Chapter 18 Flashcards
what is a virus?
- non-living
- requires assistance of living cells for reproduction
- first discovered- TMV
Viruses
- a small infectious particle
- nucleic acid enclosed in a protein coat
- small; 50 million virus particles could fit in one of our cells
host range (differences among viruses)
number of species and cell types that can be infected
structural (differences among viruses)
- all have a capsid (protein coat)- varies in shape and complexity
- some have viral envelope- from host cell plasma membrane
genome can be… (differences among viruses)
- DNA vs. RNA
- single stranded vs. double stranded
- linear vs. circular
viral reproductive cycle- basic steps
- attachement
- entry
- integration (depends on virus type)
- synthesis of viral components
- viral assembly
- release
Attachment (viral reproductive cycle)
- usually specific for certain cell type
- binds to membrane protein -> receptor for something else
Entry (viral reproductive cycle)
- bacteriophages (or phage) injects nucleotides into bacteria
- may also inject proteins to copy or integrate genes
- some viral genes expressed immediately
- virus may proceed to synthesis of viral components OR integrate into host chromosome
Integration (viral reproductive cycle)
- viral gene codes for integrase- cuts host DNA, inserts viral genome
- phage in bacterial DNA called prophage
- may excise later and proceed to synthesis
Synthesis (viral reproductive cycle)
-host cell enzymes make many copies of the phage DNA and transcribe the genes within these copies into mRNA
Assembly (viral reproductive cycle)
- some self-assemble- TMV
- other are too complicated to self-assemble, require help from proteins
release (viral reproductive cycle)
-phages must lyse hose cell to escape
latency
- some viruses integrate their genomes into a host chromosome
- every time this cell divides, each daughter cell is carrying the integrated viral genome..
- prophage or provirus remains inactive or latent
- HIV and some herpes viruses can remain silent for years
- most viral genes silenced
- known as lysogenic cycle
Lysogenic cycle
- phage injects its DNA into cytoplasm
- phage DNA integrates into host chromosome
- prophage DNA is copied when cell divides
- on rare occasions, a prophage may be excised from host chromosome
lytic cycle
- phage injects its DNA into cytoplasm
- phage DNA directs the synthesis of many new phages
- cell lyses and releases the new phages
- new phages can bind to bacterial cells
HIV
- 2 copies of single stranded RNA and two molecules of reverse transcriptase
- packaged into a capsid
- surrounded by a viral envelope
the case of HIV
- entry: HIV fuses with host cell membrane
- it binds to receptors on T-cells
- integration: HIV is an RNA virus, i.e. a retrovirus
- uses viral reverse transcriptase to make complementary DNA strand that will be template for double stranded viral DNA
- synthesis: HIV DNA is not excised from host chromosome, but it transcribed in the nucleus to produce many copies of viral RNA
- translated to make viral proteins and RNA for new viral particles
retrovirus
- use reverse transcription
- reveres transcriptase lacks a proofreading function
- makes more errors and tends to create mutant strains of HIV
- constant production of new viral proteins which our immune system has difficulty keeping up with
controlling HIV
- because HIV mutates so frequently
- vaccines hard to develop
- “cocktail” of drugs
- hard for immune system to recognize/ combat
genetics of Bacteria
- circular chromosome
- 1 origin of replication
- sometimes > 1 copy
- 1-4 identical chromosomes
- # depends on bacterial species and growth conditions
bacterial chromosomes
-may contain repetitive sequences interspersed throughout
nucleoid ( genetics of bacteria)
- where DNA hangs out
- not membrane bound
bacterial chromosomes (genetics of bacteria)
- small
- few thousand genes
- mostly structural genes
bacteria can also have plasmids
- small, circular pieces of DNA, NOT part of bacterial
- what sort of advantages do plasmids provide?
- occur natural in many strains of bacteria
- also found in a few eukaryotes, such as yeast
- replication independently of bacterial chromosome
- not usually necessary for survival but can provide growth advantages
- episome- plasmid that can integrate into bacterial chromosome
resistance plasmids
(R factors
-genes that confer resistance against antibiotics/ toxins
degradative plasmids
-genes that enable bacteria to digest and use unusual substances
col-plasmids
-genes that encode colicines; proteins that kill other bacteria
virulence plasmids
-genes that turn a bacterium into a pathogenic strain
fertility plasmids (F factors)
allow bacteria to mate with each other
reproduction in bacteria
- very fast
- E.coli can divide every 20 min
- single cell-> bacterial colony in < 1 dat
- reproduce by binary fission
- no sexual reproduction -> no meiosis
- plasmids replicate independently of chromosome
binary fission
- chromosome replicates
- cell beigns to divide with one chromosome at each end
- cell wall forms between the two new cells
- gives rise to two daughter cells -> genetically identical
genetic diversity
- different types of the same bacterial species = strains
- genetic diversity caused by 1. spontaneous mutations
- alters bacterial genome and affects traits of bacterial cells
- one E. coli strain may be resistant to pencillin, another may be sensitive
- genetic diversity caused by 2. genetic transfer
- from one bacterial cell to another
- ex: plasmid conferring antibiotic resistance transferred from a resistant cell to a senseitive cell
conjugation (genetic transfer)
-direct transfer of DNA from donor to recipient cell
transformation (genetic transfer)
DNA from the environment is taken up by another bacteria
transduction (genetic transfer)
a virus transfers genetic information from one bacterium to another
conjugation: a bit more than kissing
- 5% of E.coli strains found in nature can act as donor strains
- donor strains contain a fertility factor (F factor) that can be transferred to recipients
F factors
- several genes required for conjugation
- found on a plasmid
- may also confer a growth advantage
- 2 types of cells
- F+ cells have an F factor
- F- cells do not
- sex pili made by F+ cells that bind specifically to F- cells
- sex pili shorten, brining cells close and they conjugate
- F strand in F+ cell replicates and is transferred to recipient cell
transformation
-no contact between cells
-bacteria pick up DNA from environment
-from dead bacteria
-cells need competence factors (genes) that facilitate
-binding of DNA fragments to the bacterial cell surface
0uptake of DNA
-incorporation of foreign DNA into the bacterial chromosome
transformation; competence factors
- proteins coded for by a set of genes
- a DNA receptor gene
- DNA cutting enzymes
- DNA channel/ uptake proteins
- enzymes to integrate the new DNA into the bacterial chromosome
transduction
- viruses that infect bacteria transfer bacterial genes from one bacterium to another
- occurs when virus gets stuffed with the wrong DNA and then tries to infect another cell
