The turtle’s shell provides it with a formidable defence and one that is unique in the animal world. No other animal has a structure quite like it, and the bizarre nature of the turtle’s anatomy also applies to the skeleton and muscles lying inside its bony armour.
The shell itself is made from broadened and flattened ribs, fused to parts of the turtle’s backbone (so that unlike in cartoons, you couldn’t pull a turtle out of its shell). The shoulder blades sit underneath this bony case, effectively lying within the turtle’s ribcage. In all other back-boned animals, whose shoulder blades sit outside their ribs (think of your own back for a start). The turtle’s torso muscles are even more bizarrely arranged.
This body plan – and particularly the odd location of the shoulder blades – is so radically different to that of all other back-boned animals that biologists have struggled to explain how it could have arisen gradually from the standard model, or what the intermediate ancestors might have looked like. Enter Hiroshi Nagashima from the RIKEN Center; he has found some answers by studying how the embryos of the Chinese soft-shelled turtle (Pelodiscus sinensis) shift from the standard body plan of other vertebrates to the bizarre configuration of adult turtles.
By comparing the embryos to those of mice and chickens, Nagashima showed that all three species start off with a shared pattern that their last common ancestor probably shared. It is only later that the turtle does something different, starting of a sequence of muscular origami that distorts its body design into the adult version.
DNA is most famous as a store of genetic information, but Shawn Douglas from the Dana-Farber Cancer has found a way to turn this all-important molecule into the equivalent of sculptor’s clay. Using a set of specially constructed DNA strands, his team has fashioned a series of miniscule sculptures, each just 20-40 nanometres in size. He has even sculpted works that assemble from smaller pieces, including a stunning icosahedron – a 20-sided three-dimensional cage, built from three merged parts.
Douglas’s method has more in common with block-sculpting that a mere metaphor. Sculptors will often start with a single, crystalline block that they hack away to reveal the shape of an underlying figure. Douglas does the same, at least on a computer. His starting block is a series of parallel tubes, each one representing a single DNA helix, arranged in a honeycomb lattice. By using a programme to remove sections of the block, he arrives at his design of choice.
With the basic structure set down, Douglas begins shaping his molecular clay. He builds a scaffold out of a single, long strand of DNA. For historical purposes, he uses the genome of the M13 virus. This scaffold strand is ‘threaded’ through all the tubes in the design with crossovers at specific points to give the structure some solidity. The twists and turns of the scaffold are then fixed in place by hundreds of shorter ‘staple’ strands, which hold the structure in place and prevent the scaffold from unfolding.
The sequences of both the scaffold and staple strands are tweaked so that the collection of DNA molecules will stick together in just the right way. Once all the strands are created, they’re baked together in one hotpot and slowly cooled over a week or so. During this time, the staples stick to predetermined parts of the scaffold and fold it into the right shape. The slow cooling process allows them to do this in the right way; faster drops in temperature produce more misshapen forms.
The result: a series of six structures that Douglas viewed under an electron microscope: a monolith, a square nut, a railed bridge, a slotted cross, a stacked cross and a genie bottle. These basic shapes illustrate the versatility of the nano-origami approach, and they can also be linked together to form larger structures. Using staples that bridge separate scaffolds, Douglas created a long chain of the stacked cross units. Most impressively of all, he made an icosahedron by fusing three distinct subunits.