Virus (Virion) Subcellular particle

Consists of DNA or RNA genome surrounded by protein coat (capsid)

Virus StructureRANDOM FACTS: •Walter Reed discovered the first human virus, yellow fever virus, in 1901.•The name virus was coined from the Latin word meaning slimy liquid or poison.•Viruses are not alive – they are

inanimate complex organic matter. They lack any form of energy, carbon metabolism, and cannot replicate or evolve. Viruses are reproduced and evolve only within cells.

Fig. 24-1a, p. 502

RNA inside capsid

Capsid

0.1 µm

Fig. 24-1b, p. 502

Capsid with antenna-like fibersDNA inside capsid

0.05 µm

Fig. 24-1c, p. 502

DNA inside capsid

CapsidTai

l

Tail fibers

Emerging DNA

0.1 µm

Viruses Cannot metabolize on their own

Contain nucleic acids necessary to make copies of themselves but must invade and use metabolic machinery

of living cells in order to reproduce

Origin of Viruses

Viruses may be bits of nucleic acid that originally “escaped” from animal, plant, or bacterial cells

RANDOM FACT: There are a million virus particles per milliliter of seawater – for a global total of 1030 virions! Lined up end to end, they would stretch 200 million light years into space.

Hypothesis

Viruses must have evolved before the three domains diverged It is unlikely that similar viruses that infect

archaea and bacteria evolved twice

RANDOM FACT: Some parasitic wasps lay eggs in caterpillars, where they mature into adult wasps. The wasp eggs contain a virus, encoded in the wasp genome, which prevents the caterpillar from rejecting the eggs.

Viral Reproductive Cycles Lytic cycle

virus destroys host cell

• Temperate viruses do not always destroy their hosts

• Lysogenic cycle viral genome replicated along with host DNA

Lytic Cycle

5 steps: attachment to host cell penetration of viral nucleic acid into host cell replication of viral nucleic acid assembly of components into new viruses release from host cell

Fig. 24-2a (1), p. 504

Phages

Attachment. Phage attaches to cell surface of bacterium.

Bacterium

Bacterial DNA

Penetration. Phage DNA enters bacterial cell.

Phage protein

Replication and synthesis. Phage DNA is replicated. Phage proteins are synthesized.

Phage DNA

2

1

3

Fig. 24-2a (2), p. 504

Assembly. Phage components are assembled into new viruses.

Release. Bacterial cell lyses and releases many phages that can then infect other cells.

4

5

Fig. 24-2b, p. 504

0.25 µm

Lysogenic Cycle

Fig. 24-4b, p. 508

Envelope proteins 1 Virus attaches to specific

receptors on plasma membrane of host cell.Envelope

CapsidNucleic acid

2 Membrane fusion. Viral envelope fuses with plasma membrane.

Receptors

Host-cell plasma membrane

3Virus is released into host-cell cytoplasm.

CapsidCytoplasm

NucleusNucleic acid

Viral nucleic acid separates from its capsid.

4

Ribo-somes 5 Viral nucleic acid

enters host-cell nucleus and replicates.ER

mRNA 6 Viral nucleic acid is transcribed into mRNA.

710 Viruses are released from host cell.

Host ribosomes are directed by mRNA to synthesize viral proteins.

8 Vesicles transport glycoproteins to host-cell plasma membrane.

9 New viruses are assembled and enveloped by host-cell plasma membrane.

Membrane Fusion

Viral Diseases DNA viruses cause

smallpox, herpes, respiratory infections, gastrointestinal disorders

RNA viruses cause influenza, upper respiratory infections, AIDS,

some types of cancer

Rubella An RNA virus

Plant Viruses

Mostly RNA viruses

Spread among plants by insect vectors

Spread through plant via plasmodesmata

Retroviruses Use reverse transcriptase

Transcribe RNA genome into DNA intermediate becomes integrated into host DNA

Synthesize copies of viral RNA

Fig. 24-5, p. 509

HIV Nucleic acid (RNA)

Envelope protein 1HIV attaches to host-cell plasma membrane.Envelope

CapsidEnzymes (reverse transcriptase, ribonuclease, integrase, protease)

2 HIV enters host-cell cytoplasm. CD4 Receptors

Viral nucleic acid (RNA)Reverse

transcriptase3Capsid is removed by

enzymes. Reverse transcriptase catalyzes synthesis of single-stranded (ss) DNA that is complementary to viral RNA.

Host-cell plasma membrane Cytoplasm

ssDNA4 The DNA strand then serves

as template for synthesis of comple- mentary DNA strand, resulting in double-stranded (ds) DNA.

Nucleus dsDNAHost

chromosomeViral RNA dsDNA is transferred to host

nucleus and enzyme integrase integrates DNA into host chromosome.

5

6 When activated, viral DNA uses host enzymes to transcribe viral RNA.

7 Viral RNA leaves nucleus, viral proteins are synthesized on host ribosomes, and virus is assembled.

8Virus buds from host cell, using host-cell plasma membrane to make viral envelope.

MORE RANDOM FACTS: The first human influenza virus was isolated in 1933. In 2005, the 1918 pandemic influenza virus strain was constructed from nucleic acid sequence obtained from victims of the disease.

•The biggest known viruses are mimiviruses, which are 400 nanometers (0.0004 millimeters) in diameter. The viral genome is 1,200,000 nucleotides in length and codes for over 900 proteins.

•The smallest known viruses are circoviruses, which are 20 nanometers (0.00002 millimeters) in diameter. The viral genome is 1,700 nucleotides in length and codes for two proteins.

•The HIV-1 genome, which is about 10,000 nucleotides long, can exist as 106020 different sequences. To put this number in perspective, there are 1011 stars in the Milky Way galaxy and 1080 protons in the universe.

Our “friend” H1N1(over 61,300 cases confirmed nation-wide in

2009…)