Viruses Questions and Answers

Answer: C

Attachment is due to the haemagglutinin.

Haemagglutinin (HA) is a glycoprotein spike on the envelope of the influenza virus. It binds to sialic acid on the host cell surface, starting infection, and later mediates fusion of the viral envelope with the endosome membrane. It is named for its ability to agglutinate red blood cells and is the 'H' in names such as H1N1.

Neuraminidase, the other spike, acts at the end of infection by cutting sialic acid so that new virions can be released. Pili and fimbriae are attachment structures of bacteria, not of viruses.

Answer: C

The procapsid is assembled with the aid of Scaffolding proteins.

In many viruses, including tailed bacteriophages such as P22 and T4 and the herpesviruses, scaffolding proteins guide the coat proteins into a correctly shaped empty shell, the procapsid. They are then released or broken down as the DNA is packaged and the shell matures into the capsid.

'Ladder', 'framing' and 'form' proteins are not terms used for this role.

Answer: C

The virus with a DNA genome is Cauliflower mosaic virus.

Cauliflower mosaic virus, a caulimovirus, has a circular double-stranded DNA genome, which it replicates through an RNA intermediate by reverse transcription. Its strong 35S promoter is widely used in plant genetic engineering.

HIV is a retrovirus with an RNA genome. Tobacco mosaic virus and cucumber mosaic virus are plant viruses with single-stranded RNA genomes; most plant viruses are RNA viruses.

Answer: D

The organism that does not need an insect vector is Coxiella burnetii.

Coxiella burnetii causes Q fever. It forms highly resistant spore-like small cell variants that survive in dust and animal products, and people are infected mainly by inhaling aerosols from infected livestock, especially birth products. No arthropod vector is needed, though ticks can carry it among animals.

The others depend on arthropod vectors: Rickettsia prowazekii (epidemic typhus) is spread by the body louse, Rickettsia rickettsii (Rocky Mountain spotted fever) by ticks, and Ehrlichia chaffeensis (human monocytic ehrlichiosis) by the lone star tick.

Answer: B

DNA is not present in Tobacco Mosaic Virus (TMV).

TMV is a rod-shaped plant virus whose genome is a single strand of positive-sense RNA, about 6,400 nucleotides long, wrapped in a helical coat of identical protein subunits. Fraenkel-Conrat's reconstitution experiments with TMV showed that RNA can be genetic material.

The others contain DNA. Bacteriophage T4 and adenovirus both have double-stranded DNA genomes, and chloroplasts carry their own circular DNA, about 120 to 160 kilobase pairs, which encodes the large subunit of RuBisCO among other genes.

Answer: D

Viruses enter a host cell by binding to a receptor site on the host cell.

A virus's attachment proteins recognise specific molecules on the host cell surface, which is why each virus infects only certain hosts and tissues. HIV gp120 binds CD4 and a co-receptor on T cells, and influenza haemagglutinin binds sialic acid. After attachment the virus enters by membrane fusion or endocytosis, or, in bacteriophages, injects its genome.

Viruses do not dissolve a piece of membrane, and they are far too large to diffuse through it. Antibodies are made by the host to bind viruses; the host cell does not offer an 'antibody site' for the virus to use as its entry point.

Answer: B

The suffixes are -virales, -viridae, -virinae and -virus, in that order.

Under the International Committee on Taxonomy of Viruses, an order ends in -virales (for example Mononegavirales), a family in -viridae (Coronaviridae), a subfamily in -virinae (Orthocoronavirinae) and a genus in -virus (Betacoronavirus). The paper prints the subfamily ending as 'Virnae'.

The other options place -viridae first or put -virus and -viridae in the wrong ranks, so they mix up order, family and genus.