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gresses, it impairs the ability of infected cells to squeeze through capillaries, ruptures RBC membranes, and releases newly minted malaria parasites.1

Cancer, another leading cause of death, also progresses in many mechanical ways, such as the machinery of viral infection, which is responsible for hepatitis B, human papilloma virus, and related cancers. In addition, metastasis involves the detachment of tumor cells from the initial growth site, migration, circulation, and subsequent invasion through reattachment to new locations.

Intracellular changes in cancerous cells bring about cell growth and proliferation, cell motility, and migration. These processes are currently targeted by Taxol, a potent chemotherapy drug that impedes the progression of cancer by stabilizing microtubules in a polymerized state, thus shutting down the machinery of cell division.

The human genome is contained in approximately 3 billion base pairs of DNA. If the pieces of DNA in our cells were connected end-to-end, the chain would be roughly as long as one’s outstretched arms. Compared to other cellular filaments, DNA is quite flexible. To fit into cells, it is compacted in highly ordered structures through multiple hierarchical levels.2 In addition to the conventional Watson and Crick base pairs, DNA forces and interactions include histone protein-DNA nucleosome complexes, chromatin fibers, chromatids, and, ultimately, 46 chromosomes. In general, our biological structures have many levels of structural hierarchy and are highly organized.

MOLECULAR MACHINERY

From our DNA sequence, biological motors, including DNA and RNA polymerases and the ribosome, make RNA, and ultimately proteins, through coupled mechanical and chemical steps. This impressive machinery copies and translates with great accuracy. The ribosome, a huge motor spanning 20 nanometers (nm), which is actually composed of about 65 percent ribosomal RNA, assembles proteins from messenger RNA. Many antibiotics target the critical machinery of ribosomal translation in bacteria.

Other molecular motors, such as myosins and kinesins, run on different cytoskeletal tracks—actin filaments and microtubules, respectively.3 Myosins, which are responsible for the contraction of skeletal and cardiac muscle, contribute to cell

1

A red blood cell circulates in the body about half a million times in a 120-day period.

2

Human cells are typically 10-30 µm in diameter. The smallest, sperm, is only a few microns in diameter, and the largest, the egg, is ~100 µm in diameter. The size of randomly packed DNA can be estimated from the radius of gyration ( b·N½), where N is the number of segments and b is the length of each segment. DNA has a length of 0.3 nm/bp, a persistence length of 50 nm. An average chromosome may contain 1.3×108 bp of DNA, yielding a radius of gyration of 44 µm, larger than the typical cell size. Chromosomes were first observed by Karl Wilhelm von Nägeli in 1842.

3

Mechanically, actin filaments have a Young’s modulus of 2.3 GPa, and microtubules have a persistence length on the order of mm.



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