Lecture 21-Viruses: Structure, Classification & Life Cycles

Question 1

What is the typical size of viruses, and how small can they get?

Answer 1

typically below 200nm , but can get less than 20nm (smaller than prokaryotic ribosomes!)

Question 2

Are viruses made out of cells?

Answer 2

no they’re are acellular

Question 3

Which type of microbes are viruses (free living, commensals, etc)

Answer 3

they are obligate intracellular parasites

Question 4

Describe Capsid of viruses

Answer 4

• outer casing made out of proteins , enclosing viral genome
• it can be either rod shaped, ​polyhedral, or complex (bacteriophages)
• they are assembled from one or few types of protein subunits called capsomeres, but large repeating units

Question 5

Describe inner core of viruses

Answer 5

• Made out of genetic material either DNA or RNA
• This genetic material can be single or double stranded
• The single stranded RNA can be +(sense) or -(Antisense)
• ​can be circular or linear (genetic material)
• Some contain viral enzymes

Question 6

Which type of viruses have an envelope and what is it composed of?

Answer 6

• most animal RNA viruses including HIV contain an outer envelope sorrounding the capsid
• it s derived from the plasma membrane of host cells, composed of phospholipids,membrane proteins, as well as viral proteins/glycoproteins
• help viruses in infection host (have same receptors)

Question 7

What properties of viruses are used to classify them?

Answer 7

• Genome properties
• virion morphology (size,shape)
• presence or absence of outer envelope

Question 8

What are the major groupings that viruses are grouped into called, and give some examples

Answer 8

Families - with the ending viridae (including )

• Hepadnaviridae (Hepatitis Virus)
• Herpesviridae (**Herpes simplex virus)
• Caronaviridae (SAARS virus, MERS)
• Retroviridae (HIV)

Question 9

What is the mechanism simply said, that viruses use to attach to host cells?

Answer 9

Lock & key​ (by interaction of proteins on outside of virus withnspecific receptor on host cell)

Question 10

Do all viruses have a narrow host range?

Answer 10

no some have a broad host range such as West Nile virus (humans, mosquitoes, birds)

but some have narrow host range - Measles/polio virus (only humans)

however bacteriophages usually have a narrow host range

Question 11

What are the 2 types of life cycles of phases?

Answer 11

lytic (lysis) lysogenic (phage becomes part of bacterial DNA)

Question 12

What happens during lysogenic cell cycle of phases?

Answer 12

phage DNA is integrated into host genome called prophage

viral reproduction does not occur immediately (will switch to lysic cell cycle when bacterial cell is in danger)

Question 13

What are the 5 stages of the phage lytic cycle?

Answer 13

• Attachment
• Penetration
• Component synthesis
• assembly of phage
• release by cell lysis

Question 14

Do animal viruses usually infect all tissues of body or specific?

give examples

Answer 14

they usually affect one kind of tissue

‘Human cold viruses infect upper respiratory tract

HIV infect specific WBC’S called CD4+ T Cells

Question 15

What is the simplest possible viral reproductive cycle?

Answer 15

A DNA virus with 1 type of capsomeres, contains 3 stages

Question 16

What are the three mechanisms (briefly ) that RNA viruses consume replicate in host cells?

Answer 16

• Make RNA dependent RNA polymerase +(sense) strand
• Bring RNA dependent RNA polymerase with them and convert it from - to + and then use this complementary as template to make original - strand
• Bring RNA dependent DNA polymerase with them converting RNA to DNA - can then use DNA dependent RNA polymerase (cell enzyme) to make original RNA strand of virus retrovirus do this

Question 17

How is RNA dependent RNA polymerase made from a + stranded viral genome?

Answer 17

Use the strand to act as mRNA in ribosome, make RNA polymerase enzyme(protein)

Question 18

What are the 8 steps of the - stranded RNA virus’s cycle (contains envelope)

Answer 18

• Glycoproteins on the viral envelope bind to a specific receptor molecules on host cell
• virus enters by endocytosis shedding its envelope (as it contains phospholipids, glycolipids etc) the capsid is ​also digested by cellular enzymes
• viral genome is used as a template to make complementary strand using the RNA dependent RNA polymerase that it ​brought,
• complementary strand (+) ​is usd as a template and is used to make - (original) stranded RNA again
• complementary strand (+) acts as mRNA and is used to make capsid proteins & glycoproteins
• vesicles transport these enveloped glycoproteins to plasma membrane
• Capsid assembles around the genome near cell membrane, and glycoproteins assemble around the plasma membrane
• vrirus buds off cell by exocytosis comtaining viral glycoproteins, glycolipids, and proteins derived from ER

Question 19

What is a unique feature for retroviruses?

Answer 19

They use ​reverse transcriptase to convert ssRNA to ssDNA & then use ssDNA as temperate to make DSDNA, which embeds into nucleus (provirus)

Question 20

How many strands of RNA and reverse transcriptase does HIV have?

Answer 20

2 identical single stranded RNA & 2 reverse transcriptase

Question 21

Can viruses be grown artificially?

Answer 21

yes, it can be grown in culture with animal/human cells in lab