8/6/2019 ClinMicroMRS - Virology Notes

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VIRUSES

Viruses = genetic material (DNA or RNA, not both) enclosed in a protein coat

(capsid) + (some only) lipid bilayer surrounding capsid

NO organelles or ribosomes, but some have structural proteins and enzymes inside

capsid

Viruses only have the instructions to make copies of themselves, they don't

have the enzymes themselves, so they need to hijack other cells

CLASSIFICATION OF VIRUSES

Either DNA or RNA.

The genome can also be single stranded, double stranded, linear, looped,

in separate segments or in one cts strand.

RNA viruses

These can be positive stranded (RNA is like mRNA: so when these ones enter

a cell, it can immediately start translation using host ribosomes into proteins)

Or can be negative stranded - they can't be translated immediately, so they

need to be transcribed into + strand RNA. So they carry in their capsid the enzyme

RNA dependant RNA polymerase which converts the - strand to + strand.

They need to carry it bc human cells don't have this enzyme.

They can also be retroviruses - the RNA of retroviruses is transcribed too, but in

a reverse fashion (retrograde) into RNA! They carry reverse transcriptase in order

to do this.

DNA viruses

DNA can't be translated directly into proteins - it has to be transcribed into mRNA

first, and then the mRNA needs to be translated into structural proteins and

enzymes.

Every DNA virus has a positive strand and a negative strand. The positive strand

is the one that is read, the -ve one is ignored.

Capsids

There are two types of capsid, icosahedral and helical.

Icosahedral 

Take 1+ polypeptide chains and form a ball, this is called a capsomer.

Arrange the copsomers into an equilateral triangle.

Put 20 triangles together to form an icosahedron, the put your nucleic acid inside

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Helical 

The capsomers are now bound to RNA (ONLY RNA, bc only RNA is helical).

This is then coiled into a helical capsid, mostly spherical shaped, except for

rhabdoviruses which have bullet shaped capsids.

Membranes - which are called envelopes

Where do viruses get their lipid bi-layer membrane from? When they bud

through the host cell nucleur or cytoplasmic membrane, they tear a piece of the

membrane off with them and this is what forms their membrane. Though only

some viruses have this.

Viruses with no membranes are referred to as NAKED, or non-ENVELOPED.

Without membranes are referred to as ENVELOPED.

How are viruses classified?

1) Nucleic acid type:

-DNA, RNA

-ss, ds

-single or segmented pieces of NA

-positive or negative stranded RNA

-complexity of genome

2) Capsid

-icosahedral or helical

3) Envelope

-naked or enveloped

4)SIZE

-helical capsid: their diameter determines their size

-icosahedral capsid: the number of capsomers determines their size

The different types of virus families