Figure 2.

A cartoon representation of the replication process showing the time-dependent relationship between the vRNP and trans-acting viral polymerase (PB1, PB2 and PA). Note that this model is still under investigation and the possibility remains that replication is also done in cis. The progression of the figure is from top to bottom. (Top) The vRNP has entered the nucleus of the host cell and has separated from other vRNPs. Perhaps at this stage, the vRNP has become anchored to chromatin, though this is not specified. (Next step) The 3′ terminus of the vRNA segment somehow unwinds and the trans-acting viral polymerase captures this free segment and begins copying the sequence in the 3′–5′ direction, making an antisense copy of whatever template is present. (Next frame) The process continues with the viral polymerase somehow unwinding the vRNA + NPs, all the while recruiting new NPs for the copy. (Next frame) When the replication is complete, the structure is eventually released. There are several steps in the trans-acting model that are not fully determined yet. The schematic is inspired from [44] and [42]. The structure of the vRNP is based on the right-handed helix in [42]; however, it is important to remember that [43] reports a left-handed helix. (A colour version of this figure is available online at: https://academic.oup.com/bfg)