mRNA synthesis 3'
-AAAAAAAAAAAAA(PolyA) © mRNA
FIGURE 4.13 Relationship between genome RNAs, mRNAs, and vcRNAs of influenza virus. Synthesis of mRNAs in the cell nucleus requires a primer of 10-13 nucleotides derived from cellular pre-mRNAs by "cap-snatching," and mRNAs terminate with a poly(A) tail. Those portions of the mRNA that are not complementary to the genome RNA are shown in red. In contrast, vcRNAs are exact complements of the genomic minus strands. [Adapted from Strauss and Strauss (1997).]
(Fig. 4.13), which is a perfect complement of the genome and serves as a template for production of genomic RNA.
Influenza virus matures by budding of nucleocapsids through the cell plasma membrane (Fig. 2.21C). The virion is often spherical, averaging 100 nm in diameter, but virus preparations are often pleomorphic, and filamentous forms may be present. During assembly, the eight genome segments are reassorted in progeny virions if the cell is infected with more than one strain of influenza. Reassortment to produce viruses with mixed genomes is very efficient—the segments are almost randomly reassorted to give all possible combinations of genome segments in the progeny virions. This process is analogous to the reassortment of chromosomes that takes place during sexual reproduction in diploid organisms.
Budding must result in the packaging of the 8 different genomic segments that constitute the viral genome into one virus particle if it is to be infectious. Reoviruses (Chapter 3) have an assembly mechanism whereby the 10-12 differ-
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