Chromosome structure

Eukaryotic chromosome structure refers to the levels of packaging from the raw DNA molecules to the chromosomal structures seen during metaphase in mitosis or meiosis. Chromosomes contain long strands of DNA containing genetic information. Compared to prokaryotic chromosomes, eukaryotic chromosomes are much larger in size and are linear chromosomes. Eukaryotic chromosomes are also stored in the nucleus of the cell, while chromosomes of prokaryotic cells are not stored in a nucleus.

Some of the first scientists to recognize the structures now known as chromosomes were Schleiden, Virchow, and Bütschli, O. The term was coined by Heinrich Wilhelm Gottfried von Waldeyer-Hartz, referring to the term chromatin, which was introduced by Walther Flemming.

In the 1900s, many scientists concluded the same ideas of heredity that Gregor Mendel had previously. Scientists also discovered plant and animal cells have a central compartment called the nucleus. They soon realized chromosomes were found inside the nucleus and contained different information for many different traits.

In eukaryotes, such as humans, roughly 3.2 billion nucleotides are spread out over 23 different chromosomes (males have a Y chromosome instead of X compared to females). Each chromosome consists enormously long linear DNA molecule associated with proteins that fold and pack the fine thread of DNA into a more compact structure.

The nucleosome consists of a DNA double helix bound to an octamer of core histones (2 dimers of H2A and H2B, and an H3/H4 tetramer). About 147 base pairs of DNA coil around 1 octamer, and ~20 base pairs are sequestered by the addition of the linker histone (H1), and various length of "linker" DNA (~0-100 bp) separate the nucleosomes. The double helix was discovered in 1953 by James Watson and Francis Crick. Other researchers made very important, but unconnected findings about the composition of DNA. But it was up to Watson and Crick to put all of those findings together to come up with a model for DNA. Later, chemist Alexander Todd determined that the backbone of a DNA molecule contained repeating phosphate and deoxyribose sugar groups. The biochemist Erwin Chargaff found that adenine and thymine always paired while cytosine and guanine always paired. High resolution X-ray images of DNA that were obtained by Maurice Wilkins and Rosalind Franklin suggested a helical, or corkscrew like shape.

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