Changes

===Virus Structure===

===Virus Structure===

 

The BVDV genome comprises a single strand of positive sense RNA. By analysing the sequence of BVDV cDNA cloned into plasmid vectors, the genome has been found to be around 12.3Kb long (Donis, 1995). In infected cells, this is the only viral RNA present- no molecules of a subgenomic size are found. Figure 1.1 shows a diagrammatic interpretation of the BVDV genome.

The BVDV genome comprises a single strand of positive sense RNA which is around 12.3 kilobases in length³. The genome is read in one 3898-codon open reading frame, commencing at nucleotide 386, that contains no non-coding sequences. BVDV polyprotein is translated directly from the ORF and is cleaved by viral and cellular proteinases to form mature viral proteins^{3, 4}.  

Genomic information is read in a single, large open reading frame (ORF) that commences at nucleotide 386 of the RNA. The ORF is 3898 codons long, and contains no non-coding sequences. BVDV polyprotein is translated directly from the ORF, and mature viral proteins arise from this following endoproteinase cleavages by both viral and cellular mechanisms (Dubovi, 1990; Donis, 1995).  

The 5’ and 3’ untranslated regions (UTRs) are located at either end of the ORF. Unlike eukaryotic mRNA, BVDV genomic RNA has neither the 5’ methylated cap nor the 3’ poly-A tail at these positions. Instead, the UTRs are significantly long, at approximately 385 and 226 nucleotides for the 5’ and 3’ UTRs respectively (Donis, 1995). This allows them to accommodate a variety of functions normally conferred by the 5’ cap and the poly-A region, including the control of translation initiation and RNA stability. Entry of replicases required to produce viral progeny from the RNA template is also guided by the UTRs.

The 5’ and 3’ untranslated regions (UTRs) are located at either end of the ORF. Unlike eukaryotic mRNA, BVDV genomic RNA has neither the 5’ methylated cap nor the 3’ poly-A tail at these positions. Instead, the UTRs are significantly long, at approximately 385 and 226 nucleotides for the 5’ and 3’ UTRs respectively (Donis, 1995). This allows them to accommodate a variety of functions normally conferred by the 5’ cap and the poly-A region, including the control of translation initiation and RNA stability. Entry of replicases required to produce viral progeny from the RNA template is also guided by the UTRs.