VIRUS GR 11

What is a virus

• a virus is a non-living submicroscopic

• infectious agent

• only capable of reproducing within a host cell

• viruses are strands of nucleic acid (genetic material)(DNA/RNA) encased within a protein coat (consist / made of nucleic acid)

• virus is latin for toxin or poison

• not cellular (no cytoplasm, membrane bound organelles or cell membrane) (no characteristic of life)

virus introduction

-debate whether they are living or not

-some say not because they are not composed of cells/ dont fit any other criteria of life

-viral infection in higher organsims (complex) usually invokes immune response, viruses cause disease / illness

• antibiotics dont work against viral infections, only antivirals or vaccines keep virus at bay

• some viruses dont kill the host cell and remain in a dormant state inside the cell (lysogeny)

virus vs living cell

• viruses have no metabolic apparatus and dont digest, respire, etc

• not made of cells, no cell memberane, nucleus, or cytoplasm'

• they are crystalline, solutions of viruses leave behind crystals when evaporated

• can reproduce ONLY inside a host

• contain genes either RNA or DNA

• can take over cell activity of hosts they invade not just kill them

• can cause transmittable (contagious) diseases

viral structure (all viruses have the same basic structure(

• core of nucleic acid is surrounded by protein (each virus contains a single type of nucleic acid DNA OR RNA)

• nearly all viruses have a a protective coat / protein sheath called capsid around their nuclear acid and core

• many animal viruses form an envelop around the capsid that is rich in proteins, lipids and glycoproteins (has sugar attached to amino acid)

• microscopic, measured in nanometers

• 100x smaller than bacteria

• 5000 flu viruses can fit on head of pin

type of genetic material

RNA OR DNA

• if they have RNA may have to convert it to DNA depending on reproductive method

• DNA is never converted

viral structure (varies)

head- contains genetic material (DNA)

collar- attach and control tail fibers

sheath- channel for genetic material for delivery from head into host cell

base plate- transmits signals to initiate infection into host cell

tail fibers- aid in binding of viruses to host cells

viruses in history

• first virus discovered was the tobacco mosaic virus

• Dmitry Ivanovsky filtered sap from tobbaco plant, it remained infectious

• Matrinus Beijernick called the infectious substance the virus

• considered to be the begining of virology

classification of viruses

• linnean classification system modified for viruses since they do not fit into current 3 domains

• talk of new domain “acytota” meaning without cells for viruses

• currently have phylum down to species

• David Baltimore has developed the Baltimore classification system for viruses

Baltimore classification system

classifies viruses based on

1. type of nucleic acid (DNA OR RNA)

2. single stranded or double stranded

3. whether or not they use REVERSE TRANSCRIPTASE (work backwards going from RNA to DNA)

lytic cycle

-SHORT

1. attachment: virus must attach to cell wall or cell membrane of host

2. penetration / insertion: virus must enter the cell throguh cell membrane fusion or endocytosis

3. uncoating / make parts: protein coat is worn away exposing viral genome

4. replication and assemvly: viral genome takes over host cells machinery and creates more viruses

5. lysis: new virus particles rupture forth from host cell

lysogenic first 2 steps same

3. insertion into DNA: DNA integrates itself into the bacterial chromosome and becomes a non infective prophage

4. mitosis / replication: the chromosome with the integrated prophage replicated through mitosis / binary fission

5. excise viral DNA: the prophage can excise itself from the hosts chromosome and cells can enter the lytic cycle

lysogenic cycle

• viral genetic material enters and becomes PART of the host cell chromosomes

• if its RNA virus itll convert to DNA (RETROVIRUSES)

• one this happens the host cell has viral dna permenanently

• the viral dna that is now incorporated into the host DNA is reffered to as a provirus

virus reproduction

• retrovirus - RNA virus that goes through lysogenic cycle and needs to convert the RNA to DNA

• viruses that go through lysogenic cycles can convert back and forth to the lytic cycle

lambda phage

• a phage that can flip from lytic to lysogenic is called a lambda phage

• infects e.coli bacteria

Similarities and differences

• lytic cycle,

• entire replication process occurs in cytoplasm of host,

• viral genetic material enters the host cell and cell replicates viral DNA or RNA,

• the host cell makes new capsids and assembles new viral particles.

• host cell then lyses and virus leaves

• lysogenic cycle

• viral dna enters the host cells chromosome (RNA has to be converetd to DNA

• in many cases genes arent activated until later

• allows for many generations of cells with the viral instructions to be made

• activation results in continuation of lytic cycle

DNA viruses

• usually infect a host through a chance encouter

• include viruses such as hepres, any pox viruses (chickenpox and small pox)

• becasue these viruses have the same genetic material as humans, once DNA is inserted there are no extra steps for incorporation into the human nucleus

• can be lytic or lysogenic

RNA viruses

• use RNA as their genetic material or use an RNA intermediate to replicate

• rubella virus which causes german measles

• those that go through lysogenic will need to convert to DNA (retroviruses)

reverse transcripting viruses

• convert RNA into DNA before incorportation into a host cell

• called retroviruses because they work bakeards (natural flow is DNA to RNA) once RNA is converetd to DNA and inserted its now a provirus

• go through lysogenic cycles

• HIV is an ex

bacteriophage (Phage)

• type of virus that infects bacteria

• means bacteria eater

• attaches itself to a suspectible bacterium and infects it

• bacteriophage hijack bacteria and injects its dna this causes bacteria to make viral parts

virus and disease

• the virulence of a virus is its ability to cause disease

• deadliest diseases ex ebola, avian, flu, aids, covid 19