Chapters 19-20

What is a Virus?

An infectious particle consisting of genes (nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope​) packaged in a protein coat​

• Much simpler in structure than even prokaryotic cells​

• Not cells

• Can cause a wide variety of diseases​

• Cannot reproduce or carry out metabolism outside of a host cell ​

• Exist in a shady area between life-forms and chemicals, leading a kind of “borrowed life

What did Wendell Stanley do?

• Later work suggested that the infectious agent did not share features with bacteria (such as the ability to grow on nutrient media)​

• Stanley confirmed this latter hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)​

What can Viral genomes consist of?

• double- or single-stranded DNA or​

• double- or single-stranded RNA​

• The genome is either a single linear or circular molecule of the nucleic acid​

What is a Capsid?

• The protein shell that encloses the viral genome​

• Built from protein subunits called capsomeres​

• Can have a variety of structures; associated viruses may be referred to as helical or icosahedral viruses​

What are Viral envelopes?

• (derived from membranes of host cells) surround the capsids of influenza viruses and many other viruses found in animals​

• Contain a combination of viral and host cell molecules​

What are Bacteriophages?

• Viruses that infect bacteria​

• They have an elongated capsid head that encloses their DNA​

• A protein tail piece attaches the phage to the host and injects the phage DNA inside​

What does it mean that Viruses can replicate only within a host cell​?

Viruses are obligate intracellular parasites

What is a Host range?

• Each virus has one, a limited number of host species that it can infect​

• For example, measles virus only infects humans​

What is the Viral Replicative Cycle?

• The viral genome enters the host cell in a variety of ways​

• Once a viral genome has entered a cell, the cell begins to manufacture viral proteins​

• The virus makes use of host enzymes, ribosomes, tRNAs, amino acids, ATP, and other molecules​

• Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses​

What are the two alternative reproductive mechanisms of Phages?

The lytic cycle and the lysogenic cycle

What is the Lytic cycle?

• A phage replicative cycle that culminates in the death of the host cell​

• Produces new phages and lyses(breaks open) the host’s cell wall, releasing the progeny viruses​

What is a Virulent phage?

A phage that reproduces only by the lytic cycle

What is the Lysogenic cycle?

• Replicates the phage genome without destroying the host​

• The viral DNA molecule is incorporated into the host cell’s chromosome​

What are Temperate phages?

• Phages that use both the lytic and lysogenic cycles

• Also called lambda (l) is widely used in biological research

What is a Prophage?

• The integrated viral DNA ​

• Every time the host divides, it copies the phage DNA and passes the copies to daughter cells​

• An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode ​

• Some prophages are expressed during lysogeny,and some cause the host bacteria to secrete toxins that are harmful to humans​

Natural selection favors bacterial mutants with?

Surface proteins that cannot be recognized as receptors by a particular type of phage​

What are Restriction enzymes?

• Foreign DNA identified and cut up by cellular enzymes​

• The bacterium’s own DNA is protected from the restriction enzymes by being methylated

Both bacteria and archaea can protect themselves from viral infection with the?

CRISPR-Cas system

What is the CRISPR-Cas system based on?

• Based on sequences called clustered regularly interspaced short palindromic repeats (CRISPRs)​

• Each “spacer” sequence between the repeats corresponds to DNA from a phage that had infected the cell​

• Particular nuclease proteins interact with the CRISPR region; these are called CRISPR-associated (Cas) proteins​

When a phage infects a bacterial cell that has the CRISPR-Cas system?

The phage DNA is integrated between two repeat sequences​

• If the cell survives the infection, it can block any attempt of the same type of phage to re-infect it​

• The attempt of the phage to infect the cell triggers transcription of the CRISPR region ​

• The resulting RNAs are cut into pieces and bound by Cas proteins​

What do Cas proteins do?

• Use the phage-related RNA to target the invading phage DNA ​

• The phage DNA is cut and destroyed​

• Natural selection favors phage mutants that can bind to altered cell surface receptors or that are resistant to enzymes​

What are the two key variables used to classify viruses that infect animals?

• An RNA or DNA genome, either single-stranded or double-stranded​

• The presence or absence of a membranous envelope​

  • Whereas few bacteriophages have an envelope or an RNA genome, many animal viruses have both​

What is the Viral envelope (usually in animals)?

• Many viruses that infect animals have a membranous envelope​

• Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell​

• Usually derived from the host cell’s plasma membrane as the viral capsids exit

• Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane​ → herpesvirus​

The broadest variety of RNA genomes is found in?

Viruses that infect animals​

What do Retroviruses do?

Use reverse transcriptase to copy their RNA genome into DNA​

What is HIV (human immunodeficiency virus)?

The retrovirus that causes AIDS (acquired immunodeficiency syndrome)​

What is a Provirus?

The viral DNA that is integrated into the host genome

• Unlike a prophage, a provirus remains a permanent resident of the host cell​

RNA polymerase transcribes the proviral DNA into?

RNA molecules → function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell​

Since viruses can replicate only within cells, they?

Probably evolved as bits of cellular nucleic acid → candidates for the source of viral genomes include plasmids and transposons​; all mobile genetic elements​

What is the story of the largest virus?

• The largest virus identified about 20 years ago is the size of a small bacterium​

• Its genome encodes proteins involved in translation, DNA repair, protein folding, and polysaccharide synthesis​

• There is controversy about whether this virus evolved before or after cells​

• In the past decade several even larger viruses have been identified

Viruses may damage or kill cells by?

• Causing the release of hydrolytic enzymes from lysosomes​

• Some viruses cause infected cells to produce toxins that lead to disease symptoms​

• Others have molecular components such as envelope proteins that are toxic​

What is a Vaccine?

• A harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen​

• Can prevent certain viral illnesses, such as smallpox, rubella, mumps, and others​

• Viral infections cannot be treated by antibiotics

What are Antiviral drugs?

Can help to treat, not cure, viral infections by inhibiting synthesis of viral DNA and by interfering with viral assembly​

What is the Ebola virus?

One of several emerging viruses that cause hemorrhagic fever, an often fatal illness​

3 causes of rapidly emerging viral disease in humans are?

1. Mutation of existing viruses into new ones that can spread more easily​

2. The spread of a viral disease from a small, isolated human population​

3. The spread of existing viruses from other animals​ (75%)

What are Flu epidemics caused by?

Type A influenza viruses; these infect a wide variety of animals including birds, pigs, horses, and humans​

• Strains of influenza A are given standardized names based on the viral surface proteins hemagglutinin (HA) and neuraminidase (NA)​

What is the H5N1 strain?

• Quite deadly, because it is very different from influenza strains circulating among people for a long time​

• It is thus difficult for people to mount an effective immune response to this strain​

• However, it has not caused an epidemic because it is not transmitted from person-to-person

What is the Swine flu?

• A deadly strain of H1N1 not actually transmitted to humans from pigs​

• H1N1 was a unique combination of swine, avian, and human influenza genes​

• A global epidemic is called a pandemic​

What is the structure of the Influenza virus?

• Have 9 RNA segments in their genome, leading to many new genetic combinations​

• High rate of mutation​

• Variations of viruses thought to be most likely to occur each year are selected to generate vaccines​

Most known plant viruses have an?

• RNA genome​

• Many have a helical capsid, while others have a icosahedral capsid​

Plant viruses spread disease by two major routes?

• Horizontal transmission, entering through damaged cell walls ​

• Vertical transmission, inheriting the virus from a parent​

What are Prions?

• Infectious proteins that appear to cause degenerative brain diseases in animals​

• Scrapie in sheep, mad cow disease, and Creutzfeldt-Jakob disease in humans

• Prions are incorrectly folded proteins, can be transmitted in food, act slowly, and are virtually indestructible​

• Can convert a normal form of a protein into the misfolded version​

• Then several prions aggregate into a complex that can convert more proteins to prions, which join the chain​