Question

If the template strand has two or more identical nucleotides in a row, their complementary nucleotides will be added one after the other in the same flow step. How are two or more of the same nucleotide (in a row) detected in the flowgram? (See sample on the right.) Write out the sequence of the first 25 nucleotides in the flow-gram above, starting from the left (Ignore the very short lines.)

Short answer

A higher peak in the flow-gram shows two or more same nucleotides.

The sequence of the first 25 nucleotides would be TCAGCGTAAGGTGATGTATAGGGGC.

Step-by-step solution

Next-generation sequencing

In the next-generation sequencing technique, a DNA molecule is isolated and degraded into 300 base pair long DNA fragments. Each fragment is then isolated and copied by the PCR technique. The fragments are then sequenced parallelly.

Thus, the sequencing technique allows the sequencing of about 2 billion nucleotides within 24 hours.

Sequence of DNA fragment is shown in flow-gram

The nucleotides of the DNA fragments are exhibited in the flow-gram. Computer software analyzes the sequences of all the fragments and presents the sequence of the whole genome.

In the flow gram, the height of the peaks represents the number of identical consecutive nucleotides in the sequence. For example, if a nucleotide is present in two more numbers, that nucleotide's peak will be higher in the flow gram.

Deduction of the sequence of the DNA fragment 

The Y-axis in the flow-gram corresponds to the number of that particular nucleotide in the X-axis. On reading the nucleotides in the flow-gram, the first 25 nucleotide of the fragment sequenced is TCAGCGTAAGGTGATGTATAGGGGC.