VIRO 2

Created by

Yesu Gaveril

Cards (20)

CENTRAL DOGMA OF MOLECULAR BIOLOGY
● Major processes:
○ Replication
○ Transcription
○ Translation
Replication 
process of duplicating DNA using a DNA template
In humans, not all genetic material is replicating because they are not all actively-dividing cells
Replication only happens in actively dividing cells
Actively-dividing cells are those found in the skin, mucosa, hair, and bone marrow
Specialized cells such as brain cells and heart cells do not actively multiply and replicate
Transcription 
process of making mRNA from a DNA template for protein synthesis
Translation 
process of synthesizing proteins from the sequence of nucleotides of mRNA, The ribosome will read the sequence of mRNA to make proteins
Both transcription and translation happens in all cells due to proteins being essential to life
VIRAL REPLICATION
Viral genomes must make messenger RNA (mRNA) that can be read by host ribosomes
All viruses on this planet follow this rule; there is no exception
mRNA is the only thing that ribosomes can read
Whatever the genome of the virus is, it has to make mRNA
New proteins necessary to make new virions can only be synthesized through mRNA
DOUBLE-STRANDED DNA (dsDNA) VIRUSES
Already looks like our own DNA, making infection easier
dsDNA is transcribed to mRNA for protein production
dsDNA is also replicated to make new dsDNA to increase copies of itself
Some smaller viruses (Polyomaviridae and Papillomaviridae) use the DNA Polymerase of the host cell since they do not have genes to make their own
Larger viruses (Poxviridae, Adenoviridae, Herpesviridae) have DNA polymerase genes in their genomes
SINGLE-STRANDED DNA (ssDNA) VIRUSES (GROUP 2)
Can have (+) or (-) charge
First converted to dsDNA
Remember that the only molecule that can make mRNA is dsDNA
We cannot make mRNA from a single-strand
Single-stranded is converted to a double-stranded to make mRNA
Transcribed to make mRNA
mRNA is then used for translation of viral proteins
dsDNA is replicated to single-stranded products (either +/–), which are then packaged into the particles
ex. parvoviridae
THE BALTIMORE CLASSIFICATION SYSTEM
dsDNA - (+/-) can yield a (+) mRNA
ssDNA - will undergo processes to be dsDNA in order to become mRNA
dsRNA - (+/-) can yield a (+) mRNA
(+) ssRNA - needs to undergo processes to be (-) ssRNA to turn into a (+) mRNA
(-) ssRNA - simply converted to (+) mRNA
(+) ssRNA with DNA intermediate - is first converted to ssDNA, which turns into a dsDNA, which then yields the (+) mRNA
dsDNA with an RNA intermediate - gapped DNA viruses
GAPPED DNA VIRUSES (Group 7)
Has both double-stranded and single-stranded portions, small area of RNA and an attached protein at the 3’ end
Genome cannot be copied to mRNA due to having single-stranded parts
First, it has to be repaired to remove the gap.
dsDNA → mRNA → proteins
To make new viral dsDNA:
mRNA is converted to (-) ssDNA via RT found only in viruses.
(-) ssDNA is used as the template to reconstruct the whole gapped genome.
cell enzymes 
convert the whole genome to dsDNA and remove the protein and RNA.
Reverse transcriptase (RT) 
a DNA polymerase enzyme that transcribes single-stranded RNA into DNA
In the Baltimore scheme, gapped DNA viruses are the dsDNA viruses with an RNA intermediate.
RNA VIRUSES
Only viruses have RNA genomes, all other living things have DNA genomes
Cells cannot copy an RNA molecule into another RNA molecule (cells can only do DNA-to-DNA) because they do not have RNA-dependent RNA polymerase (RdRp)
Because of this, RNA viruses encode RdRp in their genomes or carry it in the virus particles
RdRp can produce RNA replicates and mRNA from RNA templates
RNA GENOMES
Only mRNAs can be read by ribosomes to make proteins
(+) stranded RNAs - mRNAs that are read from the 5’-3’
This is the strand that is read by ribosomes
(-) stranded RNAs - RNAs that are complementary to the mRNA sequence
Does not carry significant information because what the ribosomes read is the (+) strand
RNA viruses carry (+) ssRNA or (-) ssRNA or dsRNA
dsRNA VIRUSES (group 3)
dsDNA viruses contain both (+) and (-) strands
Ribosomes cannot translate the RNA even if it has the (+) strand in it
Hence, an mRNA must be made from the dsRNA through the viral polymerase (this enzyme is carried inside the virion itself)
This mRNA is then translated to protein to make new virions
To replicate this genome, some of mRNAs are copied to make new dsRNA genomes
ex. reoviridae
(+) ssRNA VIRUSES (group 4)
(+) ssRNA can be readily translated by the ribosomes to make more proteins
To make more genomes, the (+) strand is copied to a (-) strand which is used as a template to make more (+) strands. This is done by a viral polymerase that has been translated from the RNA genome (the virions do not carry polymerase enzymes inside them because viruses with (+) ssRNA can be readily translated, unlike viruses with dsRNA).
ex. picornaviridae, astroviridae, caliciviridae, togaviridae, flaviviridae, coronaviridae
(+) ssRNA VIRUSES with DNA Intermediate (group 6)
(+) ssRNA Viruses can be translated but this does not happen
The (+) ssRNA is converted into a (-) DNA through reverse transcriptase
(-) DNA is converted to a dsDNA
mRNA is transcribed from dsDNA and used for translation to proteins and reproduction of more viral genomes.
ex. retroviridae
(-) ssRNA VIRUSES (group 5)
(-) RNA cannot be translated, and the cell cannot make mRNA from it
RNA viruses carry polymerases in the virus particle to make (+) RNA
The (+) RNA can be used as mRNA to make proteins. It can also be used as a template to make more (-) RNAs for new virions
ex. orthomyxoviridae, rhabdoviridae, paramyxoviridae, filoviridae, arenaviridae
virus examples (all -viridae)
dsDNA - pox, polyoma, papilloma, adeno, herpes
ssDNA - parvo
dsRNA - reo
(+) ssRNA - picorna, astro, calici, toga, flavi, corona
(-) ssRNA - orthomyxo, rhabdo, paramyxo, filo, arena
(+) ssRNA w/ DNA intermediate - retro
dsDNA w/ RNA intermediate - hepadna