Ch. 19

What is reverse transcriptase?

an enzyme that catalyzes the synthesis of viral DNA starting with viral RNA as a template

in some cases, the prophage or provirus may remain inactive or

latent

the genome can be ____ or _____ depending on the type of virus

linear or circular

a prophage can be excised from the bacterial chromosome to end the

lysogenic cycle

if nutrients are short in supply, what cycle is favored because sufficient material may not be available to make new viruses

lysogenic cycle

new phages are not made during the _______ _______, and the host cell is not destroyed

lysogenic cycle

the viral reproductive cycle consists of how many steps

5 or six

once integrated, the phage DNA in a bacterium is called

prophage

Two ways latency occurs in humans

1. for HIV, latency occurs because the virus has integrated into the host genome and may remain dominant for a long time 2. genome of some other viruses can exist as an episome

Two major challenges in AIDS research

1. to discover drugs that inhibit viral proteins without also binding to the host cell proteins and inhibiting normal cellular function 2. develop drugs to which mutant strains will not become resistant

the extra genes of phage ____ encode many different proteins that are involved in the formation of the elaborate structure

T4

latency in bacteriophages is also called

lysogeny

an excised prophage may then proceed to the

lytic cycle

if nutrients are readily available, phage y usually proceeds directly to the ______ ______ after its DNA enters the cell

lytic cycle

what are viruses?

nonliving particles with nucleic acid genomes

what is host range

number of species and cell types that can be infected

what happens once a viral genome has entered the cell in step 2

one or a few viral genes are expressed immediately due to the action of host cell enzymes and ribosomes

what are capsids composed of

one or several different protein subunits called capsomers

how many viruses have been identified

over 4,000

what is an example of temperate phages

phage y

what happens in phage y

tail fibers bind to proteins in the outer bacterial cell membrane of E. coli cells

bacteriophages that can follow either a lysogenic or lytic cycle are called

temperate phages

other phages that only have lytic cycles are called

virulent phages

the capsid and envelope enable

viruses to infect their hosts

where is reverse transcriptase carried

within the capsid and released into the host cell along with viral RNA

similarities of viruses

-small size -reliance on a living cell for replication

the assembly of HIV occurs in two phases

1. capsid proteins assemble around two molecules of viral RNA and molecules of reverse transcriptase and integrase 2. the new capsid acquires its outer envelope in a budding process

what is an episome

a genetic element that replicated independently of the chromosomal DNA but also can occasionally integrate into chromosomal DNA

what is a virus?

a small infectious particle that consists of a nucleic acid enclosed in a protein coat

what is a viral envelope

a structure enclosing a viral capsid that consists of a membrane derived fro the plasma membrane of the host cell and embedded with virally encoded spike glycoproteins

What causes the effects from AIDS

a viral destruction of helper T cells

what happens in the first step of a viral reproductive cycle

a virus attached to the surface of a host cell, which the attachment is usually specific for one or a few types of cells because proteins in the virus recognize and bind to specific molecules on the cell surface

variations in the structure of viruses shown by transmission electron micrographs (left) and schematic diagrams (right)

all viruses contain nucleic acid (DNA or RNA) surrounded by a protein capsid -may or may not have an outer envelope surrounding the capsid

viruses that infect bacteria are called _______ that may have more complex protein _______ with accessory structures used for _______ the virus to a host cell and transferring vital nucleic acid into the bacterial ________

bacteriophages or phages; coat; anchoring; cytoplasm

what occurs in lysogeny

both the prophage and its host cell are said to be lysogenic

all viruses have a protein coat called a

capsid

what does integrase do

cuts the host's chromosomal DNA and inserts the viral genome into the chromosome

what is a capsid

encloses a genome consisting of one or more molecules of nucleic acid (DNA or RNA)

in temperature phages, what conditions influence whether a viral DNA is integrated into a host chromosome and how long the virus remains in the lysogenic style

environmental

capsids have a variety of shapes including

helical and polyhedral

a cell infected by a virus

host cell

a species that can be infected by a specific virus

host species

where is integrase carried

in the HIV capsid and released into the host cell during uncoating

the viral double stranded DNA enters the host cell nucleus and is inserted into a host chromosome via

integrase

viruses capable of integration carry a gene that encodes an enzyme called

integrase

the genome of some viruses, including both phage y and HIV can

integrate into a chromosome of the host cell

the genome of a virulent phage is not capable of

integrating into a host chromosome

What must a virus do to replicate?

must be taken up by a living cell

once integrated, the viral DNA in a eukaryotic cell is called a

provirus

an rna virus that utilized reverse transcription to produce viral DNA that can be integrated into a chromosome of the host cell

retrovirus

what is the viral reproductive cycle

series of steps that result in the production of new viruses during a viral infection

In some viruses, the nucleic acid is _______-______, and in others, it is _______-________

single-stranded; double-stranded

viral genomes also vary in ______, ranging from a few thousand to more than a hundred thousand _______ in length

size; nucleotides

what happens in HIV

spike glycoproteins in the viral envelope bind to protein receptors in the plasma membrane of human white blood cells called helper T cells

what is prophage

the DNA of a phage that has become integrated into a bacterial chromosome

what is the lysogenic cycle

the phase of a viral reproductive cycle when a prophage is integrated into a bacterial chromosome

what happens when a bacterial cell divides (prophage)

the prophage DNA is copied and transmitted to daughter cells along with the bacterial chromosomal DNA

if more nutrients become available, then

the prophage may become activated

the production of new viruses by a host cell involves

the replication of the viral genome and the synthesis of viral proteins that make up a protein coat

what does reverse transcriptase use

the viral RNA strand to make a complementary copy of DNA, which is then used as a template to make double-stranded DNA

how can an RNA virus integrate its genome to the host cell's DNA?

the viral genome must be copied into DNA

genetic material in a virus is called a

viral genome

differences of viruses

-host range -structure -genome composition

Why are viruses considered non-living?

-they do not exhibit key properties associated with living organisms -not composed of cells -do not use energy or carry out metabolism -do not maintain homeostasis -do not reproduce

steps of viral reproductive cycles

1) attachment - virus attaches to the surface of a host cell 2) entry - virus or viral genome enters the host cell 3) integration - some but not all viruses integrate their genome into the genome of the host cell 4) synthesis of viral components - viral proteins and DNA or RNA are made by the host cell 5) viral assembly - the viral components assemble into virus particles 6) release - viruses are released from the host cell

virus ranges in diameter

20 to 400 nm

what happens when a lysogenic bacterium prepares to divide

it copies the prophage DNA along with its own DNA so each daughter cell inherits a copy of the prophafe

why is reverse transcription named as so

it is the reverse of the usual transcription process in which the DNA strand is used to make a complementary strand of RNA

many viruses have protein fibers with a _____ or _____ glycoproteins that help the _____ to the surface of a host cell

kob; spike; bind

how are mutant strains of HIV created

through reverse transcriptase due to it lacking the function of DNA polymerase identifying and removal of mismatched nucleotides

the first virus to be discovered was

tobacco mosaic virus (TMV)

when a virus infects a host cell, the expression of viral genes leads to a series of steps called a _____ _________ _______, which results in the production of new viruses

viral reproductive cycle