Intro to Viruses

Question 1

virus

Answer 1

an infectious particle consisting of genes (nucleic acid) packaged in a protein coat (or membranous evelope)

• viruses are much simpler in structure than even prokaryotic cells
• can cause a wide variety of diseases
• cannot reproduce or carry out metabolism outside of a host cell
• exist in the gray area between life forms and chemicals “borrowed life”

Question 2

Viral genomes consist of either

Answer 2

double or single stranded DNA or double or single stranded RNA viruses

the genome is either a single linear or circular molecule of the nucleic acid

viruses have between 3-2000 genes in their genome`

Question 3

capsid

Answer 3

the protein shell that encloses the viral genome

variety of structures; associated viruses may be referred to as helical or icosahedral

Question 4

capsomeres

Answer 4

protein subunits

Question 5

viral envelopes

Answer 5

derived from membranes of host cells (ACCESSORY STRUCTURE TO HELP INFECT HOST)

surround the capsids of influenza viruses and many other viruses found in animals

viral envelopes contain a combination of viral and host cell molecules

Question 6

bacteriophages

Answer 6

AKA phages

• viruses that infect bacteria
• elongated capsid head that encloses their DNA
• a protein tail piece attaches the phage to the host and injects the phage DNA inside

Question 7

obligate intracellular parasites

Answer 7

viruses are^; meaning they can only replicate within a host cell

Question 8

host range

Answer 8

a limited number of host species that it can infect

i.e measles only infects humans

Question 9

simplified viral replicative cycle

Answer 9

The parasite is a DNA virus with a capsid consisting of a single type of protein which enters the cell

1. the virus enters the cell and is uncoated, releasing viral DNA and capsid proteins
2. host enzymes replicate the viral genome in side the cell
3. meanwhile host enzymes inside the cell transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins
4. viral genomes and capsid proteins self assemble into new virus particles which exit the cell

Question 10

best understood of all viruses

Answer 10

phages

Question 11

lytic cycle

Answer 11

a phage replicative cycle that culminates in the death of the host cell

the lytic cycle produces new phages and lyses (breaks open) the hosts cell wall, releasing the progeny viruses

Question 12

virulent phage

Answer 12

a phage that reproduces only by the lytic cycle

Question 13

steps of the lytic cycle

Answer 13

1. Attachment
   the t4 phage uses its tail fibers to bind to specific surface proteins on a cell that act as receptors
   2 Entry of phase DNA and degradation of host DNA
   the sheath of the tail contracts injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed
2. synthesis of viral genomes and proteins
   the phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell
3. self assembly
   three separate sets of protein self assemble to form phage heads tails and tail fibers. the phage genome is packaged inside the capsid as the head forms
4. release
   the phage directs production of an enzyme that damages the bacterial wall, allowing fluid to enter. the cell swells and finally bursts, releasing 100 to 200 phage particles

Question 14

lysogenic cycle

Answer 14

replicates the phage genome WITHOUT destroying the host

the viral DNA molecule is incorporated into the host cells chromosome

a temperate phage called lambda is widely used in biological research

Question 15

temperate phages

Answer 15

phages that use both the lytic and lysogenic cycles are called

Question 16

prophage

Answer 16

the integrated viral DNA

Question 17

lysogenic cycle cont.

Answer 17

every time the host divides, it copies the phage DNA and passes the copes to daughter cells

an environmental signal can trigger the virus genome to exit the bacterial chromosome and switch it to the lytic mode

some prophages are expressed during lysogeny and some cause the host bacteria to secrete toxins that are harmful to humans

Question 18

bacterial defenses against phages

Answer 18

bacteria have their own defenses against phages

natural selection favors bacterial mutants with surface proteins that cannot be recognized as receptors by a particular type of phage

the bacteriums own DNA is protected from the restriction enzymes by being methylated

Question 19

restriction enzymes

Answer 19

foreign DNA can be identified as such and cut up by cellular enzymes called

Question 20

CRISPRs Cas system

Answer 20

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs)

Particular nuclease proteins interact with the CHRISPR region : CRISPR-associated (Cas)

^^^^ a type of protection from viral infection used by bacteria and archaea

each spacer sequence between the repeats corresponds to DNA from a phage that had infected the cell

Question 21

What happens when a phage infects a bacterial cell that has the CRISPR Cas system?

Answer 21

The phage DNA is integrated between 2 repeat sequences

• if the cell survives the infection, it can block any attempt of the same type of phage to reinfect it
• the attempt of the phage to infect the cell trigger transcription of the CRISPR region
• the resulting RNAs are cut into pieces and bound by Cas proteins
• the Cas proteins use the phage related RNA to target the invading phage DNA
• the phage DNA is cut and destroyed
• natural selection favors phage mutants that can bind to altered cell surface receptors or that are resistant to enzymes

the relationship between phage and bacteria is in constant evolutionary flux