Viruses Questions and Answers

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.

Answer: A

A virion is the complete infectious particle: the nucleic acid together with its protein coat and, in some families, an envelope. A protomer is a single coat protein subunit and a capsomere a cluster of them, so both are parts of the particle rather than the whole. A viroid is a separate class of agent altogether.

Answer: B

The defining constraint is metabolic, not physical: a virus carries no machinery for generating energy or synthesising proteins, so it can only replicate inside a cell that supplies both. Size and visibility are consequences of its simplicity, and many viruses leave the host cell alive.

Answer: C

A virion carries one kind of genome, either DNA or RNA, and this is one of the clearest differences from a cell, which always contains both. Whichever it carries may be single or double stranded, which is what the classification into genome classes records.

Answer: D

The capsid is the virus-coded protein shell enclosing the nucleic acid, and the two together are called the nucleocapsid. The envelope is a lipid layer outside that, present only in some families, and peplomers are the glycoprotein spikes projecting from it.

Answer: A

Individual subunits are protomers, and they gather into capsomeres, which are what electron microscopy resolves on the capsid surface. Counting capsomeres was historically one way of characterising a virus. The nucleocapsid is the capsid with the genome inside it.