Scientists are studying the structure and function of receptor x, a single-polypeptide protein that is found in the membrane around certain types of cells. receptor x contains no alpha- helices or beta sheets. a specific molecule outside the cells is recognized and bound by receptor x. the binding of the molecule to receptor x causes the cells to have a particular response. to study the structure and function of receptor x the scientists are altering the sequence of small sections of the receptor. both the normal sequence of a five-amino acid section of receptor x and an altered sequence that contains two amino acid substitutions are shown in figure 1.

answer : haversian

[explanation: -(compact bone is organized as parallel columns, known as haversian systems, which run lengthwise down the axis of long bones. these columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone)]

the oldest known insects are believed to live around 400 millions ago as suggested by the fossil records.   some plants having woody stems also evolved during this time.

after the evolution of insects the first four legged species or the tetrapods evolved in shallow freshwater habitats from the intermediate species approximately 397 million years ago. the tetrapods went on conquering the land giving rise to amphibians, reptiles, mammals and birds.

the major split in tetrapods first occurred 340 million years ago branching off from amphibians.

363 million years ago when the carboniferous period began the insects started roaming on land and taking flights, following which the four limbed tetrapods adapted in the terrestrial habitat.