Baltimore Scheme- Viral Classification System
Summary
TLDRThis video from the Brain Boost channel delves into the Baltimore scheme of viral genome classification, covering seven distinct groups. It explains how each group, from double-stranded DNA to plus single-stranded RNA with a DNA intermediate, utilizes host machinery or encodes its own polymerases to produce mRNA and replicate their genomes. The explanation highlights the strategies viruses employ to infect new hosts, emphasizing the importance of mRNA in viral protein synthesis and genome replication.
Takeaways
- 🌟 The Baltimore scheme classifies viral genomes into seven groups based on their genetic material and replication strategies.
- 🧬 Group 1: Double-stranded DNA viruses can use the host's DNA-dependent RNA polymerase to produce mRNA for viral protein synthesis.
- 🌀 Group 2: Single-stranded DNA viruses require a DNA synthesis step to form double-stranded DNA before mRNA production.
- 📚 Group 3: Double-stranded RNA viruses transcribe one of their RNA strands into a positive strand that serves as mRNA and also as a template for the negative strand.
- 📖 Group 4: Positive single-stranded RNA viruses are very infectious as their genome can be directly translated into proteins by the host's ribosomes, exemplified by Coronaviridae.
- ⏪ Group 5: Negative single-stranded RNA viruses must first be copied to a positive strand to serve as mRNA, using their virally encoded RNA-dependent RNA polymerase (RdRP).
- 🔄 Group 6: Positive single-stranded RNA viruses with a double-stranded DNA intermediate use reverse transcriptase to form DNA from their RNA genome, which then integrates into the host genome.
- 🔄 Group 7: Gapped double-stranded DNA viruses need to fill the gap in their DNA before they can proceed with mRNA production and genome replication.
- 🛠️ All viruses aim to express their genes as functional mRNA to direct the host's translational machinery to synthesize viral proteins and replicate their genomes for new infections.
- 🔑 The host's translational machinery can only translate mRNA, which is why all viral groups in the Baltimore scheme eventually produce mRNA as an intermediate step.
- 🔍 The purpose of viral replication strategies is to efficiently produce mRNA for protein synthesis and duplicate the viral genome for packaging into new virions to infect additional hosts.
Q & A
What is the Baltimore scheme of viral genome classification?
-The Baltimore scheme is a system for classifying viruses based on the type of nucleic acid they contain and their method of replication. It is named after David Baltimore and categorizes viruses into seven groups.
What are the seven groups in the Baltimore scheme?
-The seven groups are: 1) Double-stranded DNA, 2) Single-stranded DNA, 3) Double-stranded RNA, 4) Positive single-stranded RNA, 5) Negative single-stranded RNA, 6) Positive single-stranded RNA with a double-stranded DNA intermediate, and 7) Gapped double-stranded DNA.
Why is mRNA important in the context of viral replication?
-MRNA is important because it is the form of genetic material that can be directly translated by the host's ribosomes to produce viral proteins, which is a necessary step for viral replication and infection.
What is the role of the host's translational machinery in viral replication?
-The host's translational machinery is used by viruses to translate mRNA into viral proteins, which is essential for the production of new viral particles.
How do DNA viruses with limited coding capacity produce mRNA?
-DNA viruses with limited coding capacity rely on the host's DNA-dependent RNA polymerase to transcribe their DNA into mRNA.
What is the difference between positive and negative strands in the context of RNA and DNA?
-In RNA and DNA, the positive strand is the one that can be directly translated into proteins or used as a template for replication. The negative strand is the complement of the positive strand and must be converted into a positive strand to serve these functions.
Why do RNA viruses need to encode their own polymerases?
-RNA viruses need to encode their own polymerases because cells do not have RNA-dependent RNA polymerases, which are necessary for replicating the viral RNA genome or producing mRNA from an RNA template.
What is the purpose of reverse transcriptase in the replication of certain RNA viruses?
-Reverse transcriptase, also known as RNA-dependent DNA polymerase, is used by certain RNA viruses to convert their RNA genome into a DNA intermediate, which can then be integrated into the host genome or used for further replication.
How does the process of replication differ between positive and negative single-stranded RNA viruses?
-Positive single-stranded RNA viruses can be directly translated into proteins and use their genome as a template for replication. Negative single-stranded RNA viruses must first be transcribed into a positive strand to serve as mRNA and then use this positive strand as a template for replication.
What is the significance of the term 'gapped double-stranded DNA' in the context of viral genomes?
-Gapped double-stranded DNA refers to a viral genome that has a break or gap in one of the DNA strands. This gap must be filled before the DNA can be used as a template for replication or transcription into mRNA.
Can you provide an example of a virus that belongs to Group 4 of the Baltimore scheme?
-An example of a virus in Group 4 is the coronavirus, which has a positive single-stranded RNA genome that can be directly translated into proteins by the host's ribosomes.
Outlines
🌟 Introduction to the Baltimore Scheme
This paragraph introduces the Baltimore scheme for viral genome classification, which categorizes viruses into seven groups based on their genome types. The speaker provides an overview of the groups, which include double-stranded DNA, single-stranded DNA, double-stranded RNA, plus single-stranded RNA, negative single-stranded RNA, plus single-stranded RNA with a double-stranded DNA intermediate, and gapped double-stranded DNA. The importance of mRNA in the translation process is emphasized, as all viral genomes must be expressed as functional mRNA to direct the host's translational machinery to synthesize viral proteins. The ultimate goal of viruses is to replicate their genomes and package them into virions for infection of new hosts.
🧬 DNA Viral Genomes: Groups One, Two, and Seven
This section delves into the specifics of DNA viral genomes, which include groups one (double-stranded DNA), two (single-stranded DNA), and seven (gapped double-stranded DNA). The speaker explains that double-stranded DNA viruses can use the host's DNA-dependent RNA polymerase to produce mRNA. Single-stranded DNA viruses must first convert to double-stranded DNA before mRNA production, while gapped double-stranded DNA viruses need to fill in the gap to form a complete DNA molecule. The process of genome replication and the role of reverse transcriptase in converting RNA back to DNA are also discussed, with examples provided to illustrate these mechanisms.
📚 RNA Viral Genomes: Groups Three, Four, Five, and Six
The speaker transitions to RNA viral genomes, highlighting the differences between DNA and RNA viruses. RNA viruses cannot rely on the host's polymerase due to the lack of RNA-dependent RNA polymerase in cells. Instead, they must encode their own polymerases. The discussion covers group three (double-stranded RNA), where one strand is transcribed to produce mRNA and the other to duplicate the genome. Group four (positive single-stranded RNA) is particularly infectious as it can be directly translated into proteins. Group five (negative single-stranded RNA) requires the production of a positive strand to serve as mRNA. Finally, group six involves reverse transcriptase to convert the RNA genome into a double-stranded DNA intermediate, which then integrates into the host genome for mRNA production.
🔬 Conclusion of the Baltimore Scheme Overview
In conclusion, the speaker summarizes the Baltimore scheme, emphasizing the diversity of strategies employed by viruses to replicate their genomes and produce mRNA. The importance of understanding these mechanisms for virology studies is highlighted, and the speaker encourages viewers to subscribe, like, and comment with questions or topic suggestions for future videos. This paragraph wraps up the educational content, providing a comprehensive review of the Baltimore classification system and its relevance to the study of viral genomes.
Mindmap
Keywords
💡Baltimore Scheme
💡Double-Stranded DNA
💡Single-Stranded DNA
💡Double-Stranded RNA
💡Positive Single-Stranded RNA
💡Negative Single-Stranded RNA
💡Plus Single-Stranded RNA with Double-Stranded DNA Intermediate
💡Gapped Double-Stranded DNA
💡RNA Polymerase
💡Reverse Transcriptase
💡Encapsidation
Highlights
Introduction to the Baltimore scheme of viral genome classification.
Overview of seven groups of viral genomes and their unique characteristics.
Explanation of the importance of mRNA in the host's translational machinery.
Clarification of the terms 'positive strand' and 'negative strand' in the context of RNA and DNA.
The purpose of viruses to make copies of their genome and package them into virions.
Different strategies used by DNA and RNA viruses to produce mRNA and replicate their genomes.
Group 1: Double-stranded DNA viruses and their reliance on the host's polymerase.
Group 2: Single-stranded DNA viruses and the process of converting to double-stranded DNA for mRNA production.
Group 7: Gapped double-stranded DNA viruses and the necessity to fill gaps before mRNA generation.
The role of reverse transcriptase in RNA to DNA conversion for certain viral genomes.
Group 3: Double-stranded RNA viruses and their method of mRNA production and genome replication.
Group 4: Positive single-stranded RNA viruses and their direct translation into proteins by the host.
Group 5: Negative single-stranded RNA viruses and the need for initial copying to produce mRNA.
Group 6: Positive single-stranded RNA viruses with a double-stranded DNA intermediate.
The use of reverse transcriptase in Group 6 to convert RNA genomes back to DNA.
Summary of the Baltimore scheme and its significance in understanding viral genome strategies.
Transcripts
hello everyone and welcome to the brain
boost
channel so today we're going to be
looking at the baltimore scheme
of viral genome classification so let's
just
jump right into the video
okay so this is just an overview of what
we're going to be looking at today
so here we have seven groups that we're
going to start
tackling individually in a bit i just
want to give a quick
overview and a brief explanation of each
one
and a few points that we should have an
understanding of before we get into the
depth
so here we have group one which is
double-stranded dna
group 2 which is single-stranded dna
we have group 3 which is double-stranded
rna
group 4 which is plus single-stranded
rna
and if you notice something that
that's basically just mrna
we have group 5 which is negative single
stranded rna
we have group 6 which is plus single
stranded rna
with a double-stranded dna intermediate
and we have group seven which is gapped
double-stranded dna
so basically something that we should
keep in mind is that
the host's translational machinery is
it can only translate mrna right
so m this is why
all of these groups in this schematic
this is why they all point
down towards this boxed mrna because
they all have to get here
so mrna is technically defined as
the positive strand because that's what
is
translated by ribosomes and to make
a positive stranded rna from dna
that has to be negative so when we're
looking at rna or dna
the complement of the plus strand is
called the negative strand
and vice versa so
that means that plus strand plus
single stranded rna is
technically just mrna they're the same
thing which is what i mentioned
about group four here and so
uh basically before we get into all the
details about everything
the purpose of the virus is to
basically make copies of its genome
and package these genomes into virions
so that
they can go on and infect new hosts
so this means that all viruses have to
make or express their genes as a
functional mrna
right so that they can direct
you know host translational machinery to
synthesize
viral proteins which is the end product
here at the bottom
we want to get to mrna to make viral
proteins and we want to
duplicate our genome so that we can get
that packaged
and sent off to start infecting new
hosts
so let's just start with
the dna viral genomes so that's groups
one two and seven
um and before we get into the dna
genomes i just want you guys to remember
that when we're looking at viral dna
genomes
all dna viruses have to go to
double-stranded dna so group one
is already double-stranded dna so that
can be
left as is before we get to the mrna
group two is single stranded dna so it
has to become
double stranded first before we can
start working with it and messing around
and the same goes for group seven it's
gapped double-stranded dna so
we have to fill that gap in first before
we have a functional
double-stranded dna that we can mess
around with to get to mrna
so we're going to start with group one
which is our double stranded
dna genome and we're only really going
to focus on
figure a all the other figures here are
just
images of other examples that have this
genome type but we're only looking at
figure a
for the most part so in this case
of double-stranded dna there is a
limited coding capacity here so
you know some smaller genomes they don't
necessarily have space to encode
a polymerase so they're going to depend
or the virus is depending on
the hosts polymerase to produce
the mrna and to do all the work for it
so
in this case when we're starting with
double stranded dna and we're going to
produce an mrna
we're relying on the host's dna
dependent rna polymerase
this is kind of a little bit of a trick
you're going to see it
pop up later in the video as well but
essentially the name of the polymerase
is kind of based
on what your starting point is and what
you're going to so we're starting with
double-stranded dna and we're trying to
make
rna we're going from dna to rna so this
polymerase is dna dependent rna
polymerase
so that's kind of how the naming works
here and we'll get into that a bit later
but that's how we get to the mrna and
that's going to be our viral protein
like we mentioned
earlier the other objective we said of a
virus
in this case is to duplicate or
replicate
its genome so that that duplication
could be sent off
for encapsidation and infection of new
hosts
because the virus just wants to infect
and spread and work fast right
so that's what it's doing that's what
it's coming to do make an mrna
to become viral protein and then uh
[Music]
replicate its genome for encapsulation
so if we look at figure b
here polyomaviridae this
genome is very small so as you can see
it doesn't really have it has limited
coding capacity here
it can't encode its own polymerase which
is why it's going to rely on the host
another thing i want you to keep in mind
is these are dna genomes that we're
looking at right now
so that's another reason why they can
rely on the host
because host machinery is already built
to work with dna
when we look at rna genomes a bit later
you'll notice that they do
a whole different thing they use their
own polymerases because
they can't rely on the host because rna
is very foreign to the host
so this is group two this is
single-stranded
dna genome uh so for this example
we are going to once again look at
figure a and
we start off with a single strand um and
we need dna synthesis to get to our
double-stranded dna
before we can produce an mrna right so
once again we're exploiting the hosts
dna polymerase
dna-dependent rna polymerase to get to
the mrna
production which is going to be go off
and
become the viral protein in the after
future steps
and then this double-stranded dna is the
template
for the duplication of the genome to get
back to our single
strand and that's going to be sent off
for encapsidation
so group 2 is relatively simple here
moving on to our final group
for the dna viral genomes this is group
seven
gapped double stranded dna so when we're
starting with
gapped double-stranded dna this gap must
first be filled so that we can generate
a completed
double-stranded dna molecule from which
we can generate an mrna strand
so that's the first step here we're
trying to get this gap filled and that
is done
in the nucleus so
now we have a fully formed
double-stranded dna and from there we
can generate our mrna which we've seen
before and that's going to be the path
going rightwards
so the boxed strand that's our mrna
which is going to go on to be the viral
protein and
you know keep going on from there so
then
going leftwards this is where we're
duplicating our genome so
from um double-stranded dna
um it makes an mr an rna that's
basically like the mrna
because it's plus single stranded rna
and this is going to encode a reverse
transcriptase
which is going to take this rna as a
template and make
the dna strand so it is reverse
transcriptase that's going to get us
back to the gapped double stranded
dna molecule from the rna
so if you recall a little bit earlier in
the video i mentioned
that we keep up with names of the
polymerases based on what your starting
point is and what the end point is
so we said dna dependent rna polymerase
was us going from dna to rna
so here actually when we're duplicating
our genome we're going from rna to dna
and we're using reverse transcriptase
but another name
for the reverse transcriptase is the
rna-dependent
dna polymerase so hopefully that makes
sense
okay so now that we've done the dna
genomes we're going to move on to the
rna genomes
so just a quick refresher we said that
dna genome viral dna genomes
exploit the host polymerase in order to
you know duplicate their genome and to
produce an
mrna from a dna template
rna viral genomes cannot necessarily
do this and this is because cells have
no rna-dependent rna polymerase
so to replicate their genome or
to make an mrna from an rna template
because the host doesn't have anything
that goes from rna to rna
whereas in dna genomes we the host
is you know built to work with dna so it
could obviously make
an mrna from a dna template so in this
case with rna since it can't do that it
needs to
encode its own polymerase all rna
viruses have to encode their own
polymerases
which also means that they need space in
their genomes in order to do this
so they do all of this through two
strategies
the first one is the rna dependent rna
polymerase
also known as the rdrp so this is
virally encoded
and i want you to recall that little
rule
i i mentioned earlier where i said
basically the naming of the polymerase
is it's easier to keep track
of the name and where it comes into play
based on what our template is and what
we're trying to go to
so if we're going from rna to make an
rna
we're using the rdrp if we were going
from
dna like in the dna genomes to make an
mrna
we're using a dna dependent rna
polymerase okay so now let's look at the
second
strategy the retro reverse transcriptase
known as the rt so we just looked at
reverse transcriptase
um in action with the gapped
double-stranded dna
and if you notice it was used when we
were going from an
rna to dna so
you can kind of guess what another name
for reverse transcriptase is
and that is rna-dependent dna polymerase
because it goes from rna to dna like in
the case of the gapped double-stranded
dna okay so
let's move on to our first rna
viral genome group and this is group
three this is the
double stranded rna viral genome
so we're only really going to pay
attention to the left side
figure a that's all we really need to
focus on here so
we start off with a double stranded rna
genome
and what happened is what happens here
is that we get
a transcription of one of the rna
strands
of the double-stranded rna genome to
single-stranded plus
rna which is our mrna right
the boxed green rna strand that is
always going to be our mrna if you've
noticed throughout this video
and that is going to go on to make our
viral protein or our virion
so in addition to this this
single stranded plus rna can also act as
a template
to make the negative rna strand
so for that strand synthesis that's
going to basically convert us back
to our double-stranded rna genome so
we've duplicated our genome now
and that is for that's going to go up
for packaging so one thing i want you to
know
is when we're using this plus single
stranded
rna as our um
template for the negative strand that
negative strand intermediate
is called our anti-genome
okay moving on to group four
this is the positive single stranded rna
genome so with this one
the positive rna genome is because it's
already single stranded it's basically
an mrna so it can be
translated directly into proteins by the
hosts
ribosomes so the genome of a positive
single-stranded rna virus
is actually very infectious when um
introduced into the host
so i want you to look at figure b there
there's an example and it says corona
viridae
that might sound a bit familiar to those
watching this through the pandemic
but so this is technically very
infectious because
it can be translated directly into
proteins
so as you can see our starting group
our starting product there it's already
boxed that genome because it's basically
mrna
that can go off and be our viral protein
immediately
um so also what happens here when we
want to duplicate
is that the negative rna strand is
generated
by the virally encoded rdrp
and then copied back into the plus
strand so that can go off for
encapsidation
moving on to group five this is the
negative single-stranded rna genome
so this is um negative
so what happens is that it must first be
copied to make
the positive strand so that we can get
our mrna
so that is the boxed one that's mrna
which is once again going to go off
to that's going to be our virion or the
viral protein
and the other objective we said of the
virus is to
duplicate the genome so
uh basically this um genome
is making it's the template to make our
positive strand which we've done
and that can be in turn copied to make
our negative strand
so we've duplicated it and that is sent
off for encapsidation
so in this case we have the rdrp working
because we're going from rna
template to an mrna when we were
starting off so the rdrp is working here
because we're going from rna to
rna we're not switching back and forth
between rna and dna
okay moving on to our last
rna genome type this is group six
this is the plus single-stranded rna
with a double-stranded
dna intermediate so
basically um what we're dealing with
here is we're starting with rna
and we're going to dna so the polymerase
that we're using
is going to be reverse transcriptase
it's the rna-dependent dna polymerase
because we're going from rna
going back to dna so reverse
transcriptase does that
so basically um the rna genome
our single strand is converted to double
stranded dna
so it's converted into this
double-stranded dna intermediate through
the retro
the reverse transcriptase and this
uh double-stranded dna is going to
become
integrated into the host genome
and this is going to serve as the
template to then produce the mrna which
we've seen before we know that
double-stranded dna we can get an mrna
from that
so that's going to go off to be our
viral protein and then
we can produce an a single stranded rna
which is
basically the same thing as mrna they're
the same thing
and this is to duplicate the original
genome because we want to always get
back to what the original was
so we duplicate that and that's going to
be for sent off for encapsidation
okay so that is the end of this video
um we've gone through groups one through
seven of the baltimore scheme
and their details and the different
types of
is used so hopefully that covers
all that you need for this topic in your
viral
vibrology classes so be sure to
subscribe to this channel and like this
video
and comment if you have any questions
concerning this material
also feel free to comment with any other
topics you would like brain boost to
make a video on and
we will have that up for you
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