Humans and Viruses
Human Biology 115A
Robert Siegel, instructor
Replication of Bunyaviridae
This section will focus
on the nature of Bunyaviridae replication within a cell. What is known
about viral replication comes from work on the Bunyavirus
genus on vertebrate cells. There are three segments in the genome: L
(large), M (medium)
and S (small). The L segment codes for the L protein which is
used as viral transcriptase and viral replicase. The M segment codes for a
polyprotein that is cotranslationally cleaved to garner glycoproteins G1
and G2. The S segment codes for the N nucleocapsid protein.
An image of the L,M, and S segments of Hantavirus
Attachment and Entry
Viral proteins on the surface are used to bind to a receptor on the cell
surface. Current research on the LAC virus has indicated that G1 proteins
critical for binding to cells than G2 proteins. It has not been determined
cell receptors are used by the virus to gain entry into the cell.
However, it is known that entry
is dependendent upon acidic conditions.
As Bunyaviridae is segmented, three mRNA species are transcribed from the
The RNA of bunyavirus is negative sense , except for
phleboviruses and tospoviruses which are ambisense. Bunyaviridae have
their own encoded polymerases which are used for transcription.
The mechanism for transcription
initiation has not fully been determined, but there is some evidence that
similar to the flu virus,
bunyaviruses, hanta viruses and phleboviruses steal caps from host cell
mRNAs. However, unlike orthomyxovirus,
transcription occurs in the cytoplasm of the cell rather than in the
nucleus. As a result, it is
hypothesized that caps are stolen from mRNAs that are derived from
host messages within the cytoplasm.
Secondary transcription, which occurs after replication, increases
the amount of L, S and M mRNA
segments. Most of the segments found in the cell due to
are S. The the least amount of segments are
M, with an intermediate amount of L segments formed.
The L and S segments of the genome are translated by free ribosomes. M
segments are translated by membrane bound ribosomes. The L, S,
and M segments are
responsible for different structural and non-structural proteins.
Primary glycosylation and processing of envelope proteins occurs in Golgi
Viral RNA is used as a template for both replication and transcription.
Replicative intermediates of positive sense RNA are used to create more
copies of the RNA for
the progeny. Replicative intermediates are full length template strands
that have 6-8 daughter strands emerging from them.
Assembly and Egress
The viral progeny are assembled within the Golgi apparatus of the cell.
All of the viruses are given glycoproteins and undergo terminal
glycolysation. In addition, the viral progeny adopt pieces of host
membrane to form the envelope. The viruses bud into the Golgi cisternae
and then are released
from the cell by exocytosis.
This is an image of Hanta virus found in a person. The number of virus
particles found in the picture is the result of replication.
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