Virology lecture 1 | Virus structure and classification
Summary
TLDRThis 20th lecture in the microbiology series delves into the intricate world of viruses and prions, exploring their elusive nature and the challenges in their discovery since 1884. The video explains the unique structure and function of viruses, distinguishing them from living cells due to their dependence on host cells for replication. It covers the diversity in viral shapes and sizes, the protective role of the capsid, and the significance of the viral envelope in host interaction. The lecture also touches on the specificity of viruses to certain host tissues, known as tissue tropism, and their varying host ranges, offering a comprehensive introduction to these fascinating entities.
Takeaways
- đ The script introduces the topic of viruses and prions, highlighting the start of a new lecture series focusing on their structure and function.
- đ The history of virus discovery is discussed, emphasizing that viruses were not identified until 1884 due to their small size, which is beyond the resolution of light microscopes.
- đ± The script explains that viruses are non-cellular particles with a definite size, shape, and chemical composition, and they are not considered true living organisms because they require a host cell to replicate.
- 𧏠It outlines that viruses contain either DNA or RNA as their genetic material, and they lack the machinery for protein synthesis, relying on the host cell for replication.
- đ The diversity of viruses is highlighted, with nearly 1,500 known types and an estimated 400,000 more, emphasizing the vast number of viruses yet to be discovered.
- đŹ The importance of electron microscopy in studying viruses is mentioned, as it allows for detailed visualization and understanding of their structure.
- đ The script discusses the obligatory intracellular nature of viruses, their filterability, and the fact that they can contain proteins and have tissue specificity.
- đ It describes the various shapes of viruses, including round, rod-shaped, and complex structures, and how these shapes can be icosahedral, helical, or more irregular geometrical forms.
- đĄïž The functions of the viral capsid and envelope are explained, with the capsid protecting the nucleic acid and the envelope facilitating host-cell fusion and tissue specificity.
- 𧏠The core of the virus, which contains the nucleic acid, is detailed, noting that the viral genome can be either DNA or RNA but not both, and that the number of genes varies among different viruses.
- đ The concept of host range and tissue tropism is introduced, explaining that viruses have specific hosts and tissues they can infect, such as hepatitis B's specificity to liver cells.
Q & A
What is the main topic of the 20th lecture in the microbiology series?
-The main topic of the 20th lecture is the structure and function of viruses.
What is the significance of the year 1884 in the study of viruses?
-In 1884, it was postulated that rabies was caused by an agent smaller than bacteria, marking the beginning of the search for viruses.
What are the two different sets of lectures planned for viruses?
-The two sets of lectures planned are one on the structures and properties of viruses, and another on human viruses, common infections, and diseases they cause.
Why are viruses considered non-cellular particles?
-Viruses are considered non-cellular because they lack the ability to carry out metabolic processes on their own and require a host cell to replicate.
How do viruses differ from true living organisms?
-Viruses differ from true living organisms because they do not exhibit all the characteristics of life outside of a host, such as metabolism and growth.
What is the role of the capsid in a virus?
-The capsid is a protein coat that encloses and protects the viral nucleic acid, forming the nucleocapsid.
What is the difference between enveloped and naked viruses?
-Enveloped viruses have an additional lipid membrane surrounding the capsid, while naked viruses do not have this extra layer.
What are the two main types of viral capsid structures?
-The two main types of viral capsid structures are helical and icosahedral.
What is the function of viral spikes in enveloped viruses?
-Viral spikes help the virus to specifically bind to their target tissue and facilitate entry into the host cell.
What is the significance of the viral genome being either DNA or RNA but not both?
-The viral genome being either DNA or RNA signifies the virus's simplicity and specialization in carrying the necessary genes to invade the host cell and redirect its activity for viral replication.
What is tissue tropism, and how does it relate to viruses?
-Tissue tropism refers to the specificity of a virus to infect certain types of tissues or cells, such as hepatitis B being specific to liver cells.
Outlines
đ Introduction to Virology and Virus Structure
This paragraph introduces the topic of the video series on microbiology, specifically focusing on viruses and prions. The lecturer outlines the aim of the lecture series to discuss the structure, function, and diseases caused by viruses. It highlights the historical discovery of viruses, their elusive nature due to their small size, and the development of the field of virology from the late 19th century to the 1950s. The paragraph emphasizes the unique nature of viruses as non-cellular entities that require a host cell to replicate and their dual role in both biological and computer sciences.
đŹ Exploring Virus Characteristics and Diversity
The second paragraph delves into the characteristics of viruses, including their obligate intracellular parasitism, filterability, genetic makeup of DNA or RNA, and the presence of a protein coat known as a capsid. It discusses the various shapes and sizes of viruses, such as round, rod-shaped, and helical forms, and the distinction between enveloped and naked viruses. The paragraph also touches on tissue specificity and host attachment sites, illustrating the diversity of viruses with examples like hepatitis B and tobacco mosaic virus, and the use of electron microscopy to visualize these microscopic entities.
𧏠Dissecting the Components of Viral Structure
This paragraph breaks down the structure of viruses into their coverings and central core. It explains the role of the capsid in protecting the genetic material and the potential presence of an envelope in some viruses. The paragraph describes the two main types of capsid structures: helical and icosahedral, and the significance of spikes in virus-host interactions. It also discusses the composition of the viral core, which may contain DNA or RNA and occasionally enzymes, and how viruses differ from living cells due to their lack of protein-synthesizing machinery.
đĄïž The Protective Role of Capsid and Envelope
The fourth paragraph focuses on the functions of the capsid and envelope in a virus. It explains how the capsid safeguards the viral nucleic acid from external damage and how the envelope facilitates fusion with host cells, contributing to tissue specificity and ease of infection. The paragraph also describes the maturation process of the envelope and the role of spikes in binding to target tissues. Additionally, it touches on complex viruses like bacteriophages and poxviruses, which have unique structural components beyond the typical capsid and envelope.
𧏠Viral Genome and Host Specificity
The final paragraph discusses the core component of a virusâthe nucleic acid, which can be either DNA or RNA. It explains that viruses carry the necessary genes to invade host cells and redirect cellular activities to produce new viruses. The paragraph also covers the differences between DNA and RNA viruses, including the concepts of positive and negative sense RNA. Furthermore, it addresses the host range and specificity of viruses, highlighting tissue tropism as a key factor in determining which cells a virus can infect, using hepatitis B as an example of a virus with a narrow host range.
Mindmap
Keywords
đĄVirus
đĄPrions
đĄViral Structure
đĄNucleic Acid
đĄCapsid
đĄEnvelope
đĄTissue Tropism
đĄIcosahedral
đĄHelical
đĄBacteriophage
đĄViral Genome
Highlights
Introduction to the 20th lecture in the Microbiology series focusing on the structure and function of viruses.
The elusive nature of viruses and their discovery in 1884, highlighting their small size not visible under light microscope.
The differentiation between viruses and prions, and the plan for two sets of lectures on their structures, infections, and diseases.
Historical context of virus discovery, including the postulation of rabies being caused by an agent smaller than bacteria.
The identification of tobacco mosaic virus in the 1890s as an important milestone in virus research.
The multidisciplinary nature of virology by the 1950s, with the discovery of various types of viruses.
Characteristics of viruses as non-cellular particles with definite size, shape, and chemical composition.
The dual nature of viruses as non-living entities outside a host cell and behaving like living organisms within a host.
The importance of viruses in various fields, including human welfare and computer technology, as agents of infection or 'malware'.
The diversity of known and estimated viruses, with only 1,500 identified out of an estimated 400,000 types.
The use of electron microscopy in visualizing viruses and gathering information on their components.
General characteristics of viruses, including their obligatory intracellular nature and filterability.
The composition of viruses containing DNA or RNA and the presence of a protein coat known as a capsid.
The various shapes of viruses, such as round, rod-shaped, and helical, and their significance in identification.
The concept of tissue specificity in viruses, with examples like Hepatitis B infecting liver tissue.
The structure of viruses, including the nucleocapsid and the distinction between enveloped and naked viruses.
The role of viral envelopes in facilitating fusion with host cells and their contribution to tissue specificity.
The core of a virus, consisting of nucleic acid, and the absence of cellular machinery for protein synthesis.
The types of viral genomes, which can be either DNA or RNA, and the distinction between positive and negative sense RNA.
The concept of host range and specificity in viruses, with examples of tissue tropism and narrow host ranges.
Transcripts
welcome friends welcome to another video
tutorial from tsoumas Valley G and here
we are at the 20th lecture of
microbiology lecture series and from now
we're starting with Viral la G lecture
so this is the first lecture of Y roller
G and we're going to talk about the
virus structure and function in little
bit details so if you want to know about
the virus this video is just for you
stay tuned and watch it okay now in this
lecture we want to talk about viruses
and prions
okay so virus like organisms we want to
talk about along with viruses so there
will be two different set of lectures
for viruses one is this one regarding
the structures and properties of virus
and how the infect how they multiply and
there will be another lecture on human
viruses and some very common infections
and diseases caused by the human viruses
and it's cure okay so let us begin with
this first lecture on viruses and prions
the very first thing that I want to
share is the search of the virus elusive
virus and the search began not very
earlier its until 1884 not until that
time we discovered virus because Eve
iris is so small that we cannot know the
presence of virus with the help of even
light microscope light microscope is not
enough to give us the details about the
virus so when the light microscopy was
was invented many organisms which we
cannot see with naked eye is visible
like bacteria protozoa but not the
viruses until 1884 where we passed two
postulated that the rabies was caused by
something which is much smaller than
bacteria and which has a different way
of approaching and infecting organisms
and hosts at 1884 so again we pass to
started that thinking and then 1890s
even offski and base Ehrlich show
the disease in tobacco was caused by a
virus and it lists um this virus this is
tobacco mosaic virus another disease
which is very common in tobacco is
actually caused by virus which is funded
by different industries and the works
start to begin in 1890s you can imagine
it's not that far away so the research
gradually begins from the time period
and by the time at 1950s viral OG was a
multi fair like you know it's a what I
can say multi-phase discipline ok so
where we have you know we have
discovered a lot of different type of
viruses non-cellular particles with a
definite size shape and chemical
composition we know that they have
proteins in it they have some other
chemical components in it but they are
not a true living organism because they
are not behaving the same all the time
that means once we provide them with a
living cell they behave like a living
organism once we take away that cell
they are not behaving like a living
organism so the the property and the
nature of viruses depends on the
presence of another living cell which is
known as the host so whenever you find
the host the virus is living without the
host while the viruses are nonliving
material that's we cannot even put virus
in that free domain concept where this
is totally separate the foundations of
biology if you think about you know this
is a fascinating field and you probably
have heard of this term viruses and
every single lame and even heard this
term virus because whenever something
critical is thought about something
which is a very dangerous which can make
you sick and fell asleep all slick
example is virus in it because this term
is used not only in disease for a human
welfare as well as you know in computer
technology in this field the computer
virus so whenever say something can
destroy things or
bad for your health for your health of
your PC that's the virus nonliving
agents that infect all life-forms
example Farge versus animal virus animal
why this is goin attack animal cells
while Farge is attack bacteria that's
why we call them bacteriophage vial
cultivation differs from bacterial
cultivation near about 1,500 known
viruses are there and estimates to be
near about 4 lakh type of virus result
there among them we only identify and
know 1500 type and the more we develop
the electron micrograph e electron
microscopy sorry it allows us to
visualize viruses with much up close and
we gather much more information
regarding the different body component
of a virus like in case of bacterial
fast this is a picture of a typical
bacteria faj which has a head a collar
and the helical see the tail pin spikes
and all so what are the general
characteristics of a virus if you look
at this this pictures given here all
these three pictures are electron
micrographs of different size and shape
viruses these are round shape virus this
is also rod shaped virus this is like a
very rope lie coiled virus so viruses
are obligatory intracellular parasites
intracellular means they love to be
present inside the cell and they take up
nutrients from the host and they can
virtually make the host very sick and
can kill it they are filterable the
virus is Latin for the word poison they
contain DNA or RNA they contain a
protein code ok and the code is known as
cap seed made up of cap so mirrors ok
they are of various shapes this cap
seeds are of various shape some are
enclosed by an envelop some are without
the envelop those one without the
envelop are known as naked virus those
with the envelop known as enveloped
virus some viruses have spikes which are
mostly made up with proteins and most
viruses are
tissue specific so they have this tissue
specificity that means
certain virus will infect a certain type
of tissue for example hepatitis B
infects the liver tissue host arranged
it determined by the specific host
attachment sites and the cellular
factors that a virus has okay now
viruses are of different shapes and
sizes you can see this picture and that
can help you understand how much
diversify this virus is are and you can
see this equal I seems pretty big of a
cell in contrast with the size of the
viruses where you can see this is the
red blood cell 10,000 nanometer in
diameter ecoli near about 3,000 to 1000
nanometer in this case of a rectangular
rod shaped structure while the virus if
you think about most of them are very
very small you can see even the smallest
one bacteria phages near 424 nanometer
so the e.coli three thousand five
thousand and in contrast vector 524
nanometer only other than than
poliovirus 30 nanometer adenovirus 90
nanometer you can see it contains much
more spikes like that there are square
viruses or like rectangular viruses like
vaccine yeah 300 by 200 by 100 I can see
whenever we show you the different size
and shapes of virus you can see it's all
kind of geometrical shapes that are
available they are polyhedrons so they
have different planes and multiple
planes are present and we can see
different kind of structures you can see
videos like this kind of rod like
structure tobacco mosaic virus another
rod shape structure very long 250
nanometer by very thin 18 nanometer thin
you can see back to a Fajr T 4 that is
225 nanometer and bacteriophage m13 you
can see 800 into 10 nanometer only
Ravi's virus 170 to 70 nanometer so
they're of different type of shapes and
sizes and you can see ebola virus is a
pretty big 970 nanometer near about
equalise thousand nanometer range but
it's totally different of a structure so
and you
there's a huge place of a viruses and
virus community that we don't know about
we only know very fraction of that of
their virus community so maybe many
other variety of viruses are available
viruses as I said are polyhedron so why
polyhedrons because of the different
planes they have they can be either this
round shape measles virus
harpus simplex virus which is enveloped
in this case as you can see and there
are you know these are the shapes
poliovirus parvo virus okay polyhedral
shapes that are that are present always
okay so either icosahedral and different
kind of so these are icosahedral viruses
as you can see herpes simplex virus
poliovirus parvo virus now polio virus
and parvo virus are icosahedral viruses
without the capsule but so they are Nick
they are naked viruses and herpes
simplex is enveloped viruses helical
virus as you can see tobacco mosaic
virus rabies virus measles virus are
helical and there are complex virus
which does not have any fixed
geometrical structure they have multiple
structures linked for example back
shofars hades icosahedron and there is
this the rest of the region of the body
as you can see apart from this region
small linker is a collar and the rest of
the contractile body portion is of like
a tobacco mosaic virus helical pattern
so they are composed of multiple
geometrical shapes now the viral
structure can be divided into two
different type you know the covering and
the central core okay so the covering
can be of two different type capsid must
be present in a virus that is the
protein coat inside of which the genetic
material is present now there may or may
not be enveloped okay if envelop is
present enveloped virus if envelope is
absent then naked virus central core can
consist of DNA or RNA that is nucleic
acid molecule or they may also contain
along with that they also contain
enzymes not found in all viruses but in
very few cases they
enzymes present but most of the time
they only have DNA or RNA in the core
okay so viruses we are no resemblance to
since they don't have any protein
synthesizing machinery at all that's why
they are totally different
compared to any living cell they don't
have that their idea of protein
synthesis so they are not involved in
the physiological properties of a cell
they don't follow those rules so how
they survive and how they replicate how
they make their more copies they can
only make more copies when they attack
their host once they are attached with
their host they hijack the protein
synthesis machinery of their host and
then they utilizes the hosts enzymes for
the protein synthesis okay general
structure so we know the the basic
composition of a viral structure now if
you think about every single component
like capsids all viruses have cap seeds
as I told you earlier protein code that
enclose all the and protect the nucleic
acid of the virus and the capsid
together with the nucleic acid is known
as nucleo capsid and as you can see in
both this pictures this is the nucleic
acid you can see either it can be DNA or
RNA it doesn't matter but this is the
capsid this desire cos'Ăš Hedren
structure is the cap seen both occasion
but in some occasion maybe this
icosahedron is the last layer there is
no other envelope so it is naked virus
so nuclear naked nucleo capsid virus and
some other occasion envelope the virus
where we have the cap seed and it is
covered by the envelope and they envelop
may or may not carry spikes okay for few
viruses the envelope protrude some
spikes out and some other viruses there
are no spike instead of the spike there
is the very other protein components
that are present in the envelope okay
but generally this spike help them to
interact with their host cell surface
and and adherence and start to get in
guard and inserted into the host cell
Anini each capsid is made of identical
protein subunits known as caps so
meanors okay so the cap summers are the
protein subunits which are linked with
one another stitched to one another to
form the icosahedron structure now there
are 2 structural capsid types available
helical and icosahedron two different
capsid types that are present now the
helical and icosahedral the difference
between the two if you look at here that
the helical are tobacco mosaic virus one
example and icosahedral is the one that
we studied here okay
folio virus that we that we discussed
earlier poliovirus herpes simplex virus
parvo virus those one icosahedral
contains 20 side with 12 corners so it's
a complex geometrical shape where
helical structure is just made up with
protein subunits they are present in
stack of one another like a helical coil
inside of which the genetic material is
present okay
and there are complex a virus like
bacterial phase which contains the
capsid that is a protein seed the
protein head and tail and there's a
spike and the attachment plate all these
things are present and inside the capsid
head the genetic material is present now
if you think about the viral envelop we
have talked about the cap seed you know
capsid is a must for a virus because it
is the primary protective envelope
primary protective structure for a virus
but in some viruses there are enveloped
external membrane that is present
outside so most of the animal viruses
have this envelope surrounding the
capsid layer itself so ackward when the
virus leaves the host cell generally
this capsid is formed due to the
modified version of the same membrane of
the host itself so let's assume that
this is the host cell and if this is
let's say the virus and the virus is
taking a part of the vesicle out
and once they bulge out from the host a
portion of the cell membrane from the
host cell is captured and that becomes
the envelop and that envelope as we know
the host cell surface contains many
receptors and other protein molecules so
that's why that that envelope of the
virus contains many of those cell
surface protein structures known as the
spikes okay so made of how many
identical put protein subunits it
protects the genetic material may be
involved in cell entry as well as say
Lexy because you know once they have
this envelope now next time when this
virus is going to attack another host
the fusion becomes lot easier because
now the host cell surface is a cell
membrane made up with phospholipid
membrane now this envelope is also made
up with similar phospholipid component
so they can interact quite well and the
fusion can be facilitated even faster
okay now this is another thing envelop
as I said that located outside of the
capsid and as I said it is a coil during
the viral maturation so you think about
this picture you can see that this is
the genetic material inside capsid
preventing the genetic material okay
composed of many protein subunit known
as cap Samir's and the envelope outside
may or may not contain spike but they
are mostly made during the maturation
that means while the virus is going out
the virus is coming out of the host okay
and it is fascinating facilitating this
virus to attack another healthy host
okay and the spikes help this virus to
specifically bind to their target tissue
and infect it
now what are the functions of capsid
envelope
I just penchant about it the capsid
protects the nucleic acid material from
an external damage okay and the envelop
helps the virus to fuse with another
healthy host and also it has some role
in tissue specificity if the envelope
contains spike now the complex viruses
complex virus is like bacteriophage
or poxviruses why we call them complex
virus particularly if you look at
bacteria fat this is vector alpha why we
call it complex because these virus they
don't have any particular geometrical
shape they have icosahedral head they
have a very thin collar and they have a
helical sheath and they also have a base
plate tail fibers are attached to it so
it's it's very complex with multiple
components okay
let's we call them complex virus because
they have multiple components all
together while pox virus lacks the
typical capsid and are covered by a
dense layer of lipoprotein only think
about it instead of having those complex
proteins of Venus known as the caps
amoenus they are only covered by dense
layer of aleppo proteins so there is a
lipoprotein layer only for pox virus
now it's all about we've talked about
that envelope and cap seed but what
about the core the core part of a virus
which definitely is required without
this part the virus is not virus itself
is the nucleic acid the makeup and then
the makeup of the nature of virus itself
viral genome can be either DNA or RNA
but can never be both it can be either
DNA or RNA unlike animals and plants I
know we have DNA RNA both genetic
material is the DNA but RNA is also
found to carry the message but in virus
they have either DNA or RNA not the both
they carries genes necessary to invade
the host cell and then simply they
redirect cells activity to make new
viruses so the virus's job is not to do
everything on its own that's why virus
is very simple and very lightweight and
very small because they don't need to
carry proteins inside the cell
they have protein coat outside but they
don't need to carry enzymes because what
they will do
once the infect and attack a host they
will hijack the host machinery of
protein synthesis and they will produce
the necessary proteins they need to get
multiplied and to get spread number of
genes varies for each type of virus few
to hundreds okay now if we compare
between two different type of virus DNA
viruses usually double-stranded but
maybe single-stranded now the DNA can be
either circular or linear we will talk
about the example letter RNA viruses
obviously are single-stranded may be
double-stranded may be segmented into
separated RNA pieces single stranded RNA
geebo genome ready for immediate
translation are positive sense RNA they
are known as positive sense
the single son Darren is immediately
ready for the translation process and
single-stranded RNA that must be
converted into a proper form unknown as
negative since RNA because you know when
we studied the transcription we've
talked about the same strand of the RNA
antisense strand of the RNA right coding
strand non-coding strand but in this
case this idea is little different in
case of virus and viral infection virus
is carrying RNA if that RNA is
completely ready to be translated we
call it a positively sense or positive
sense RNA but if the RNA needs to be
converted that means you know if that
RNA acts as a template to make another
RNA and that another RNA is translated
into proteins then the earlier RNA is
known as negative sense that means
positive sense RNA can only be converted
into proteins negative sense RNA needs
to be converted into positive sense RNA
first and then convert it into proteins
another very important thing is host
range and specificity you can see this
picture animation in the left hand side
here host range and specificity is
another very important property of virus
virus has its specific host or range it
can be narrow or it can be broad
generally virus host range is narrower
than a bacterial host range because they
can only attack a particular type of
host this is known as tissue tropism
like you know in case of plants we know
gravitropism phototropism so similarly
for virus is we have tissue drop ISM
that means a virus one particular virus
hepatitis B infects liver
so it is specific to infant liver cells
so even if you try and check whether
this virus infect other part of the cell
you you try it with a cell line if you
provide liver cell then only will see
the virus to be thriving okay
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