Although much of the genetic makeup of humans has been mapped, hundreds of missing DNA sequences remain.

Until now.

Scientists from the U.S. National Human Genome Research Institute report they have produced the complete DNA sequence of a single human chromosome. That discovery could allow researchers to sequence the entire human genome.

"This accomplishment begins a new era in genomics research," said Dr. Eric Green, director of the institute. "The ability to generate truly complete sequences of chromosomes and genomes is a technical feat that will help us gain a comprehensive understanding of genome function and inform the use of genomic information in medical care."

A human genome is very long and includes about 6 billion bases, which DNA sequencing machines cannot read all at once.

So, researchers typically cut the genome into smaller bits, and analyze each piece. These bits of DNA sequences are then put back together.

"Imagine having to reconstruct a jigsaw puzzle. If you are working with smaller pieces, each contains less context for figuring out where it came from, especially in parts of the puzzle without any unique clues, like a blue sky," senior author Adam Phillippy explained. "The same is true for sequencing the human genome. Until now, the pieces were too small, and there was no way to put the hardest parts of the genome puzzle together."

For this study, researchers chose to complete the X chromosome sequence first, because of its link with many diseases, including hemophilia, chronic granulomatous disease and muscular dystrophy.

After mapping the X chromosome, the researchers used a new computer program to put the sequences together.

"We don't yet know what we'll find in the newly uncovered sequences. It is the exciting unknown of discovery. This is the era of complete genome sequences, and we are embracing it wholeheartedly," Phillippy said in an institute news release.

The report was published July 14 in the journal Nature.