Influenza: A case study
Influenza: A case study

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Influenza: A case study

5.1 Initial identification of influenza infection

Oral swabs or nasal aspirates are initially screened for the presence of a variety of respiratory viruses. This is done by extracting RNA from the sample and subjecting it to a reverse-transcription polymerase chain reaction (RT-PCR) as described below:

  1. Initially the RNA sample is reverse transcribed into complementary DNA (cDNA), using a commercially-available reverse transcriptase enzyme.
  2. The cDNA is then used in a standard PCR reaction to detect and amplify a short sequence of nucleotides specific to the virus. Multiple DNA sequences, each specific for a different type of virus, can be amplified in the same reaction, provided that these sequences are of different lengths.
  3. Each of the different amplified sequences is separated from the others when the entire sample is subjected to gel electrophoresis (an analytical technique in which molecules of different sizes move at various rates through a gel support in an applied electric field, thus making it possible to identify specific molecules.)

The polymerase chain reaction (PCR) technique is illustrated in Video 2.

Download this video clip.Video player: Video 2
Skip transcript: Video 2 PCR technique.

Transcript: Video 2 PCR technique.

Polymerase chain reaction, or PCR, uses repeated cycles of heating and cooling to make many copies of a specific region of DNA. First, the temperature is raised to near boiling, causing the double-stranded DNA to separate, or denature, into single strands. When the temperature is decreased, short DNA sequences known as primers bind, or anneal, to complementary matches on the target DNA sequence. The primers bracket the target sequence to be copied. At a slightly higher temperature, the enzyme Taq polymerase, shown here in blue, binds to the primed sequences and adds nucleotides to extend the second strand. This completes the first cycle.

In subsequent cycles, the process of denaturing, annealing and extending are repeated to make additional DNA copies.

After three cycles, the target sequence defined by the primers begins to accumulate.

After 30 cycles, as many as a billion copies of the target sequence are produced from a single starting molecule.

End transcript: Video 2 PCR technique.
Video 2 PCR technique.
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Typically, nucleotide sequences specific to five types of virus are searched for in each sample: influenza A, influenza B, respiratory syncytial virus (Baltimore group V, (–)ssRNA virus, and a major cause of respiratory illness in young children), adenoviruses and enteroviruses. Those samples that test positive for influenza in the RT-PCR reaction are inoculated into cells in culture. Sufficient virus for a limited number of tests can be produced from such cultures within 24 hours, but they are often maintained for up to a week.

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