prokaryotic cells. There is some DNA in the mitochondria of eukaryotic animal cells and some DNA in the chloroplasts of eukaryotic plant cells. Histones are the proteins that compact the chromosomes so they fit into the small nucleus. Histones are small proteins that intertwine between the DNA strands, allowing them to stack better to make a smaller, thicker, and shorter strand. Prokaryotes generally do not have histones. Histones may have some role in the transcription of DNA, deciding which ones will open up for transcription and which ones will not be transcribed. Figure 3 shows histone proteins attached to a DNA molecule:
Figure 3
DNA Properties As mentioned, DNA is a long polymer made from repeating nucleotides. It is extremely long and is capable of coiling into loops and other shapes to condense it within the nucleus. In all species, DNA is double-stranded, consisting of two helical strands that are bound by hydrogen bonding. Each chain is coiled around the same axis. The largest human chromosome is chromosome 1, which contains about 220 million base pairs of nucleotides. If straightened, it would be about eighty-five millimeters in length. DNA is not a single molecule but is two separate long strands of nucleotides that are connected in a double helix. The backbone of the DNA is strong, consisting of covalent bonds of alternating nucleotide bases and phosphate groups. The nucleobase that makes up the backbone of the molecule is a nucleoside (a sugar group) and a phosphate group, which makes it a nucleotide. Many nucleotides connected to one another is called a polynucleotide, which is the type of molecule a DNA molecule is.
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