Virus genetics

Viruses can have RNA, double-stranded (ds) RNA, DNA, or dsDNA.  The virus genome is usually a single linear or circular molecule of nucleic acid.  The number of genes in a virus ranges from 4 to several 100.

Types of Virus ds genome replication

          1. DNA converted to DNA
              uses host's DNA polymerase while destroying host DNA
          2. RNA converted to RNA (to RNA)
              contain RNA replicase gene
          3. RNA to DNA to RNA
              contain reverse transcripterase gene

Animal Viruses (minus strands and plus strands)

Most animal viruses contain RNA.  Class 1 RNA viruses have a minus strand of RNA that is a template for the plus strand.  A Class 2 RNA virus has a plus strand of RNA codes for proteins.  A Class 3 RNA virus is a reovirus. It is comprised of double stranded RNA genomes.  A Class 4 RNA virus is a reovirus with a single plus strand.  A Class 5 RNA virus is a reoviurs with a single minus strand.  Class 6 RNA viruses are called retroviruses, single plus strand genome with template for DNA synthesis.

Lytic viruses	vs	Lysogenic viruses
host has immediate symptoms of invasion immediate host degradation 1. sicks to receptor sites 2. viral DNA inserted when capsid breaks through the membrane (and cell wall, if applicable) 3. capsid left outside; cell's (host's) DNA is hydrolyzed 4. viral genome directs the host's metabolic machinery to create more viruses 5. enzyme (lysosyme) made to break the cell wall (if applicable) causing the cell to lyse or burst releasing the many replicate viruses		incorporates its DNA into that of the host and "hibernates" called a temperate virus Bacterial defense against viral infections 1. mutation 2. restriction enzymes cut up prophages (invaded DNA)

An Emerging Virus is one derived from another virus by mutations, spreading between species.  The host range is defined by the "lock and key" fit between proteins of virus and bacterial receptors.

Why humans are currently more susceptible to viruses/disease?
     intraveneous drug abuse
     promiscuity
     travel
     rainforests
     blood transfusions

Viral Impacts on Invaded Cells

    A.1. toxins released
A.    2. toxins produced

    B.1. damage to cell
B.    2. death of cell

    Lysis initiates fever.

    C.1. hydrolytic enzymes released from lysosomes

Phages

ds DNA viruses can reproduce by 2 alternative mechanisms: 1. Lytic Cycle and 2. Lysogenic cycle to Lytic cycle

ONE	TWO
sticks injects hydrolyzes assembly lysis	sticks injects inserts prophage bacterial reproduction prophage exits and circularizes assembly lysis

Provirus

     animal viruses with envelopes (reproduction)

Steps:

1. glycoproteins bond to each other; viral envelope fuses with cell membrane
2. capsid and genome enter the cell
3. cellular enzymes remove capsid
4. viral genome is replicated
5. copies transcribed into mRNA
6. mRNA translated into capsid and glycoproteins
7. vesicles take these to cell membrane
8. capsids assemble around viral genome copies
9. virus buds from the cell's plasma membrane

     animal viruses with RNA genomes (HIV--2 identical RNA molecules--reproduction)

Steps:

1. virus fuses with membrane
2. reverse transcripterase makes DNA complement
3. new DNA strand serves as template for the other DNA strand
4. 2 stranded DNA incorporated into cell's DNA as a proviurs
5. proviral genes transcribed into mRNA
6. mRNA translated into HIV proteins (provirus also codes for the viral genomes of the next generation)
7. capsid assembly
8. budding from cell

Polio attacks nerves; cold attacks epithelium of respiratory tract; immune system cannot differentiate between cowpox and smallpox; antiviral drugs interfere with genome synthesis (although new HIV drugs do not let the virus into the cell at all).

Viruses and Cancer

The virus responsible for hepatitis B causes liver cancer in individuals with chronic hepatitis.  The Epstein-barr virus (herpesvirus causing mononucleosis) is linked to cancer prevalent in Africa; Papillom with cervical cancer; retroviruses with leukemia.

Oncogenes

Viral genes can be directly involved in triggering cancerous characteristics.  They are also found in some species normal cells.  These genes typically code for cellular growth factors of GF proteins.  In some cases, tumor cells lack some genes so they overcompensate for that deficiency.

Plant Viruses

Plant viruses stunt growth and reduce yields.  Types of spreading include horizontal and vertical transmission. Horizontal transmission is the infection from an external source.  Vertical transmission is when the plant inherits infection from a parent (asex. or sex. reproduction).  Plant viruses can spread through the organism by plasmodesmata.

Viriods are naked RNA several 100 nucleotides long.

Prions are infectious proteins (ex. Mad Cow Disease).

Next:  "Bacteria."