Biomedical Engineering Reference
In-Depth Information
(a)
(b)
Figure 7.1
(See companion CD for color igure.)
.Twofold.(red),.threefold.(green),.and.ivefold.(blue).rotational.symmetry.
elements.of.an.icosahedron.(a).and.the.symmetric.equivalents.of.a.T.=.3.virus.capsid.(b).
axes.are.outlined.red,.green,.and.blue,.respectively..A.more.in-depth.discussion.of.quasi-
equivalence.and.icosahedral.symmetry.can.be.found.in.reference.(Baker.et.al..1999).
A.smaller.number.of.viruses.are.known.to.exist.in.a.tubular.or.ilamentous.formation,.
exhibiting.helical.symmetry..Helical.symmetry.exists.along.the.helix.axis.and.is.deined.
by.the.number.of.subunits.per.turn,.and.the.amount.of.rise.along.the.helical.axis.per.sub-
unit..In.some.tubular.viruses,.the.genetic.material.is.coiled.helically.inside.the.capsid,.that.
is,.several.nucleotides.are.associated.with.each.subunit.(as.in.the.case.of.tobacco.mosaic.
virus. [TMV]).. In. other. tubular. viruses,. such. as. the. ilamentous. bacteriophage. M13,. the.
capsid.protein.subunits.form.a.hollow.tube.that.encompasses.the.circular.ssDNA.genome.
within.it..In.this.case,.there.is.no.direct.link.between.the.number.of.nucleotides.that.each.
protein.subunit.interacts.with..In.fact,.the.length.of.the.M13.phage.is.a.direct.result.of.the.
amount. of. DNA. packaged. within. it.. Longer. segments. of. DNA. will. result. in. additional.
subunits.in.the.capsid.structure,.and.longer.length.(Knipe.et.al..2007).
One.reason.that.viruses.are.such.an.inspiration.to.biologists.and.material.scientists.alike.
is. their. inherent. ability. to. self-assemble. at. any. physiological. conditions.. With. respect. to.
self-assembly,.an.integral.part.of.virion.formation.is.packaging.the.genome.into.the.cap-
sid.structure..It.is.possible.to.realize.that.a.large.number.of.genetically.identical.subunits.
with.slightly.different.conformations.enhance.the.eficiency.of.building.a.macromolecular.
structure..Yet,.in.addition.to.just.assembling.the.capsid,.a.forming.virus.must.also.ensure.
the.preferential.packaging.of.its.own.genetic.material.over.other.endogenous.nucleic.acids.
that.are.present.in.the.host.cell.
Many.viruses.solve.this.by.including.a.unique.“packaging.signal”.in.their.own.genomes.
and.some.type.of.structural.recognition.component.in.the.coat.protein.to.allow.for.virus-
speciic.genome.assembly..The.packaging.signal.is.typically.a.short.genetic.sequence,.or.
series.of.sequences,.that.promotes.encapsidation.of.the.genome..It.is.common.for.some.virus.
capsids.to.form.around.the.genome,.after.a.speciic.interaction.with.the.packaging.sequence.
has.occurred..In.some.more.complex.viral.systems,.the.capsid.is.irst.formed,.followed.by.
insertion.of.the.genome.inside..However,.the.ability.of.viruses.with.segmented.genomes,.
that.is,.multiple.nucleic.acid.segments.in.each.mature.virion,.to.package.a.complete.set.of.
