Deoxyribonucleic Acid (DNA)
Deoxyribose, the sugar component in each nucleotide, is so called because it has one less oxygen atom than ribose, which is present in ribonucleic acid (RNA). Deoxyribose contains five carbon atoms, four of which lie in a ring along with one oxygen atom. The fifth carbon atom is linked to a specific carbon atom in the ring. A phosphate group is always linked to deoxyribose via a chemical bond between an oxygen atom in the phosphate group and the carbon atom in deoxyribose by a chemical bond between a nitrogen atom in the base and a specific carbon atom in the deoxyribose ring.
The nucleotide components of DNA are connected to form a linear polymer in a very specific way. A phosphate group always connects the sugar component of a nucleotide with the sugar component of the next nucleotide in the chain. Consequently, the first nucleotide bears an unattached phosphate group, and the last nucleotide has a free hydroxyl group. Therefore, DNA is not the same at both ends. This directionality plays an important role in the replication of DNA.
DNA molecules contain two polymer chains or strands of nucleotides and so are said to be double-stranded. (In contrast, RNA is typically single-stranded.) Their shape resembles two intertwined spiral staircases in which the alternating sugar and phosphate groups of the nucleotides compose the sidepieces. The steps consist of pairs of bases, each attached to the sugars on their respective strands. The bases are held together by weak attractive forces called hydrogen bonds. The two strands in DNA are antiparallel, which means that one strand goes in one direction (first to last nucleotide from top to bottom) and the other strand goes in the opposite direction (first to last nucleotide from bottom to top).
Because the sugar and phosphate components which make up the sidepieces are always attached in the same way, the same alternating phosphate-sugar sequence repeats over and over again. The bases attached to each sugar may be one of four possible types. Because of the geometry of the DNA molecule, the only possible base pairs that will fit are adenine (A) paired with thymine (T), and cytosine (C) paired with guanine (G).
The DNA in our cells is a masterpiece of packing. The double helix coils itself around protein cores to form nucleosomes. These DNA-protein structures resemble beads on a string. Flexible regains between nucleosomes allows these structures to be wound around themselves to produce an even more compact fiber. The fibers can then be coiled for even further compactness. Ultimately, DNA is paced into the highly condensed chromosomes. If the DNA in a human cell is stretched, it is approximately 6 ft (1.82 m) long. If all 46 chromosomes are laid end-to-end, their total length is still only about eight-thousandths of an inch. This means that DNA in chromosomes is condensed about 10,000 times more than that in the double helix. Why all this packing? The likely answer is that the fragile DNA molecule would get broken in its extended form. Also, if not for this painstaking compression, the cell might be mired in its own DNA.
- Deoxyribonucleic Acid (DNA) - Function
- Deoxyribonucleic Acid (DNA) - History
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