Download Answers to Mastering Concepts Questions

Answers to Mastering Concepts Questions
Chapter 17
17.1
1. Viruses are similar to bacteria in that they are small, have some kind of genetic material, can
be infectious, and can evolve. But viruses are much smaller than bacteria, and they cannot
reproduce independently or respond to stimuli. Most biologists do not consider viruses to be
alive.
2. An average virus is 12 times smaller than a bacterium. An average bacterium is 10 times
smaller than a eukaryotic cell.
3. An enveloped virus gains its outer membrane from the host cell membrane.
4. A virus’s host range is the array of cells (hosts) that it will be able to infect.
5. It is important to know the reservoir species for a virus that causes a disease in humans
because this knowledge can be used to control an outbreak of the viral disease.
6. Viruses evolve as their DNA or RNA mutate, and some variants are more successful that
others at infecting cells and leaving descendants.
7. Viruses don’t belong to a domain or a kingdom of life because they are extremely variable and
don’t share a single common ancestor.
17.2
1. It is inaccurate to refer to the “growth” of viruses because viruses cannot grow. Instead, they
are assembled inside cells (much as cars are assembled in a factory). Viruses do not increase in
size or develop.
2. If a virus can attach to a cell and penetrate it, the virus can infect a cell. Generally viruses can
only enter cells in which they can reproduce.
3. A virus’s genetic material enters a cell after the virus attaches to a receptor on the cell surface.
The virus may inject its genetic material through a hole in the cell wall, or the virus may enter
plant cells on contaminated mouthparts of plant-eating insects. Viruses can also enter animal
cells by endocytosis.
4. The source of energy and raw materials is the host cell’s ATP and its stores of ribosomes,
nucleotides, amino acids, and enzymes.
5. The assembly and release of completed viruses follows this pattern:
- Viral genetic material is transcribed to mRNA, which is translated to viral proteins at the host’s
ribosomes.
- Viral genetic material is copied for assembly into new viruses.
- New viruses assemble from coat proteins, enzymes, and nucleic acids.
- New viruses are released as enzymes break down the host cell’s wall, as viruses simply burst
out of the cell, or as new viruses bud off of the cell membrane.
17.3
1. A lytic viral infection is similar to a lysogenic viral cycle in that both allow viruses to reproduce.
Whereas a lytic infection destroys a cell immediately, a lysogenic virus inserts its DNA into the
host chromosome, where it is carried on into daughter cells. Eventually an environmental change
triggers reproduction of viruses and the lysogenic virus follows a lytic pathway, destroying the
host cell.
2. A latent animal virus follows a lysogenic cycle. Some latent animal viruses include the
herpesvirus (cold sores), HIV, human papillomavirus, and the Epstein-Barr virus.
3. HIV replicates following this sequence:
- the virus enters the host T cell after binding to the appropriate receptor protein;
- the enzyme reverse transcriptase transcribes HIV RNA to DNA;
- the viral DNA inserts into the chromosome of the host cell;
- eventually, viral genes are transcribed into mRNA that is translated into viral proteins and copied
to viral RNA;
- these viral components are packaged into new viruses that bud off of the host cell.
4. Some latent viral infections lead to human cancers. For example, the human papillomavirus
that causes genital warts can lead to cervical cancer, and the Epstein-Barr virus that leads to
mononucleosis may cause Burkitt lymphoma.
17.4
1. One person can transmit a viral infection to another person by coughing or sneezing, which
expel respiratory droplets that others may inhale or ingest in food or water. Viruses can also be
transmitted in blood transfusions, with sexual contact, or on contaminated needles.
2. Different viral infections lead to different symptoms because the viruses destroy different kinds
of host cells.
3. The immune system responds to a viral infection with symptoms that discourage the
development of the virus. These include higher body temperature, inflammation, and increased
mucus secretion.
4. Antibiotics target the metabolic processes of bacteria, the integrity of their ribosomes, or their
enzymes or cell walls. Viruses don’t share these targets and are therefore unaffected by antibiotic
drugs.
5. Viruses enter plant tissues when a virus-contaminated insect, tool, or mechanical damage
breaches the plant’s cell walls.
17.5
1. A viroid is made only of naked RNA. It has no protein coat or enzymes.
2. Prions convert normal proteins into abnormal ones that do not fold properly.
3. The best way to avoid prion disease is to keep animal brains and spinal cords out of the human
food chain.