A2.3: Viruses

Structure of Viruses (5)

Small Size: need to be small to enter host (20-300 nm)

Fixed Size: Do not grow, assembled in full size in host

Nucleic Acid as Genetic Material: either single/double stranded DNA/RNA, hosts machinery must read it to make proteins

Capsid: Protein coat that encloses genetic material

No cytoplasm, very few enzymes: Relies on hosts metabolism. Own enzymes used to replicate genetic material and infect/burst host

Genetic Material Basics

Single or double-stranded DNA or RNA

Circular or linear

Genetic material varies greatly in length

Positive-Sense RNA Virus

Genes are used directly as mRNA

Positive-sense is the complement to template/antisense strand, therefore looks like mRNA

Negative-Sense RNA Virus

Genes need to be transcribed to mRNA

Retrovirus

RNA gene changes to double-stranded DNA (w/ Reverse Transcriptase), then to mRNA

Drawing of DNA to Showcase Template vs. Nontemplate

Envelope in Viruses

-Envelope is membrane acquired during lysis, or bursting process

-Takes plasma membrane of animal cell host

-helps make contact w/ other hosts (b/c looks similar to disguise itself)

Enveloped Viruses

Infect Animal cells

Non-enveloped Viruses

Infect bacteria or plant cells

Lose envelope as break through cell wall

Bacteriophage Lambda Characteristics (5)

Type: DNA Virus

Envelope: Non-envelope

Genetic Material: one double-stranded, positive & negative sense DNA (AKA regular DNA)

32 Genes code for 29 proteins, 4 enzymes

Features: can engage in lytic and lysogenic cycles

Host: Escherichia coli (E. coli)

HIV (human immunodeficiency virus) Characteristics (5)

Type: Retrovirus

Envelope: Enveloped

Genetic Material: Two single-stranded positive-sense RNA (but must be reverse-transcribed)

9 Genes or 15 proteins, 4 enzymes

Features: Contains reverse transcriptase, makes DNA from RNA

Host: Human Helper T Cell

Lytic Cycle Characteristics

Virus is virulent, or kills host, spreads extremely fast from host to host

Disease becomes more severe as it spreads

Disadvantages: Host can produce antibodies to destroy the virus, may kill host to fast and not be able to replicate

Lytic Cycle Process (in Bacteriophage Lambda) (7)

Attachment: proteins on virus tail bind to maltoporin on host's (E. coli) membrane

DNA entry - viral DNA enters host

DNA replication - linear, viral DNA turns circular and replicates

DNA Transcription - mRNA produced

Protein Synthesis - Host ribosomes synthesize viral proteins, head and tail proteins self-assemble into capsid

Lysis - Viral proteins puncture host's cell wall

Spread - Host bursts, releasing about 100 viruses

Lytic Cycle Process Image

Lysogenic Cycle Characteristics

Virus is temperate, so host is not destroyed

-remains this way until lytic cycle induced, this occurs when a stimulate activates viral genes

Benefit to Host: due to prophage (viral + bacterial DNA), new host can receive previous host's DNA

-this increased bacterial genetic diversity

*Typically used w/ hosts w/ longer lifespans so the virus doesn't go through the host population too quickly and kill off all it's potential hosts*

Lysogenic Cycle Process (in Bacteriophage Lambda (4)

Attachment, DNA Entry (same as Lytic)

Integration - linear, viral DNA turns circular

-enzyme Integrase inserts DNA into specific location in bacterial DNA

-now viral DNA only exists as prophage, or part of bacterial DNA

Cell Division: bacterial cell divides as normal, and prophage (viral genes) copy as well

Lysogenic Cycle Image

Viral Evolution

Viruses must have evolved from cells

Viruses evolved through convergent evolution (not common ancestor)

-bc high structural and genetic diversity suggests multiple origins

Obligate Parasite

What a virus is, means that it requires a host to replicate

Progressive Hypothesis

Viruses developed from cells via modification

Evidence: cells contain virus-like components such as retrotransposons

Retrotransposons

-Sequence of nucleotides common in DNA

-Transcribed into mRNA, then translated into several enzymes

-Enzymes make copies of the transposon DNA in reverse transcription (similar to revers transcriptase)

-Transposon copies inserted into various chromosomes at random places, similar to HIV, a retrovirus

*basically just copies DNA to RNA, then back to DNA and put in random place*

Regressive Hypothesis

Virus developed from cells via loss (basically just remove everything unnecessary in cell)

Evidence: Viruses vary regarding complexity and self-reliance

-ex. some larger/complex viruses (smallpox) perform host functions

Rapid Evolution of Viruses Causes (3)

Quick Generation Time, High Mutation Rate, High Intensity for Natural Selection

Quick Generation Time

Evolution occurs between generations, each virus generation is about 1 hour (very fast)

High Mutation Rate

Variation must exist for evolution, RNA Polymerase doesn't proofread replication errors, leads to lots of variation

High Intensity for Natural Selection

-Host has immune mechanisms for detection and disposal of viruses

-host's antibodies target viruses' antigens

-Viruses w/ new antigens can survive (leads to very fast natural selection)

Influenza Virus Characteristics

Enveloped virus w/ negative-sense, single stranded RNA w/ a HIGH MUTATION RATE

Transmitted between species

Why does Influenza mutate quickly? (6)

-Genome exists as 8 separate molecules

-Has two antigens on membrane surface:

-hemagglutinin - binds to host

-neuraminidase - used to break free form host

New strain/mutation surfaces if new combinations of genome or antigen evolve

Allows for high potential for a pandemic

HIV (human immunodeficiency virus) Characteristics Part II

Retrovirus w/ reverse-transcriptase that converts single-stranded RNA into DNA

HIGH MUTATION RATE

Why does HIV mutate quickly? (4)

-Reverse transcriptase does not proofread

-HIV affected by cytidine deaminase, enzyme in host that converts cytosine to uracil (relevant b/c viral DNA can be mutated more when incorporated into host DNA)

-env gene - surface protein used to enter host, mutations in it allow it to enter different human cells

-New strain surfaces from any different combinations, leads to resistance to antiretroviral drugs