What is a Contig?

Maggie J. Hall

A contig, short for contiguous, refers to either a deoxyribonucleic acid (DNA) segment or the product of overlapping segments that form a continuous, extensive, uninterrupted DNA sequence. By studying maps of these segments, it is possible to discover the order of segments comprising various sequences, and contigs can be added, removed, or rearranged to form new sequences. DNA contig mapping and libraries allow researchers to identify alignments which might cause disease, provide a better understanding of how cells react under certain conditions, and enable the ability to clone favorable characteristics in living organisms.

DNA contig mapping allows researchers to identify alignments that might cause disease.
DNA contig mapping allows researchers to identify alignments that might cause disease.

Genomic contigs are connected to one other by the overlapping of matching sequence tagged sets or by matching expressed sequence tags. By understanding this building block process used by the bases of DNA segments, contigs may replicated or cloned into desirable sequences, adding or eliminating less favorable segments. Researchers view these sequences as maps on highly detailed, computerized DNA programs. The program provides a visual breakdown of the bases used to construct the segments, and the segments used to build particular sequences. Using different color schemes, various contig programs display this information as low, medium, and high quality sequences.

Contig mapping frequently sheds light on the causes of genetic disorders. Williams syndrome, for example, produces multi-system symptoms in children, including cardiovascular problems, elevated blood calcium levels, and failure to thrive. Researchers discovered band deletions affecting certain genes, which may be caused by inappropriate overlapping of contigs. Another genetic childhood disorder, cri-du-chat syndrome, also produces multiple symptoms, each of which scientists believe correlate with deletions and abnormal positioning.

Researchers also analyze contig sequences in plant species. Each plant contains sequencing unique to that plant, and by obtaining maps of individual plant species, scientists discover similar and varying characteristics that may contribute to the health or deterioration of a particular plant type. By analyzing contig segments and sequences in disease carrying organisms, physicians can determine traits which can then be used in treatment development. Mapping provides visual insight into possible genetic manipulations through sequence deletions or cloning.

Scientists might grow cloned DNA samples in bacterial artificial chromosome, yeast artificial chromosome, or in P1 or phage artificial chromosome media. These chromosomes contain anywhere from 50 to hundreds of thousands of bases, which compile contig segments. After a period of colonization, samples are extracted using a syringe and placed in DNA sequencing gel, which undergoes electrophoresis and dries. An autoradiograph reads the sample and fingerprints the sequences. Researchers then scan the fingerprint into an image file, which may later be loaded into a contig program.

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