In recent years, scientists have decoded the DNA of humans and a menagerie（兽群，动物园） of creatures but none with genes as complex as a stalk（茎） of corn, the latest genome to be unraveled（解释，解开）. A team of scientists led by The Genome Center at Washington University School of Medicine in St. Louis published the completed corn genome in the Nov. 20 journal Science, an accomplishment that will speed efforts to develop better crop varieties to meet the world's growing demands for food, livestock feed（畜牧饲料） and fuel.

"Seed companies and maize（玉米） geneticists will pounce on（突然袭击，猛扑） this data to find their favorite genes," says senior author Richard K. Wilson, Ph.D., director of Washington University's Genome Center, who led the multi-institutional sequencing effort. "Now they'll know exactly where those genes are. Having the complete genome in hand will make it easier to breed new varieties of corn that produce higher yields or are more tolerant to extreme heat, drought, or other conditions."

Corn, also known as maize, is the top U.S. crop and the basis of products ranging from breakfast cereal（早餐谷类食品） to toothpaste, shoe polish and ethanol（乙醇）. The corn genome is a hodgepodge（混淆，杂菜） of some 32,000 genes crammed into just 10 chromosomes. In comparison, humans have 20,000 genes dispersed among 23 chromosomes.

The $29.5 million maize sequencing project began in 2005 and is funded by the National Science Foundation and the U.S. departments of agriculture and energy. The genome was sequenced at Washington University's Genome Center. The overall effort involved more than 150 U.S. scientists with those at the University of Arizona in Tucson, Cold Spring Harbor Laboratory in New York and Iowa State University in Ames playing key roles.

The group sequenced a variety of corn known as B73, developed at Iowa State decades ago. It is known for its high grain yields and has been used extensively in both commercial corn breeding and in research laboratories.

The genetic code of corn consists of 2 billion bases of DNA, the chemical units that are represented by the letters T, C, G and A, making it similar in size to the human genome, which is 2.9 billion letters long.

But that's where much of the similarity ends. The challenge for Wilson and his colleagues was to string together the order of the letters, an immense and daunting task（令人生畏的艰巨任务） both because of the corn genome's size and its complex genetic arrangements. About 85 percent of the DNA segments are repeated. Jumping genes, or transposons（转位子）, that move from place to place make up a significant portion of the genome, further complicating sequencing efforts.

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