chapter 13 learning objectives

Question 1

discuss viral genomes in terms of dsDNA, ssDNA, dsRNA, and ssRNA, and number of segments of nucleic acid

Answer 1

viral genomes may be DNA or RNA but never both. can be double-stranded DNA like all cells, or single-stranded DNA, single-stranded RNA, double-stranded RNA. (ssDNA and dsRNA are almost non-existent in cells). the genome of any particular virus may be linear and composed of several molecules of nucleic acid, or circular and singular. The genomes of viruses are usually much smaller than the genomes of cells.

Question 2

explain the mechanism by which viruses are specific for their host cells

Answer 2

most viruses only infect particular host’s cells due to attraction of viral attachment molecules for complementary proteins or glycoproteins on host cell surface. may be so specific that they only infect a particular kind of cell in a particular host.

there are GENERALISTS which infect many kinds of cells in one host or many different hosts

all cells are susceptible to some sort of viral infection

Question 3

compare and contrast viruses of animals and bacteria

Answer 3

viruses that infect animals can have an envelope and avoid detection by animal immune systems. the key differences between phage and animal viral replication are budding, chromosomes are inside nucleus and transcription can occur within the nucleus.

Question 4

discuss the structure and function of the viral capsid

Answer 4

capsids are protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells

Question 5

discuss the origin, structure, and function of the viral envelope

Answer 5

similar in composition to a cell membrane, found particularly in animal viruses. enveloped virus acquires its envelope from its host cell during viral replication or release. composed of phospholipid bilayer and proteins. provides some protection to the virus.

Question 6

sketch and describe the 5 stages of the lytic replication cycle as it typically occurs in bacteriophages

Answer 6

1. attachment- random collision, tail fibers are responsible for attachment, chemical attraction and precise fit to receptor sites
2. entry- virus must overcome barrier posed by the cell wall and cytoplasmic membrane, tail sheath contracts and forces internal hollow tube through the cell wall and cytoplasmic membrane. viral genome moves through tail sheath into the bacterial cell
3. synthesis- viral enzymes degrade bacterial DNA into constituent nucleotides, viral genome takes control of host cell’s metabolic machinery, transcription and translation of phage mRNA completed by bacterial cell, phage directs host cell to make only viral DNA and proteins.
4. assembly- not completely understood
5. release- bacterium disintegrates

Question 7

compare and contrast the lysogenic replication cycle of viruses with the lytic cycle

Answer 7

lysogenic cycle- modified replication cycle in which infected host cells grow and reproduce normally for generations before they lyse. has 8 steps.

key differences:
in lytic cycle- lysozyme enzyme degrades the bacterial cell wall prior to penetration.
in lysogenic cycle- prophage inserted into bacterial chromosome (lysogenic conversion)

in lytic cycle- host cell lysis at the end of the cycle
in lysogenic cycle- host cells replicate; daughter cells contain prophage

in lytic cycle- very rapid release of virions because of cell lysis
in lysogenic cycle- prophage is excised from chromosome, then back to lytic cycle

Question 8

explain the differences between bacteriophage replication and animal viral replication

Answer 8

budding, chromosomes are inside nucleus, transcription can occur within the nucleus. these differences result from presence of envelope around some (animal) viruses, eukaryotic nature of animal cells, and lack of cell wall in animal cells.

Question 9

compare and contrast the release of viral particles by lysis and budding

Question 10

compare and contrast latency in animal viruses with phage lysogeny