Within the influenza A virion are eight segments of viral RNA. These molecules carry the all the information needed to make new influenza virus particles. These eight RNAs are shown schematically as olive green lines at the top of the illustration. RNAs are chains of four different nucleotides, A, C, G, U. In the case of influenza virus, the eight RNAs are a total of about 14,000 nucleotides in length. The nucleotides make up the genetic code – it is read by the cell’s translation machinery in groups of three, with each triplet specifying an amino acid.
There are two important aspects of these viral RNA that we must consider. First, you can see that the ends of the RNAs are labeled 3′ and 5′. Nucleic acids have polarity, in that one end of the chain is chemically different from the other. Such polarity is represented by 5′ or 3′. The second point is that when a nucleic acid is copied, or duplicated, by enzymes called polymerases, a strand of the complementary polarity is produced. Influenza viral RNAs are called (-), or negative strand RNAs, because they are the opposite polarity of the RNA that is translated to make protein. The RNA molecules that are templates for the synthesis of proteins are defined as having having (+), or positive polarity. Upon entering the cell, the (-) strand influenza viral RNAs must be copied into complementary (+) strands, so that they can serve as templates for proteins. The viral RNAs are copied by an enzyme – called RNA polymerase – that is carried into the cell with the virus.
In the above scheme, the olive green lines are the (-) strand RNAs found in the influenza virion. Once the virion enters the cell, these 8 RNAs are copied into (+) strand mRNAs. Finally, the mRNAs can serve as templates for the synthesis of proteins. The specific viral proteins that are produced by each viral mRNA are shown at the bottom of the illustration. From this picture we see that, for example, RNA segment 4 codes for the viral HA protein, and RNA segment 6 codes for the viral NA protein. Note also that some RNA segments encode for more than one protein. Both influenza A and B viruses have 8 RNA segments, while the influenza C viruses have 7.
Influenza viruses are called (-) strand RNA viruses because of the polarity of the RNA that is carried in the virion. Other RNA viruses – such as poliovirus – are (+) strand RNA viruses, because their genomic RNA can be translated into protein immediately upon entering the cell.
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