NOVA Online | Cracking the Code of Life

Sequence for Yourself
Part V: Assembly and Finishing
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Assembly of 500-Base Segments
Perhaps you noticed in the last section that the human DNA incorporated in the vector is 4,000 base pairs long and that the 500 bases of human DNA we can read has to be adjacent to the vector DNA. So how do we read the rest of the human DNA? The answer is by piecing together overlapping sequences.

Because we have many overlapping pieces, we also have many starting points for the 4,000-base sequences -- enough to allow us to read every base.

Assembly of 150,000-Base Segments
With the help of computers, we assemble the 500-base sequences into the 150,000-base segments from which they were derived.

Rebuilding the Chromosomes
Finally, we determine the chromosome that the 150,000-base segment belongs to as well as where along the chromosome it belongs. We do this by looking for overlaps and by looking for matches in banding between the segments and chromosomes.

Using this approach -- called the "map-based shotgun approach" -- we can sequence the entire genome. To learn about the differences between this approach and the "whole genome shotgun" approach, check out the links in our Resources section.

Final Notes
Although the job of piecing together overlapping fragments sounds straightforward, the task is a challenging one. Many gaps will need to be filled in, and there are areas where sequences are repeated many times, making it almost impossible to determine where some of the fragments belong.

In this explanation, some of the descriptions of the techniques used were simplified. Also, for each of the processes described here, researchers can use a variety of techniques to get the same results.

Watch the Program Here | Our Genetic Future (A Survey)
Manipulating Genes: How Much is Too Much? | Understanding Heredity
Explore a Stretch of Code | Nature vs Nurture Revisited
Sequence for Yourself | Journey into DNA | Meet the Decoders
Resources | Update to Program | Teacher's Guide | Transcript
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