ds DNA viruses

Question 1

what class do dsDNA viruses belong to

Answer 1

Class I baltimore

Question 2

Important families and examples of dsDNA viruses

Answer 2

Herpes

papillomaviruses

t4 phage

Question 3

Taxonomy of Herpesviruses

Answer 3

Herpesvirusesbelongs to the family Herpesviridae, order Herpesvirales.

• Derived their family name from the Greek word herpin(to creep).
• Viruses in the family Herpesviridaetypically infect mammals, reptiles and birds while other herpesviruseshave been found to infect fish and amphibians.

Question 4

Characteristics fo herpesviruses

Persistent infections

Answer 4

• Persistent infection: Infections typically persist for a life time in their host.
• Infections are usually latent which could be activated from time to time especially when the host immunity is compromised.
• Both primary or reactivated infection could be asymptomatic or can result in disease of varying severity.
• Disease severity depends on the interaction between the virus and the host or the host’s immune status.

Question 5

Human herpesviruses

Answer 5

•Eight species of the herpesvirusesare known to infect man persistently

Question 6

Herpes Simples viruses 1 and 2

infection

Answer 6

Infection: Upon entering the host, these viruses first infect epithelial cells of the oral and genital mucosa, the cornea or the skin.Thereafter, the viruses are spread to neurons where it may be transported to the nuclei, where it may establish a latent infection

Question 7

Herpes simplex virus 1

infection

Answer 7

• The HSV-1 virus generally infects the lips or the nose of children between 6 and 18 months.
• Thereafter, a latent infection might become reactivated if the child becomes stressed or the immune system is compromised.
• Once reactivation of a latent infection occurs, mass production of virionsis triggered, of which 20-40% of the cases are transported within the neuron, to the initial site of infection (epithelial cells) resulting in the cold sores.
• Sometimes, infection may result in serious complications like encephalitis, especially in immune suppressed perso

Question 8

Herpes Simplex virus 2 tissue trophism

Answer 8

• HSV-2 is a sexually transmitted disease caused by the genital herpes.
• Infection in new born babies could be serious and result in fatality of approximately 54%.

Note: Although, HSV-2 and HSV-1 commonly infect the genitals and nose respectively, few cases could result in HSV-2 infecting the nose and HSV-1 infecting the genitals

Question 9

3. Varicella-zoster virus

Answer 9

• The virus commonly cause infection in children by spreading from the blood to the skin, resulting in a rash.
• Infection in children results in varicella (chickenpox).
• Sometimes, the virus may spread to the nerve cells resulting in latent infection.
• Nerves commonly infected are those of the face or the trunk, which are common areas affected by the zoster (shingles) when a latent infection is reactivated.

Question 10

4. Epstein-Barr Virus (EBV)

Answer 10

• EBV is transmitted in salivaand first infect the epithelialcells before spreading to B cells, the major host cell types for the virus.
• Usually, greater than 90% of individuals become infected with HBV at the first years of life, resulting in little or no symptoms.
• Some individuals in developed countries do not get infected until about adolescent or adulthood.
• These individuals develop a condition known as mononucleosisor glandular fevercommonly referred to by physicians as ‘’kissing disease’’.
• Also, EBV in humans has been linked with a number of tumors

Question 11

Human cytomegalovirus

transmission

Answer 11

contact with virus-contaminated body fluids such as, urine, saliva and genital secretions

Question 12

Human cytomegalovirus

symptoms

Answer 12

majority of infected persons show mild or no symptoms. However, infected pregnant woman can infect their fetus leading to severe consequences

Question 13

Human cytomegalovirus

Pathology

Answer 13

Pathology: Research showed that 0.7% of children are born with virus, some of which show virus-induced damages such as enlargement of the brain and organs and small brain size.

• 0.5% of babies with congenital infections die.
• About 20% of persons who overcome the virus develop complications such as mental retardation and hearing loss.
• Immuno-compromised persons such as AIDS patients, cancer patients or patients with organ transplant could show severe symptoms involving hepatitis and pneumonia

Question 14

Human Herpesvirus(HHV) 6

transmission, types and symptoms

Answer 14

• Transmission: Is through person to person contact with secretions from respiratory droplets.
• Two types of human herpesvirus6 exist, HHV-6A and HHV-6B
• Symptoms: HHV-6B infection in children show symptoms of fever and sudden appearance of a rash known as a exanthemsubitum.

Question 15

7. Human Herpesvirus(HHV) 7

transmission, infection and symptoms

Answer 15

• Transmission: HHV-7 has been found in the mouth and blood of humans. Over 95% of humans have had previous infection during childhood and have developed immunity.
• Infection: Affect human CD4+ T cells.
• Symptoms: causes skin rash known as a exanthemsubitumsimilarly as HHV-6B infection. However, it could result to other symptoms such as fever, vomiting, diarrhea, low lymphocyte count, etc

Question 16

8. Kaposi’s sarcoma-associated herpesvirus

transmission and symptoms

Answer 16

• The virus was discovered in 1994 and named after the tumor which is associated with it.
• Transmission: In saliva and contaminated bloodthrough sexual and non-sexual means such as, blood transfusion and tissue transplant.
• Symptoms: The virus is associated with a cause of cancer among HIV patients commonly known as Kaposi’s sarcom

Question 17

Generalized structure of a herpes virion

Answer 17

•The viruses in this group have relatively complex structures composed of a large number of proteins arranged into three distinct structures namely;

1. capsid,
2. tegument,
3. envelope

•Majority of the envelop glycoprotein molecules are arranged into spikes of varying dimensio

Question 18

Generalized herpesvirus genome

Answer 18

• The genome of herpesvirusesis a linear dsDNAmolecule, which is approximately 125 kbpamongst the smallest virus members and 290 kbpamongst the largest viruses.
• The dsDNAis enclosed in an isosahedralshaped capsid which is surrounded by a tegument

Question 19

The virionof human simplex virus 1 (HSV-1)

capsid, tegument and envelope

Answer 19

• Capsid: The capsid of HSV-1 is arranged asymmetrically within the tegument
• Tegument: The tegument contains 26 or more species of virus proteins, some of which forms the virus proteins, cell proteins and others forming the virus mRNA molecules.
• Envelope: The virionenvelope is composed of 16 or more protein species, majority of which are glycoproteins forming 600-750 spikes

Question 20

Structure of HSV-1 virion

capsomers

Answer 20

• The HSV-1 capsid is organized from 162 capsomeres, out of which 150 are hexons.
• Eleven of the twelve vertices of the icosahedron each has a penton (composed of 5 protein molecules), while one of the vertices has a structure referred to as a portal.
• The portal have been shown in electron microscopy to play a role in assembly and packaging of progeny virioninto individual procapsidsas well as release of viriongenome from the capsid after invading a new ce

Question 21

HSV VP proteins

Answer 21

• HSV-1 proteins are represented with numbers with the prefix virus protein (VP).
• VP16is the mostabundantprotein in the virus tegument.
• VP5forms the hexons(6 molecules of VP5) and pentons (5 molecules of VP5).
• Other proteins forms structures called triplexeswhich form links between capsomeres.
• The envelope-containing glycoproteins are designated by alphabets; glycoprotein B (gB), glycoprotein C (gC) and glycoprotein D (gD)

Question 22

Replication in HSV-1

steps

Answer 22

So far, HSV-1 is only known to infect humans in nature. However, the virus have been found to replicate in other cell cultures from monkeys, dogs and mice.

The steps in replication are:

•Attachment and Entry•Transcription•Translation•Genome replication •Assembly•Exit

Question 23

HSV-1 Attachment and Entry

hint = glycoprotein

Answer 23

• Attachmentof HSV-1 to host cell involved initial binding of the virionto heparansulfate (cell surface glycoprotein) prior to binding to main cell receptors.
• The main cell receptors could be several types of molecule such as nectins(cell adhesion proteins).
• Entry: Depending on the type of host cell, the HSV-1 can enter the cell in two steps:

1. Fusion of virionenvelope with plasma membrane or by endocytosis
2. Fusion of the virionenvelope with endosome membrane

•Five or more of the virionenvelope glycoproteins are involved in virus attachment and entry

Question 24

HSV-1 Entry into the Cytoplasm and transport to the nucleus

Answer 24

• Some tegument proteins are released upon entry of the virus into the cytoplasm.
• Other tegument proteins remain fused with the nucleocapsidwhich must be transported to the cell nucleus before replication can occur.
• Microtubules: Invaded virionswith tegument still associated with nucleocapsidare easily transported to the nucleus via microtubules than in cell neurons based on research finding

Question 25

HSV-1 Entry into the cell nucleus and activation of viral genes

Answer 25

* Once the virionarrives at a nuclear pore with the aide of a microtubule, it binds to the nuclear pore, inducing a conformational change, releasing its genome (dsDNA) into the cell nucleus. * Covalent bonding binds the two ends of the linear DNA forming a circular molecule. * Association of virionDNA with histones: The virionDNA has been found to be associated with cell histones(proteins)which have been detected in several locations of the cell using antibody assay. * At the various sites, histonesplay various roles in virus replication such as:1. activation of virus genes2. down-regulation of host DNA, RNA and protein synthesis * Virus host shutoff (VHS) protein: a type of tegument protein that breakdown certain cell mRNAs. * VP16: Is transported to the nucleus to form association with virionDNA where it plays a role in the activation of virus gen

Question 26

HSV-1: Transcription and translation 3 PHASES

Answer 26

•The genes of herpesvirusesreplicate their genes in 3 phases: 1. Immediate early (IE) 2. Early (E) 3. Late (L) Note that a small number of intron in HSV-1 genes, especially in the intermediate early genes

Question 27

HSV-1 IE genes

Answer 27

* The IE genes of HSV-1 are activated by VP16 * VP16 functions as a transcription factor by recruiting host cell RNA polymerase II and associated initiation components of the gene. * Five IE proteins exist, for which all act as transcription factors in switching on early (E) and late (L) genes. * Note that IE proteins may play more that one role in virionreplication

Question 28

HSV-1 Early (E) and Late (L) protein

Answer 28

* A number of E proteins play roles in virus DNA replication, which usually occur in discrete areas of the nucleus referred to as replicativecompartments. * VirionDNA and proteins concentrate in this compartment together with RNA polymerase II. * Majority of virionL proteinsfunction as virus structural protein.

Question 29

HSV-1 Genome replication

Answer 29

•Replication of HSV-1 virus DNA requires seven essential E proteins. Copies of an origin-binding protein bind at one of three ori sites in the virus DNA; the protein has helicase activity, causing the DNA to unwind at that site. * A ssDNAbinding protein binds to each strand of DNA, to prevent them from reforming double helix. * Thereafter, a complex of three proteins also act as helicase to further unwind the double helix to form replicative fork. * Also, a complex of the same three proteins (now acting as a primase) to synthesize short RNA sequences complementary to the DNA. * The synthesized RNA acts as a primer and together with DNA polymerase synthesize the leading strand of DN

Question 30

Figure 7: Functions of HSV-1 proteins in DNA replication

Answer 30

Note that the helicase and primaseare synthesized from a complex of three virus proteins. Helicase functions in unwinding dsDNAinto single strands resulting in a replication fork. Primasesynthesizes short RNA sequence complementary to the DNA which functions as a primer, which in association with DNA processivityfactors synthesize the leading DNA strand. On the other strand also, primasesynthesizes short RNA sequence which function as primer in synthesizing

Question 31

Assembly and exist of dsDNA virions from cells

Answer 31

* The virus envelopes, composed of glycoproteins are synthesized in the rough endoplasm reticulumprior to transport to the Golgi complex (processorting, . * Procapsidformation: Capsid proteins (e.g. VP5), some tegument proteins and scaffolding proteins accumulate in the nuclear replication compartment where procapsidare constructed. * Note that the scaffoldingproteinshelps to maintain the circularshapeof the procapsidwhich are lost before or during DNA packagin

Question 32

dsDNA Virionpackaging: Entry of DNA into procapsid

Answer 32

* The progeny DNA enters the procapsidvia a hole at one of the vertices of the icosahedron. * Each procapsidacquires a genome length of DNA, which is cut from a concatemer(long DNA molecule formed by herpesviruses). * Entry is facilitated by a complex of virus proteins called terminase, which pumps the progeny DNA into the procapsids, recognition of packaging signal and cleaves the long DNA molecule to form the nucleocapsid

Question 33

Assembly and exit of dsDNA virions

Answer 33

* The nucleocapsidmust acquire a tegument, membrane and envelope before exiting the cells to infect new cells. * The nucleocapsidacquires tegument protein in the nucleus. * The nucleocapsidthen buds into the perinuclearmembrane (space between the inner and outer nuclear membrane) and acquires a temporary envelope. * The temporary envelope binds with the outer nuclear membrane, releasing the nucleocapsidinto the cytoplasm. * The virionenvelope is acquired through budding into vesicles derived from the Golgi complex.

Question 34

Figure 8: Packaging of the nucelocapsid into the envalope of a dsDNA virus

Answer 34

virionenvelope is acquired through budding into vesicles derived from the Golgi complex. The enveloped virionis transported in the vesicle to the plasma membrane, fuses with it and exit the cell.Source:Adapted from Carter and Saunders 2013; Virology: Princ