Problem of the Week

Chapter 19:

Problem of the Week

A Spider's Web
Spider silk is one of the strongest naturally occurring fibers. The strength lies in the protein structure of the fiber, which changes as the silk goes from the liquid state inside the spider to the solid state outside the spider. Every spider produces silk in several different kinds of glands, each gland is responsible for its own protein chemistry. While in the gland the spider silk is a water-soluble, amorphous, viscous liquid, but when it is drawn out through the spinnerets, the molecular arrangement becomes ordered and uniform and the molecule becomes insoluble, increasing in density. Both male and female spiders produce spider silk and it is not unusual for a spider to produce 2,000 feet of spider silk at one time. The pulling action that the spider uses to weave and stretch the web strengthens the individual strands. Compared to steel of the same diameter, black widow spiders produce silk twice as strong as steel and it can stretch 25 percent more than steel.

Spider silk has been important to indigenous people of the Pacific, Asia, and Australia. Natives have used spider silk to weave water-shedding rain gear, make fishing line and construct nets. In fact, legend has it that Genghis Khan and his soldiers wore a leather garment interwoven with spider silk to protect them from enemy arrows. Scientists are trying to develop spider dragline filament because of its strength. When spider silk can be mass-produced, these fibers will be used in the next generation of bulletproof vests. Spider farms are not a possibility because spiders tend to eat one another, therefore industrial methods must be used. Two methods of bioengineering spider silk are currently underway. The first involves the introduction of a cloned silk gene into bacteria such as Escherichia coli. The bacteria can then manufacture the silk polymer through protein synthesis. A second method involves using transgenic animal technology. The silk producing gene is added to New Zealand miniature goats in a form of recombinant DNA, which can be passed on to the offspring. The goat milk will contain the spider silk polymer and extraction of the silk will involve the use of a milking machine. Once the cream and whey are skimmed from the milk and the milk protein fat has been separated, the milk will be pure enough to produce a film of spider silk.

Spider silk is a primarily made up of two simple amino acids, alanine and gylcine, in a 2:1 ratio.

a. Identify the R- group, the carboxyl group, and the amino group in the amino acids that make up spider silk.
b. Draw the structure of the dipeptide Ala-Gly and circle the peptide bond.

Explain the role of the DNA and RNA from the inserted gene in the production of spider silk.

Dragline spider silk is stronger than Kevlar synthetic fibers. Compare the ala-gly dipeptide of spider silk to that of Kevlar.

Useful Web Sites:
The Spider Page
Secondary Protein Structure of the Dragline Silk of the Black Widow Spider Using C¹³NMR
Scientists Weave Spider Silk Into New Bulletproof Vests
Macromolecule & Carbon Chemistry
Spider Engineering
Amino Acids
Chemistry of Kevlar

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Chapter 19:

Problem of the Week