355Z-DNA (zigzag DNA) A region of DNA that is
“flipped” into a left-handed helix. DNA adopts the Z
configuration when purines and pyrimidines alternate
on a single strand. It may be important in regulating
gene expression in eukaryotes.
zinc finger A DOMAIN, found in certain DNA-binding
proteins, comprising a HELIX-loop structure in which a
zinc ion is coordinated to two to four cysteine sulfurs,
the remaining LIGANDs being histidines. In many pro-
teins of this type the domain is repeated several times.
See alsoCOORDINATION.
zoned reserve system An ecosystem preservation
concept where certain lands are preserved and managed
as wild or natural, while areas around it are used and
altered by human activity. Several nations, such as Costa
Rica, have adopted this strategy. Municipal zoning in the
United States is another approach where undeveloped
lands within a community are zoned for various uses
such as industrial, residential, and conservation.
zones A series of life zones developed in 1889 by
mammalogist Clinton Hart Merriam, who studied the
distribution patterns of plants and animal. His developed
the concept of a life zone. He recognized that similar
zones of vegetation occurred with both increasing lati-
tude and increasing elevation. He named them Arctic-
Alpine, Hudsonian, Canadian, Transition, Upper
Sonoran, and Lower Sonoran.zoological nomenclature SeeBINOMIAL.zooplankton Small floating or weakly swimming
invertebrates that are transported by water currents
and eat other plankton. Zooplankton are foraminifera
and radiolarians, tiny crustaceans (Daphnia), as well as
the larval or immature stages of animals like mollusks
(snails and squid), crustaceans (crabs and lobsters),
fish, jellyfish, and sea cucumbers.zoospore An asexually produced flagellated, swim-
ming spore.zwitterionic compound A neutral compound hav-
ing electrical charges of opposite sign, delocalized or
not, on adjacent or nonadjacent atoms.zygodactyl The trait of having two toes pointing
forward and two toes pointing backward, found in