The helix will make a complete turn about every 10 base pairs. There is a wider major groove to the molecule and a narrow minor groove to the molecule. The major groove is wide enough to allow proteins to interact with the DNA molecule, which help to regulate
things like DNA super-folding, replication of DNA, and translation of DNA to RNA. DNA that is naturally occurring in the cell may be linear or circular. DNA Is linear in prokaryotic cells, such as archaea and bacteria, as well as in mitochondria and chloroplasts of eukaryotic cells. Plasmids, which are pieces of DNA passed from one cell to another, are also circular. Eukaryotes, such as fungi, plants, and animals, have linear DNA arranged in a linear fashion in the form of chromosomes. Each type of organism has its own number of chromosomes, located in the cell nucleus. Humans have 23 pairs of chromosomes, of which 22 pairs are homologous or roughly the same structure, while one pair consists of the sex chromosomes, labeled X and Y, that do not share the same
genes on them. DNA does not exist as naked DNA in the cell but is tightly wound due to the presence of proteins that make chromatin. There are histone proteins in eukaryotic cells that wrap around the DNA molecule in order to condense it. Each histone-DNA segment is called a nucleosome, which looks like a bead on a string. These need to be disassembled to some degree before the DNA can be replicated or transcribed into RNA. Figure 6 shows a
nucleosome:
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