The Genetics of Viruses and Bacteria Quiz

Answer: Tobacco mosaic virus

The discovery of viruses began in 1883 with Adolf Mayer, a German scientist seeking the cause of tobacco mosaic disease. This disease stunts the growth of tobacco plants and gives their leaves a mottled, or mosaic, coloration. Mayer discovered that the disease was contagious when he found he could transmit it from plant to plant by spraying sap extracted from diseased leaves onto healthy plants.

He searched for a microbe in the infectious sap but found none. Mayer concluded that the disease was caused by unusually small bacteria that could not be seen with the microscope.

This hypothesis was tested a decade later by Dimitri Ivanowsky, a Russian who passed sap from infected tobacco leaves through a filter designed to remove bacteria. After filtering, the sap still produced mosaic disease. Later, David Beijerinck imagined a reproducing particle much smaller and simpler than bacteria.

His suspicions were confirmed in 1935, when the American scientist Wendell Stanley crystallized the infectious particle, now known as tobacco mosaic virus (TMV). Subsequently, TMV and many other viruses were actually seen with the help of the electron microscope.

Answer: A strand composed of both RNA and DNA

Genes are usually thought of as being made of double-stranded DNA--the conventional double helix--but many viruses defy this convention. Their genomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the specific type of virus. A virus is called a DNA virus or an RNA virus, according to the kind of nucleic acid that makes up its genome. In either case, the genome is usually organized as a single linear or circular molecule of nucleic acid. The smallest viruses have only four genes, while the largest have several hundred.

Answer: Protein

The protein shell that encloses the viral genome is called a capsid. Depending on the type of virus, the capsid may be rod-shaped (more precisely, helical), polyhedral, or more complex in shape. Capsids are built from a large number of protein subunits called capsomeres, but the number of different kinds of proteins is usually small. Tobacco mosaic virus, for example, has a rigid, rod-shaped capsid made from over a thousand molecules of a single type of protein. Adenoviruses, which infect the respiratory tracts of animals, have 252 identical protein molecules arranged into a polyhedral capsid with 20 triangular facets--an icosahedron.

Answer: Lysogenic cycle

In contrast to the lytic cycle, which kills the host cell, the lysogenic cycle replicates the phage genome without destroying the host. Phages that are capable of using both modes of reproducing within a bacterium are called temperate phages. To compare the lytic and lysogenic cycles, we will examine a temperate phage called lambda, written with the Greek letter l. After entering the bacterial cell and circularizing, the l DNA can either integrate into the bacterial chromosome (lysogenic cycle) or immediately initiate the production of a large number of progeny phages (lytic cycle). In most cases, the lytic pathway is followed, but once a lysogenic cycle begins, the prophage may be carried in the host cell's chromosome for many generations.

Answer: Reverse transcriptase

The RNA viruses with the most complicated reproductive cycles are the retroviruses. Retro, meaning "backward," refers to the reverse direction in which genetic information flows for these viruses. Retroviruses are equipped with an enzyme called reverse transcriptase, which transcribes DNA from an RNA template, providing an RNA to DNA information flow.

The newly made DNA then integrates as a provirus into a chromosome within the nucleus of the animal cell. The host's RNA polymerase transcribes the viral DNA into RNA molecules, which can function both as mRNA for the synthesis of viral proteins and as genomes for new virus particles released from the cell.

A retrovirus of particular importance is HIV (human immunodeficiency virus), the virus that causes AIDS (acquired immunodeficiency syndrome).

Answer: Viroids

As small and simple as viruses are, they dwarf another class of pathogens, viroids. These are tiny molecules of naked circular RNA that infect plants. Only several hundred nucleotides long, viroids do not encode proteins but can replicate in host plant cells, apparently using cellular enzymes. Somehow, these RNA molecules can disrupt the metabolism of a plant cell and stunt the growth of the whole plant. One viroid disease has killed over 10 million coconut palms in the Philippines. Viroids seem to cause errors in the regulatory systems that control plant growth, and the symptoms that are typically associated with viroid diseases are abnormal development and stunted growth.

Answer: One double-stranded, circular DNA molecule

The major component of the bacterial genome is one double-stranded, circular DNA molecule. Although I will refer to this structure as the bacterial chromosome, it is very different from eukaryotic chromosomes, which have linear DNA molecules associated with a large amount of protein.

In the case of the well-studied intestinal bacterium E. coli, the chromosomal DNA consists of about 4.6 million nucleotide pairs, representing about 4,300 genes. This is 100 times more DNA than is found in a typical virus, but only about one-thousandth as much DNA as in an average eukaryotic cell. Still, this is a lot of DNA to be packaged in such a small container. Stretched out, the DNA of an E. coli cell would measure about a millimeter in length, 500 times longer than the cell. Within a bacterium, however, the chromosome is so tightly packed that it fills only part of the cell.

This dense region of DNA, called the nucleoid, is not bounded by membrane like the nucleus of a eukaryotic cell. In addition to the chromosome, many bacteria also have plasmids, much smaller circles of DNA. Each plasmid has only a small number of genes, from just a few to several dozen.

Answer: Binary fission

Bacterial cells divide by binary fission, which is preceded by replication of the bacterial chromosome. From a single origin of replication, DNA synthesis progresses in both directions around the circular chromosome. Because binary fission is an asexual process--the production of offspring from a single parent--most of the bacteria in a colony are genetically identical to the parent cell.

As a result of mutation, however, some of the offspring do differ slightly in genetic makeup. For a given E. coli gene, the probability of a spontaneous mutation averages about 1 X 10^-7 per cell division, only one in 10 million.

But among the 2 X 10^10 new E. coli cells that arise each day in a single human colon, there will be approximately (2 X 10^10)(1 X 10^-7) = 2,000 bacteria that have a mutation in that gene.

The total number of mutations when all 4,300 E. coli genes are considered is about 4,300 X 2,000 = 9 million per day per human host. The important point is that new mutations, though individually rare, can have a significant impact on genetic diversity when reproductive rates are very high because of short generation spans.

This diversity, in turn, affects the evolution of bacterial populations: Individual bacteria that are genetically well equipped for the local environment clone themselves more prolifically than do less fit individuals.

Answer: True

How can you detect genetic recombination in bacteria? Consider two mutant E. coli strains (genetic varieties), each unable to synthesize one of its required amino acids. Wild-type E. coli can grow on a minimal medium containing only glucose, as a source of organic carbon, and salts.

The mutant strains cannot grow on this culture medium of minimal nutrients because one of them cannot synthesize tryptophan and the other cannot synthesize arginine. If you mix bacteria from the two strains together in a liquid medium and allow them to incubate for an hour or so and then spread a small sample of this culture on solid (agar-thickened) minimal medium in a petri dish and incubate the dish overnight.

The next morning you observe numerous colonies of bacteria on the minimal medium. Each of these colonies must have started with a cell capable of making both tryptophan and arginine, but their number far exceeds what can be accounted for by mutation. Most of the cells that can synthesize both amino acids must have acquired one or more genes from the other strain.

This is evidence that genetic recombination has occurred.

Answer: Conjugation

Conjugation is the direct transfer of genetic material between two bacterial cells that are temporarily joined. This process, the bacterial version of sex, has been studied most extensively in E. coli . The DNA transfer is one-way: one cell donating DNA, and its "mate" receiving the genes.

The DNA donor, referred to informally as the "male," uses appendages called sex pili to attach to the DNA recipient, the "female". A sex pilus acts like a grappling hook: After contacting a female cell, it retracts, pulling the two cells together.

A temporary cytoplasmic bridge then forms between the two cells, providing an avenue for DNA transfer. In most cases, "maleness," the ability to form sex pili and donate DNA during conjugation, results from the presence of a special piece of DNA called an F factor (F for fertility). An F factor can exist either as a segment of DNA within the bacterial chromosome or as a plasmid.