viruses MORPHOLOGY AND ULTRASTRUCTURE OF VIRUSES CLASSIFICATION OF VIRUSES

Question 1

Viruses form the kingdom

Answer 1

Viruses form the kingdom Vira,

Question 2

is classified into two subkingdoms by the type of —–1) &2).

Answer 2

by the type of nucleic acid: DNA viruses and RNA viruses

Question 3

Subkingdoms consists from families -

Answer 3

DNA viruses united into six families, RNA viruses are united into 14 families

Question 4

Viruses are characterized by the following features (4)

Answer 4

1. They haven’t cell structure (do not have nucleus, and do not have organelles such as ribosome, mitochondria and etc.).
2. They contain only one kind of nucleic acid either DNA or RNA.
3. Viruses replicate in a manner different from that of cells; i.e., viruses do not undergo binary fission or mitosis. The type of replication is reproduction (disjunctive way), and they are not capable to reproduce independently. Viruses must replicate within cells, because they cannot generate energy to synthesize proteins. Because they can reproduce only within cells, viruses are obligate intracellular parasites.
4. Viruses are smaller, they are measured in nanometers (nm) which is the 1/1000th part of micrometer /µm/ and cannot be seen in the light microscope

Question 5

Viruses replicate in a manner different from that of cells;

Answer 5

viruses do not undergo binary fission or
mitosis. The type of replication is reproduction (disjunctive way), and they are not capable to reproduce
independently

Question 6

The type of replication is

Answer 6

The type of replication is reproduction (disjunctive way), and they are not capable to reproduce
independently

Question 7

The intracellular forms are named and extra-cellular forms –

Answer 7

The intracellular forms are named viruses, extra-cellular forms – virion.

Question 8

The virion consists essentially of a— -and surrounded by a

Answer 8

The virion consists essentially of a nucleic acid core surrounded by a protein coat called a capsid

Question 9

The capsid is composed of –and each one consists of

Answer 9

repeating protein units called capsomers
Each capsomer consists of one or
several proteins

Question 10

Viral proteins are divided into structural and functional.

The structural proteins are in the —Their major purpose(goal) is to 1/2/3/4

Answer 10

The structural proteins are in
the capsid composition.
Their major purpose is to 1) facilitate transfer of the viral nucleic acid from one host cell to another.
2) They serve protective function,
3)participate in the attachment of the virus particle to a
susceptible cell, and
4)provide the structural symmetry of the virus particle

Question 11

Functional proteins

1) what is it
2) amount
3) essential for

Answer 11

Functional proteins
1)are enzymes,
2)which are present in very small amounts and are probably not important in the structure of the
virus particles;
3) they are essential for the initiation of the viral replicative cycle when the virion enters a host cell.

Question 12

The capsid with the enclosed nucleic acid is known as the . These viruses are called.

Answer 12

The capsid with the enclosed nucleic acid is known as the nucleocapsid
These viruses are called simple viruses.

Question 13

The arrangement of capsomers gives the virus structure its geometric symmetry.
There are following types of symmetry in virus capsids

Answer 13

1. Icosahedral,
   in which the capsomers are arranged in 20 triangles that form a symmetric figure (an
   icosahedron) with the approximate outline of a sphere /cubic symmetry/.
2. Helikal,
   in which the capsomers are arranged in a hollow coil that appears rod-shape (spiral
   symmetry).
3. Complex structure:
   Some virus particles do not exhibit simple cubic or helical symmetry but are more
   complicated in structure (poxviruses are brick-shaped with ridges on the external surface and a core and
   lateral bodies inside).

Question 14

The functions of nucleic acid:

Answer 14

 are carriers of genetic information

 infectivity

Question 15

The functions of capsid are:

Answer 15

 protective (protection of nucleic acid)
 adhesive and invasive (make attachment to the host cell and invasion)
 antigenicity

Question 16

Some viruses have supercapsid membrane (enveloped viruses). The envelope is —-membrane
composed of——-derived from the host cell membrane and—— that is virus specific.
Furthermore, there are frequently—– in the form of spike-like projections on the surface, which —– to host cell receptors during the entry of the virus into the cell.

Answer 16

Some viruses have supercapsid membrane (enveloped viruses).
The envelope is a lipoprotein
membrane composed of lipid derived from the host cell membrane and protein that is virus specific.
Furthermore, there are frequently glycoproteins in the form of spike-like projections on the surface, which
attach to host cell receptors during the entry of the virus into the cel

Question 17

The functions of supercapsid

Answer 17

are the same as the capsid’s:
adhesive and invasive, protective, and
they are the principal antigens against which the host mounts its immune response to viruses.

the presence of an envelope makes the virus more sensitive to heat, detergents, and lipid solvents such as
alcohol and ether than are non-enveloped (nucleocapsid) viruses.

Question 18

The classification of viruses is based on the following properties (9)

Answer 18

1.The type of nucleic acid – either DNA or RNA (but not both), the number of strands in nucleic acid.
Viral nucleic acid (genome) is located internally and can be either single or double strand DNA or single or double strand RNA.
Only viruses have genetic material composed of single stranded DNA or of single stranded or double stranded RNA. The functions of the nucleic acid are: they carried genetic information and secure infectious properties. Polarity (sense)-single stranded RNA can be single stranded RNA (+);
single stranded RNA(-). Nuclear weight of the nucleic acid, its percentage in the virion, number of
capsomers in the capsid.
2. According to nucleocapsid symmetry (icosahedral, helical)
3. According to the structure: presence or absence of the second membrane /supercapsid/ -simple virus, enveloped virus.
4. According to the size the viruses fall into 3 groups:
 Small-size viruses – 30-40 nm (poliomyelitis virus)
 Middle-size viruses – 80-150 nm (viruses of influenza, AIDS)
 Large-size viruses – from 200 and over till 400 nm (Paramyxoviruses, virus of parotitis)
The size of viruses can be determined by: a) Filtration through colloid membranes, b) Centrifugation in
high speed centrifuges, c) Electron microscopy, d) Diffusion in gel.
5. According to their shape:
 Spherical form (viruses of influenza, AIDS)
 Rod-shaped form (causative agents of tobacco mosaic disease)
 Spermatozoid form (phages)
 Bullet-shaped (rabies)
 Star shaped
 Crown shaped
6. According to the host which they can affect:
 plants (causative agent of tobacco mosaic disease)
 animals and humans (animal viruses)
 bacteria ( bacteriophages)
7. According to the tropism. By this classification they may be neurotropic (rabies virus); pneumotropicaffect the respiratory system (Orthomixoviruses, Paramixoviruses); enterotropic (Epidemic poliomyelitis
virus, Hepatitis virus A).
8.According to the formation of inclusion bodies (intranuclear, cytoplasmic)
9.According to the antigenic structure: there are internal core nucleocapsid “S” antigen and superficial
viral “V” antigen

Question 19

According to the structure

Answer 19

: presence or absence of the second membrane /supercapsid/ -simple virus,
enveloped virus

Question 20

(THE CLASSIFICATION OF VIRUSES )According to the size the viruses fall into 3 groups:

Answer 20

 Small-size viruses – 30-40 nm (poliomyelitis virus)
 Middle-size viruses – 80-150 nm (viruses of influenza, AIDS)
 Large-size viruses – from 200 and over till 400 nm (Paramyxoviruses, virus of parotitis)

Question 21

The size of viruses can be determined by:

Answer 21

a) Filtration through colloid membranes, b) Centrifugation in
high speed centrifuges, c) Electron microscopy, d) Diffusion in gel.

Question 22

(THE CLASSIFICATION OF VIRUSES )According to their shape:

Answer 22

 Spherical form (viruses of influenza, AIDS)
 Rod-shaped form (causative agents of tobacco mosaic disease)
 Spermatozoid form (phages)
 Bullet-shaped (rabies)
 Star shaped
 Crown shaped

Question 23

(THE CLASSIFICATION OF VIRUSES )According to the host which they can affect:

Answer 23

 plants (causative agent of tobacco mosaic disease)
 animals and humans (animal viruses)
 bacteria ( bacteriophages)

Question 24

(THE CLASSIFICATION OF VIRUSES ) According to the tropism.

Answer 24

By this classification they may be neurotropic (rabies virus); pneumotropicaffect the respiratory syste

Question 25

Viruses are known to be intracellular obligate parasites. (yes or no)

Answer 25

(yes )Viruses are known to be intracellular obligate parasites.

Question 26

A single virus can give rise to several or even

thousands of similar viruses in a single host cell (yes or no)

Answer 26

(yes )A single virus can give rise to several or even

thousands of similar viruses in a single host cell

Question 27

In the result of this interaction following forms of infection develop:

Answer 27

1. Productive – when reproduction of viruses takes place (formation of new virion).
2. Abortive form – death of virions or cession of their reproduction.
3. Integrative – mutual adaptation of the virus and the cell (virogeny) or the development of a neoplastic
   process in which intensive growth and reproduction of cells are observed (oncogenic function of viruses).

Question 28

There are three types of persistent viral infections of clinical importance:

Answer 28

 Chronic - carrier infection:
 Latent infections:
 Slow Virus infections:

Question 29

 Chronic - carrier infection:

Answer 29

Some patients who have been infected with certain viruses continue to produce significant amount of the virus for long periods.
This carrier state can follow an asymptomatic infection as well as the actual disease and it can itself be either asymptomatic or result in chronic illness.
Important clinical examples are chronic hepatitis, which occurs in hepatitis B and hepatitis C virus in carriers, in which carriers can produce virus for years.

Question 30

 Latent infections:

Answer 30

Best of all these infections are illustrated by the herpes virus group, the patient
recovers from the initial infection and virus production stops.
Subsequently, the symptoms may recur, accompanied by the production of virus. The molecular nature
of the latent state is unknown.

Question 31

 Slow Virus infections:

Answer 31

The term “slow virus” refers to the prolonged period between the initial infections and the onset of disease, which usually last for years.
The incubation period and the progress of the disease are prolonged
(e.g. subacute sclerosing panencephalitis, which follows several years after measles virus infections).

Question 32

Infected cells during viral infections frequently contain ——–, which are discrete areas containing ——They have either———–, or
—-location or in the———-
. Some inclusion bodies serve as

Answer 32

Infected cells during viral infections frequently contain inclusion bodies,
which are discrete areas containing viral proteins or viral particles.
They have either intranuclear (in herpes, chickenpox),

or intracytoplasmic location (Guarnieri’s inclusion bodies in small pox; Babes-Negri bodies in rabies),
or in the nucleus and cytoplasm (in smallpox).

Some inclusion bodies serve as a diagnostic character.

Question 33

the multiplication cycle of viruses can be divided into following stages:

Answer 33

1. Attachment (adsorption) -
2. Penetration –
3. Uncoating –
4. Eclipse period (
5. Biosynthesis –

Question 34

multiplication cycle of viruses

1.Attachment (adsorption) :

Answer 34

adsorption takes place only if there is an affinity between virions and host
cells. This attachment is a chemical interaction in which weak bonds are formed between the attachment
and receptor sites. The cell surface should contain specific receptor sites to which the virus can gain
attachment. However, the receptor sites of animal cells are proteins and glycoproteins of the plasma
membrane. The attachment sites of animal viruses are distributed over the surface of the virus. The sites
themselves vary from one group of viruses to another. In icosahedral viruses, the attachment sites are
small fibers at the corners of the icosahedron. In many of the envelope viruses, the attachment sites are
spikes located on the surface of the envelope. As soon as one spike attaches to a host receptor, additional
receptor sites on the same migrate to the virus. Attachment is completed when many sites are bound.

Question 35

multiplication cycle of viruses
1.Attachment (adsorption) :

adsorption takes place only if—–
This attachment is a—–
The cell surface should contain——-
the receptor sites of animal cells are———
The attachment sites of animal viruses are distributed over the surface of the virus.

The sites themselves vary from one group of viruses to anothern ,in icosahedral viruses——In many of the envelope viruses,———
As soon as one spike attaches to a host receptor, additional receptor sites on the same migrate to the virus. Attachment is completed when ——–

Answer 35

adsorption takes place only if there is an affinity between virions and host cells.

This attachment is a chemical interaction in which weak bonds are formed between the attachment
and receptor sites.

The cell surface should contain specific receptor sites to which the virus can gain attachment. ,

the receptor sites of animal cells are proteins and glycoproteins of the plasma membrane.

The attachment sites of animal viruses are distributed over the surface of the virus
.
The sites themselves vary from one group of viruses to another.
In icosahedral viruses, the attachment sites are
small fibers at the corners of the icosahedron.
In many of the envelope viruses, the attachment sites are spikes located on the surface of the envelope.

As soon as one spike attaches to a host receptor, additional receptor sites on the same migrate to the virus. Attachment is completed when many sites are bound.

Question 36

multiplication cycle of viruses

2. Penetration –

Answer 36

– occurs by several ways: endocytosis or viropexis – penetration of envelope animal
viruses occurs by endocytosis, an active cellular process by which nutrients and other molecules are
brought into a cell. A cell plasma membrane continuously folds inward to form vesicles. These vesicles
contain elements that originate outside the cell and are brought into the interior of the cell to be digested.
If a virion attaches to a small out-folding on the plasma membrane of a potential host cell, the host cell
will enfold the virion into a fold of plasma membrane, forming a vesicle. Once the virion is enclosed
within the vesicle, its viral envelope is destroyed. Penetration sometimes occurs by an alternative method
called fusion, in which the viral envelope fuses with the plasma membrane and released the capsid into
the cell cytoplasm.

Question 37

multiplication cycle of viruses
2. Penetration 
 occurs by several ways: -------
 Penetration sometimes occurs by an alternative method
called -----which

Answer 37

occurs by several ways: endocytosis or viropexis – penetration of envelope animal
viruses occurs by endocytosis, an active cellular process by which nutrients and other molecules are
brought into a cell. A cell plasma membrane continuously folds inward to form vesicles. These vesicles
contain elements that originate outside the cell and are brought into the interior of the cell to be digested.
If a virion attaches to a small out-folding on the plasma membrane of a potential host cell, the host cell
will enfold the virion into a fold of plasma membrane, forming a vesicle. Once the virion is enclosed
within the vesicle, its viral envelope is destroyed. Penetration sometimes occurs by an alternative method
called fusion, in which the viral envelope fuses with the plasma membrane and released the capsid into
the cell cytoplasm.

Question 38

multiplication cycle of viruses
3. Uncoating
is the process of—-

Answer 38

– is the process of viral remove all cover from outer layers and capsid so the nucleic acid is released into the cell.
It occurred by the proteolytic enzymes.

Question 39

multiplication cycle of viruses

4. Eclipse period (

Answer 39

(the time during which no virus is found inside the cell). – from the stage of
penetration till the appearance of mature daughter virions, the virus cannot be demonstrated inside the
host cell. This period during which the virus seems to disappear or go “underground” is known as the
eclipse phase. In this step the synthesis of the virus components begins, so reproduction begins.

Question 40

multiplication cycle of viruses
5. Biosynthesis –
The first step in viral gene expression is ——-
It is at this point that viruses follow different pathway depending—–

Steps in biosynthesis:–
DNA viruses —-
 RNA viruses – RNA viruses fall into following groups with quite
different strategies for mRNA synthesizing—-

Answer 40

The first step in viral gene expression is mRNA synthesis.

It is at this point that viruses follow different pathway depending on the nature of their nucleic acid and the part of the cell in which they replicate

Question 41

multiplication cycle of viruses
5. Biosynthesis Steps in biosynthesis:–
DNA viruses —-
 RNA viruses – RNA viruses fall into following groups with quite
different strategies for mRNA synthesizing—-

Answer 41

1.Transcription of messenger RNA (mRNA) from the viral nucleic acid
2.Translation of the mRNA into “early proteins”. These early or non structural proteins are enzymes
which initiate and maintain the synthesis of virus components. They may also induce shutdown of host
protein and nucleic acid synthesis.
3.Replication of viral nucleic acid.
4.Synthesis of “late” or structural proteins, which are the components of daughter virion capsid

DNA viruses – generally, DNA containing viruses replicate their DNA in the nucleus of the host cell
by using the host cell DNA-dependent RNA polymerase to synthesize their mRNA. The genome of all DNA viruses consists of double-stranded DNA, except for the parvoviruses, which have a singlestranded DNA genome. The reproduction of DNA viruses takes place in the classic way:
DNA – mRNA – viral proteins
DNA viruses that replicate in the nucleus generally use host cell DNA and RNA polymerases and processing enzymes. The poxviruses are the exception because they replicate in the cytoplasm, where
they do not have access to the host cell RNA polymerase. They therefore carry their own polymerase
within the virus particle.
 RNA viruses – Most RNA viruses undergo their entire replicative cycle in the cytoplasm (the two
exceptions are retroviruses, influenza viruses). RNA viruses fall into following groups with quite
different strategies for mRNA synthesizing
: a) The simplest strategy is the viruses, which has singlestranded RNA of positive polarity as its genetic material. These viruses use their RNA genome directly as mRNA (poliovirus): RNA(+)—viral proteins
 b) The second group has single-stranded RNA of negative polarity as its genetic material. An
mRNA must be transcribed by using the negative strand capable of using RNA as a template; the virus
carries its own RNA-dependent RNA polymerase (Paramyxoviruses, Rabdoviruses):
 RNA(-) – mRNA – viral proteins
 c) The third group has double-stranded RNA as its genetic material. Because the cell has no enzyme
capable of transcribing this RNA into mRNA, the virus carries its own polymerase (Reovirus).
 d) The fourth group, exemplified by retroviruses, has single – stranded RNA of positive polarity that
is transcribed into double-stranded DNA by the RNA-dependent DNA polymerase (reverse transcriptase, so called because it carries out the reaction: RNA-DNA that is exactly the reverse of the familiar
transcription of DNA-RNA) carried by the virus. This DNA copy is then transcribed into viral mRNA
by the regular host cell RNA polymerase (polymerase II), before transcription the viral DNA must be
integrated into the DNA of a host cell chromosome. In this integrated state, the viral DNA is called a
provirus: RNA – DNA(copy) – RNA(+) – viral proteins

Question 42

multiplication cycle of viruses
6. Assembly and Release
the first step is an—
The progeny particles are assembled by—-
Virus particles are released from the cell by either of the two processes.—-

Answer 42

• the first step is an assembly of the protein capsid. The progeny particles are
  assembled by packaging the viral nucleic acid within the capsid proteins.
  Virus particles are released from the cell by either of the two processes. One is the rupture of the
  cell membrane and the release of the mature particles; this usually occurs with non-enveloped viruses.
  The other, which occurs with enveloped viruses, is the release of viruses by “budding” through the outer
  cell membrane. Budding does not immediately kill the host cell, and in some cases the host cell survives.

Question 43

Viral enzymes – in contrast of Prokaryotes and other organism viruses deprived of enzymes, which are capable to participate into metabolic processes.
But many viruses in their capsid structure contain one
or two groups of enzymes:
The first group and exp :
The second group exp :

Answer 43

The first group: contains virus-specific enzymes which are the replication and transcription enzymes and the enzymes which contribute to penetration of viruses into the host cell and virus particles released from the host cell (RNA dependent RNA polymerase, reverse transcriptase, neuraminidase, lysozyme).
The second group contains virus- induced enzymes. The structure of these enzymes is encoded in the virus genome but they are synthesized in the host ribosomes (RNApolymerase, DNA-polymerase)