins and this complex is called chromatin. Euchromatin refers to parts of the chromosome that have coding regions or genes, while heterchromatin refers to regions that are devoid of genes or regions where gene transcription is turned off. DNA binding proteins can attach to specific regions of chromatin. These proteins mediate DNA replication, gene expression, or represent structural proteins important in packaging the chromosomes. Histones are structural proteins of chromatin and are the most abundant protein in the nucleus. In fact, the mass of histones in a chromosome is almost equal to that of DNA. Chromosomes contain five types of these small Figure 1. A human karyotype. Illustration by Hans & Cassidy. Courtesy of Gale Group.

proteins: H1, H2A, H2B, H3, and H4. There are two of each of latter four histones that form a structure called the octomeric histone core. The H1 histone is larger than the other histones, and performs a structural role separate from the octomeric histone core in organizing DNA within the chromosome.

The octomeric histone core functions as a spool from which DNA is wound two times. Each histone-DNA spool is called a nucleosome. Nucleosomes occur at intervals of every 200 bases pairs of the DNA helix. In photographs taken with the help of powerful microscopes, DNA wrapped around nucleosomes resembles beads (the nucleosome) threaded on a string (the DNA molecule). The DNA that exists between nucleosomes is called linker DNA. Chromosomes can contain some very long stretches of linker DNA. Often, these long linker DNA sequences are the regulatory portions of genes. These regulatory portions switch genes on when certain molecules bind to them.

Nucleosomes are only the most fundamental organizing structure in the chromosome. They are packaged into structures that are 30 nanometers in size and called the chromatin fiber (compared to the 2 nm DNA double helix, and 11 nm histone core). The 30 nanometer fibers are then Figure 2. How DNA is packaged within chromosomes. Illustration by Hans & Cassidy. Courtesy of Gale Group. further folded into a larger chromatin fiber sometimes that is approximately 300 nanometers thick and represent on of the arms of the chromsome. The chromatin fibers are formed into loops by another structural protein. Each loop contains 20,000–30,000 nucleotide pairs. These loops are then arranged within the chromosomes, held in place by more structural proteins. Metaphase chromosomes are approximately 1400 nm wide.