About Histones

Histones are important proteins that enclose the nucleic acid, DNA (deoxyribonucleic acid), into the chromosomes of a nucleus. They play an important role in the expression of proteins from nucleic acid.

The discovery of histones dates back in 1884, but their real value and function in gene regulation was understood only in recent times.

Histones have many functions. They wind DNA within the chromosomes; wound DNA are 50,000 times shorter than unwound ones, so without histones DNA would be very long in chromosomes. Histones are also essential in several other cellular processes such as ADP-ribosylation, phosphorylation and ubiquitination.

Classes

• Previously, it was thought that there were only five classes of histones: one linker histone, H1, and four core histones, H2A, H2B, H3 and H4. The newly discovered variety of histones (H5) is a highly conserved protein. The recent research has discovered several other histone proteins that also play an important role in different cellular processes. Histone proteins create nucleosome core particles called "chromatin" by packaging DNA around their nuclear proteins.

Histones in Chromosomes

• The nucleus contains the chromosomes, which are made of two types of chemical substances: protein and deoxyribonucleic acid, or DNA. Each nuclear protein consists of multiple varieties of histones. The histones found in chromosomes are some of the most conserved proteins in the eukaryotic cells. Chromosomal histones are essentially basic amino acids. However, the amino acid sequences vary depending on the organisms.

Histones Modifications

• The primary amino acid sequence of histones does not vary, but the individual histone molecules can be different in their structures due to the chemical changes that take place later to individual amino acids. These chemical changes mainly occur in the H3 and H4 varieties, and most of these changes are reversible. Modified histones have been found to play an important role in the formation of telomere--that is, the natural end of a eukaryotic chromosome--centromere and Barr body--a chromatin mass found in females.

Novel Histone--H5

• The discovery of the new histone variant, H5, is the result of extensive research. Histone H5 was first observed in chicken erythrocytes. Later, it was detected in avian red blood cells. Recent research has found a close resemblance between histone H5 and the catabolite gene activator (CAP) protein in their structures.

Histones and Their Functions

• Every human cell contains about 300 million histone molecules. Histones help in enclosing an entire DNA content of a cell into the chromosomes of the nucleus. They are also associated with the formation of a basic chromatin subunit called "nucleosome."