Internet Protocol version 6 (IPv6) is the next-generation Internet Layer protocol for packet-switched internetworks and the Internet. IPv4 is currently the dominant Internet Protocol version, and was the first to receive widespread use. In December 1998, the Internet Engineering Task Force (IETF) designated IPv6 as the successor to version 4 by the publication of a Standards Track specification, RFC 2460.

In December 2008, despite celebrating its 10th anniversary as a Standards Track protocol, IPv6 was only in its infancy in terms of general worldwide deployment. A recent study by Google indicates that penetration is still less than one percent of Internet traffic in any country. The leaders are Russia (0.76%), France (0.65%), Ukraine (0.64%), Norway (0.49%), and the United States (0.45%). Although Asia leads in terms of absolute deployment numbers, the relative penetration is smaller (e.g., China: 0.24%). IPv6 is implemented on all major operating systems in use in commercial, business, and home consumer environments. According to the study, Mac OS X leads in IPv6 penetration of 2.44%, followed by Linux (0.93%) and Windows Vista (0.32%).

IPv6 has a much larger address space than IPv4. This results from the use of a 128-bit address, where IPv4 uses only 32 bits. The new address space thus supports 2^128 (about 3.4×10^38) addresses. This expansion provides flexibility in allocating addresses and routing traffic and eliminates the need for network address translation (NAT). NAT gained widespread deployment as an effort to alleviate IPv4 address exhaustion.

IPv6 also implements new features that simplify aspects of address assignment (stateless address autoconfiguration) and network renumbering (prefix and router announcements) when changing Internet connectivity providers. The IPv6 subnet size has been standardized by fixing the size of the host identifier portion of an address to 64 bits to facilitate an automatic mechanism for forming the host identifier from Link Layer media addressing information (MAC address).

Network security is integrated into the design of the IPv6 architecture. Internet Protocol Security (IPsec) was originally developed for IPv6, but found widespread optional deployment first in IPv4 (into which it was back-engineered). The IPv6 specifications mandate IPsec implementation as a fundamental interoperability requirement.

The general requirements for implementing IPv6 on a network host are specified in RFC 4294.

Enzymes are generally globular proteins and range from just 62 amino acid residues in size, for the monomer of 4-oxalocrotonate tautomerase, to over 2,500 residues in the animal fatty acid synthase. A small number of RNA-based biological catalysts exist, with the most common being the ribosome; these are referred to as either RNA-enzymes or ribozymes. The activities of enzymes are determined by their three-dimensional structure. However, although structure does determine function, predicting a novel enzyme's activity just from its structure is a very difficult problem that has not yet been solved.

Most enzymes are much larger than the substrates they act on, and only a small portion of the enzyme (around 3–4 amino acids) is directly involved in catalysis. The region that contains these catalytic residues, binds the substrate, and then carries out the reaction is known as the active site. Enzymes can also contain sites that bind cofactors, which are needed for catalysis. Some enzymes also have binding sites for small molecules, which are often direct or indirect products or substrates of the reaction catalyzed. This binding can serve to increase or decrease the enzyme's activity, providing a means for feedback regulation.

Like all proteins, enzymes are made as long, linear chains of amino acids that fold to produce a three-dimensional product. Each unique amino acid sequence produces a specific structure, which has unique properties. Individual protein chains may sometimes group together to form a protein complex. Most enzymes can be denatured — that is, unfolded and inactivated — by heating or chemical denaturants, which disrupt the three-dimensional structure of the protein. Depending on the enzyme, denaturation may be reversible or irreversible.

Tyrannosaurus rex was one of the largest land carnivores of all time; the largest complete specimen, FMNH PR2081 ("Sue"), measured 12.8 metres (42 ft) long, and was 4.0 metres (13 ft) tall at the hips. Mass estimates have varied widely over the years, from more than 7.2 metric tons (7.9 short tons), to less than 4.5 metric tons (5.0 short tons), with most modern estimates ranging between 5.4 and 6.8 metric tons (6.0 and 7.5 short tons). Although Tyrannosaurus rex was larger than the well known Jurassic theropod Allosaurus, it was slightly smaller than Cretaceous carnivores Spinosaurus and Giganotosaurus.

The neck of T. rex formed a natural S-shaped curve like that of other theropods, but was short and muscular to support the massive head. The forelimbs had only two clawed fingers, along with an additional small metacarpal representing the remnant of a third digit. In contrast the hind limbs were among the longest in proportion to body size of any theropod. The tail was heavy and long, sometimes containing over forty vertebrae, in order to balance the massive head and torso. To compensate for the immense bulk of the animal, many bones throughout the skeleton were hollow, reducing its weight without significant loss of strength.

The largest known T. rex skulls measure up to 5 feet (1.5 m) in length. Large fenestrae (openings) in the skull reduced weight and provided areas for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus’ skull was significantly different from those of large non-tyrannosauroid theropods. It was extremely wide at the rear but had a narrow snout, allowing unusually good binocular vision. The skull bones were massive and the nasals and some other bones were fused, preventing movement between them; but many were pneumatized (contained a "honeycomb" of tiny air spaces) which may have made the bones more flexible as well as lighter. These and other skull-strengthening features are part of the tyrannosaurid trend towards an increasingly powerful bite, which easily surpassed that of all non-tyrannosaurids.The tip of the upper jaw was U-shaped (most non-tyrannosauroid carnivores had V-shaped upper jaws), which increased the amount of tissue and bone a tyrannosaur could rip out with one bite, although it also increased the stresses on the front teeth.

The teeth of T. rex displayed marked heterodonty (differences in shape). The premaxillary teeth at the front of the upper jaw were closely packed, D-shaped in cross-section, had reinforcing ridges on the rear surface, were incisiform (their tips were chisel-like blades) and curved backwards. The D-shaped cross-section, reinforcing ridges and backwards curve reduced the risk that the teeth would snap when Tyrannosaurus bit and pulled. The remaining teeth were robust, like "lethal bananas" rather than daggers; more widely spaced and also had reinforcing ridges.Those in the upper jaw were larger than those in all but the rear of the lower jaw. The largest found so far is estimated to have been 30 centimetres (12 in) long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur.

The story is set in the mid-21st century. Global warming has led to ecological disasters all over the world, and a drastic reduction of the human population. Mankind's efforts to maintain civilization lead to the creation of "mechas," advanced humanoid robots capable of emulating human thoughts and emotions. Among these androids there is an advanced prototype model named David, a mecha created by the Cybertronics company to resemble a human child and to "feel" love for its human owners. They test their creation on one of their employees, Henry Swinton and his wife Monica. The Swintons have a son, Martin, who has been placed in suspended animation until a cure can be found for his rare disease. Although Monica is initially frightened of David, she eventually warms to him after activating his imprinting protocol, which irreversibly causes David to feel love for her as a child loves a parent. As he continues to live with the Swintons, David is befriended by Teddy, a mecha toy, who takes upon himself the responsibility of David's well being.

Martin is suddenly cured and brought home; a sibling rivalry ensues between Martin and David. Martin's scheming behavior backfires when he and his friends activate David's self-protection programming at a pool party. Martin is saved from drowning but David's actions prove too much for Henry. It is decided for David to be destroyed at the factory where he was built, but Monica rather leaves him (alongside Teddy) in a forest to live as unregistered mechas. David is captured for an anti-mecha Flesh Fair, an event where useless mechas are destroyed before cheering crowds. David is nearly killed, but the crowd is swayed by his realistic nature and he escapes, along with Gigolo Joe (Jude Law), a male prostitute mecha on the run after being framed for murder.

The two set out to find the Blue Fairy, whom David remembers from the story The Adventures of Pinocchio. As in the story, he believes that she will transform him into a real boy, so Monica will love him and take him back. Joe and David make their way to the decadent metropolis of Rouge City. Information from a holographic volumetric display personality called "Dr. Know" eventually leads them to the top of the Rockefeller Center in the flooded ruins of Manhattan. David's human creator, Professor Hobby, enters and excitedly tells David that finding him was a test, which has demonstrated the reality of his love and desire. A disheartened David attempts to commit suicide by falling from a ledge into the ocean, but Joe rescues him, just as he is captured by the authorities.

David and Teddy take a submersible to the fairy, which turns out to be a statue from a submerged attraction at Coney Island. Teddy and David become trapped when the park's ferris wheel falls on their vehicle. Believing the Blue Fairy to be real, he asks to be turned into a real boy, repeating his wish without end, until the ocean freezes. 2000 years later, Manhattan is buried under several hundred feet of glacial ice, and humans are extinct. Mechas have evolved into an alien-looking humanoid form. They find David and Teddy: functional mechas who knew living humans. David wakes up and realizes the fairy was fake. Using David's memories, the mechas reconstruct the Swinton home, and explain to him via a mecha of the Blue Fairy that he cannot become human. However, they recreate Monica from a lock of her hair which has been faithfully saved by Teddy, but she will live for only a single day and the process cannot be repeated. David spends the happiest day of his life playing with Monica and Teddy. Monica tells David that she loves him and has always loved him as she drifts to sleep for her final time. This was the "everlasting moment" he had been looking for, he closes his eyes, falls asleep for his first time, and goes "to that place where dreams are born".