Transcript lytic cycle - Cloudfront.net

1950’S- VIRUSES
OBSERVED
• THE SMALLEST VIRUSES ARE ONLY 20
nm IN DIAMETER
• THE VIRUS PARTICLE, CONSISTS OF
NUCLEIC ACID ENCLOSED BY A
PROTEIN COAT
• MAY BE DOUBLE-STRANDED DNA
• SINGLE-STRANDED DNA
• DOUBLE-STRANDED RNA
• SINGLE-STRANDED RNA
COMPARING SIZES
CAPSIDS
• CAPSIDS ARE THE PROTEIN COAT THAT
ENCLOSES THE VIRAL GENOME
• MAY BE ROD-SHAPED, POLYHEDRAL OR
COMPLEX
• COMPOSED OF MANY CAPSOMERES,
PROTEIN SUBUNITS MADE FROM ONLY
ONE OR A FEW TYPES OF PROTEINS
ENVELOPE
• THE ENVELOPE IS THE MEMBRNE
THAT COVERS SOME VIRAL CAPSIDS
• IT HELPS VIRUSES INFECT THEIR
HOST
• IT IS DERIVED FROM HOST CELL
MEMBRANE
VIRAL STRUCTURE
VIRAL REPRODUCTION
OVERVIEW
• VIRUSES CAN ONLY REPRODUCE INSIDE
OF A HOST CELL
• OBLIGATE INTRACELLULAR
PARASITES-CAN EXPRESS THEIR GENES
AND REPRODUCE ONLY WITHIN A LIVING
CELL
• HOST RANGE = LIMITED NUMBER OR
RANGE OF HOST CELLS THAT A PARASITE
CAN INFECT
VIRAL LIFE CYCLES
GENERAL PATTERNS:
• 1) INFECTING HOST CELL WITH VIRAL GENOME
• 2) CO-OPTING HOST CELL’S RESOURCES TO:
A) REPLICATE THE VIRAL GENOME
B) MANUFACTURE CAPSID PROTEIN
• 3) ASSEMBLING NEWLY PRODUCES VIRAL
NUCLIC ACID AND CAPSOMERES INTO THE
NEXT GENERATION OF VIRUSES
SIMPLIFIED VIRAL REPRODUCTIVE CYCLE
VIDEO: SIMPLIFIED VIRUS LIFE CYCLE
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BACTERIOPHAGE
• A PHAGE IS A VIRUS THAT ATTACKS
BACTERIA
• PHAGES ARE THE BEST
UNDERSTOOD OF ALL VIRUSES
THE LYTIC CYCLE
• VIRULENT BACTERIOPHAGES
REPRODUCE ONLY BY A LYTIC
REPLICATION CYCLE
• VIRULENT PHAGES - PHAGES THAT
LYSE THEIR HOST CELLS
• LYTIC CYCLE = A VIRAL
REPLICATION CYCLE THAT RESULTS
IN DEATY (LYSIS) OF THE HOST CELL
STEPS OF THE LYTIC CYCLE
• 1) PHAGE ATTACHES TO CELL SURFACE
• 2) PHAGE CONTRACTS SHEATH AND
INJECTS DNA
• 3) HYDROLYTIC ENZYMES DESTROY
HOST CELL’S DNA
• 4) PHAGE GENOME DIRECTS HOST CELL
TO PRODUCE PHAGE COMPONENTS: DNA
AND CAPSID PROTEINS
• 5) CELL LYSES AND RELEASES PHAGE
PARTICLES
LYTIC CYCLE OF PHAGE T4
LYTIC CYCLE VIDEO
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BACTERIAL DEFENSES
• BACTERIAL MUTATIONS CAN CHANGE
RECEPTOR SITES USED BY PHAGES FOR
RECOGNITION, AND THUS AVOID
INFECTION
• BACTERIAL RESTRICTION NUCLEASES
RECOGNIZE AND CUT UP FOREIGN DNA,
INCLUDING CERTAIN PHAGE DNA.
BACTERIAL DNA IS CHEMICALLY
ALTERED, SO IT IS NOT DESTROYED BY
THE CELL’S OWN RESTRICTION
ENZYMES
LYSOGENIC CYCLE
• SOME VIRUSES CAN COEXIST WITH
THEIR HOSTS BY INCORPORATING THEIR
GENOME INTO THE HOST’S GENOME
• TEMPERATE VIRUSES - VIRUSES THAT
CAN INTEGRATE THEIR GENOME INTO A
HOST CHROMOSOME AND REMAIN
LATENT UNTIL THEY INITIATE A LYTIC
CYCLE
– THEY HAVE TWO POSSIBLE MODES OF
REPRODUCTION, THE LYTIC CYCLE AND
THE LYSOGENIC CYCLE
STEPS OF LYSOGENIC CYCLE
•
•
•
•
»PHAGE L LIFE CYCLE
1) PHAGE L BINDS TO THE SURFACE OF
E.COLI CELL
2) PHAGE L INJECTS ITS DNA INTO THE
BACTERIAL HOST CELL
3) L DNA FORMS A CIRCLE AND EITHER
BEGINS A LYTIC OR LYSOGENIC CYCLE
4) L DNA INSERTS BY GENETIC
RECOMBINATION (CROSSING OVER) INTO
A SPECIFIC SITE ON THE BACTERIAL
CHROMOSOME
REPRODUCTIVE CYCLES OF PHAGE L
LYSOGENIC CYCLE VIDEO
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PROPHAGES
• A PHAGE GENOME THAT IS
INCORPORATED INTO A SPECIFIC
SITE ON THE BACTERIAL
CHROMOSOME
VIRAL ENVELOPES
• SOME ANIMAL VIRUSES ARE
SURROUNDED BY A MEMBRANOUS
ENVELOPE, WHICH IS UNIQUE TO
SEVERAL GROUPS OF ANIMAL VIRUSES.
THIS ENVELOPE IS:
– OUTSIDE THE CAPSID AND HELPS THE
VIRUS ENTER HOST CELLS
– A LIPID BILAYER WITH GLYCOPROTEIN
SPIKES PROTRUDING FROM THE OUTER
SURFACE
ENVELOPED VIRUSES LIFE
CYCLE
• 1) ATTACHMENT - GLYCOPROTEIN
SPIKES PROTRUDING FROM THE VIRAL
ENVELOPE ATTACH TO RECEPTOR SITES
ON THE HOST’S PLASMA MEMBRANE
• 2) ENTRY - AS THE ENVELOPE FUSES
WITH THE MEMBRANE, THE ENTIRE
VIRUS (CAPSID AND GENOME) IS
TRANSPORTED INTO THE CYTOPLASM BY
RECEPTOR-MEDIATED ENDOCYTOSIS
• 3) UNCOATING - CELLULAR ENZYMES
UNCOAT THE GENOME BY REMOVING
THE PROTEIN CAPSID FROM THE VIRAL
RNA
• 4) VIRAL RNA AND PROTEIN
SYNTHESIS-VIRAL ENZYMES ARE
REQUIRED TO REPLICATE THE RNA
GENOME AND TO TRANSCRIBE mRNA
• 5) ASSEMBLY AND RELEASE - NEW
CAPSIDS SURROUND VIRAL GENOMES.
ONCE ASSEMBLED, THE VIRIONS
ENVELOP WITH HOST PLASMA
MEMBRANE AS THEY BUD OFF FROM
THE CELL’S SURFACE
REPRODUCTIVE CYCLE OF AN
ENVELOPED VIRUS
PROVIRUS
• A PROVIRUS IS FORMED WHEN
VIRAL DNA INSERTS INTO A HOST
CELL CHROMOSOME. (IF IT IS A
BACTERIA CELL, IT IS CALLED A
PROPHAGE). PROVIRUS’ REMAIN
DORMANT UNTIL SOME TRIGGER
(USUALLY ENVIRONMENTAL)
CAUSES THE VIRUS TO BEGIN THE
DESTRUCTIVE LYTIC CYCLE
RNA VIRUSES
• USE RNA TO CARRY GENETIC
INFORMATION
• THE REPRODUCTIVE CYCLES
DEVIATE FROM THE STANDARD
LYTIC AND LYSOGENIC CYCLES
• IN SOME RNA VIRUSES, THE VIRAL
RNA IS USED DIRECTLY AS mRNA
RETROVIRUSES
• RETRO = BACKWARD
• RETROVIRUS= RNA VIRUS THAT USES
REVERSE TRANSCRIPTASE TO
TRANSCRIBE DNA FROM THE VIRAL RNA
GENOME
• HIV-THE VIRUS THAT CAUSES AIDS IS A
RETROVIRUS
• RETROVIRUSES HAVE COMPLEX CYCLES
BECAUSE THEY MUST FIRST CARRY OUT
REVERSE TRANSCRIPTION
HIV-THE REPRODUCTIVE CYCLE
HIV RETROVIRUS VIDEO
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VIRUSES AND CANCER
• SOME TUMOR VIRUSES CAUSE CANCER IN
ANIMALS
• WHEN ANIMAL CELLS GROWN IN TISSUE
CULTURE ARE INFECTED WITH TUMOR
VIRUSES, THEY TRANSFORM TO A
CANCEROUS STATE
• EXAMPLES ARE MEMBERS OF THE
RETROVIRUS, PAPOVAVIRUS,
ADENOVIRUS, HERPESVIRUS GROUPS
• CERTAIN VIRUSES ARE IMPLICATED IN
HUMAN CANCERS
VIRUSES AND CANCER
•
•
•
•
RETROVIRUS - LEUKEMIA
HERPESVIRUS - BURKITT’S LYMPHOMA
PAPILOMA VIRUS - CERVICAL CANCER
HEPATITIS B - LIVER CANCER
• ONCOGENES - GENES FOUND IN VIRUSES OR AS
PART OF THE NORMAL GENOME, THAT TRIGGER
TRANSFORMATION OF A CELL TO A CANCEROUS
STATE
PLANT VIRUSES
• AS SERIOUS AGRICUTURAL PESTS, MANY
OF THE PLANT VIRUSES:
– STUNT PLANT GROWTH AND DIMINISH
CROP YIELDS
– ARE RNA VIRUSES
– HAVE ROD-SHAPED CAPSIDS WITH
CAPSOMERES ARRANGED IN A SPIRAL
CAPSOMERE - COMPLEX CAPSID SUBUNIT
CONSISTING OF SEVERAL IDENTICAL OR
DIFFERENT PROTEIN MOLECULES
PLANT VIRUSES
• PLANT VIRUSES SPREAD FROM PLANT TO
PLANT BY TWO MAJOR ROUTES:
• 1) HORIZONTAL TRANSMISSION - AN
ORGANISM RECEIVES THE VIRUS FROM
AN EXTERNAL SOURCE SUCH AS
INSECTS OR GARDENING TOOLS
• 2) VERTICAL TRANSMISSION - AN
ORGANISM INHERITS A VIRAL
INFECTION FROM ITS PARENT
VIROIDS
• ANOTHER CLASS OF PLANT PATHOGENS, ARE
SMALLER AND SIMPLER THAN VIRUSES
• THEY ARE SMALL, NAKED, CIRCULAR RNA
MOLECULES THAT DO NOT ENCODE
PROTEIN, BUT CAN REPLICATE IN HOST
PLANT CELLS
• THEY DISRUPT NORMAL PLANT
METABOLISM, DEVELOPMENT, AND GROWTH
BY CAUSING ERRORS IN GENE EXPRESSION
• VIROID DISEASES AFFECT MANY
COMMERCIALLY IMPORTANT PLANTS SUCH
AS COCONUT PALMS, POTATOES, TOMATOES
PRIONS
• PRIONS ARE PATHOGENS THAT ARE
PROTEINS, AND THEY APPEAR TO CAUSE
A NUMBER OF DEGENERATIVE BRAIN
DISEASES SUCH AS:
– SCAPIE IN SHEEP
– MAD COW DISEASE
– CREUTZFELDT-JAKOB DISEASE IN HUMANS
HOW PRIONS MAY PROPAGATE
WHEN A PRION CONTACTS ITS NORMAL “TWIN”, IT
MAY INDUCE THE NORMAL PROTEIN TO ASSUME THE
ABNORMAL SHAPE. THE RESULTING CHAIN REACTION
MAY CONTINUE UNTIL PRIONS ACCUMULATE TO
DANGEROUS LEVELS, CAUSING CELLULAR MALFUNCTION
GENETICS OF BACTERIA
• BACTERIA ARE PROKARYOTES
• THEY DO NOT POSSESS ANY OF THE
SPECIALIZED ORGANELLES OF
EUKARYOTES
• HAS A SINGLE, CIRCULAR DNA
• REPRODUCES BY BINARY FISSION-THE
CHROMOSOME REPLICATES AND THE
CELL DIVIDES INTO TWO CELLS
BACTERIAL REPLICATION
PLASMIDS
• BACTERIA ALSO CONTAIN PLASMIDS, SHORT,
CIRCULAR DNA MOLECULES OUTSIDE THE
CHROMOSOME
• PLASMIDS CARRY GENES THAT ARE
BENEFICIAL BUT NOT NORMALLY ESSENTIAL
TO THE SURVIVAL OF THE BACTERIUM
• PLASMIDS REPLICATE INDEPENDENTLY OF
THE CHROMOSOME
• EPISOMES = PLASMIDS THAT BECOME
INCORPORATED INTO THE BACTERIAL
CHROMOSOME
GENETIC RECOMBINATION
IN BACTERIA
• THERE ARE 3 NATURAL PROCESSES OF
GENETIC RECOMBINATION IN BACTERIA:
• TRANSFORMATION
• TRANSDUCTION
• CONJUGATION
TRANSFORMATION
• THE PROCESS OF GENE TRANSFER
DURING WHICH A BACTERIAL CELL
ASSIMILATES FOREIGN DNA FROM THE
SURROUNDINGS
• ASSIMILATED FOREIGN DNA MAY BE
INTEGRATED INTO THE BACTERIAL
CHROMOSOME BY RECOMBINATION
• PROGENY WILL CARRY A NEW
COMBINATION OF GENES
TRANSDUCTION
• GENE TRANSFER FROM ONE BACTERIUM
TO ANOTHER BY A BACTERIOPHAGE
• RANDOM PIECES OF HOST CELL DNA ARE
PACKAGED WITHIN A PHAGE CAPSID
DURING THE LYTIC CYCLE OF A PHAGE
• THIS PROCESS CAN TRANSFER ALMOST
ANY HOST GENE AND LITTLE OR NO
PHAGE GENES
• WHEN THE PHAGE PARTICLE INFECTS A
NEW HOST CELL, THE DONOR CELL DNA
CAN RECOMBINE WITH THE RECIPIENT
CELL DNA
TRANSDUCTION
CONJUGATION
• THE DIRECT TRANSFER OF GENES BETWEEN
TWO CELLS THAT ARE TEMPORARILY JOINED
• CYCLE: DNA-DONATING CELL EXTENDS
EXTERNAL APPENDAGES CALLED SEX PILI
• SEX PILI ATTACH TO A DNA-RECEIVING CELL
• A CYTOPLASMIC BRIDGE FORMS THROUGH
WHICH DNA TRANSFER OCCURS
PLASMIDS
• PLASMIDS ARE SMALL, CIRCULAR, DOUBLESTRANDED, SELF-REPLICATING MOLECULAR
RINGS OF DNA THAT CARRY
EXTRACHROMOSOMAL GENES IN SOME
BACTERIA
• PLASMIDS CARRY GENES THT ARE BENEFICIAL
BUT NOT NORMALLY ESSENTIAL TO SURVIVAL
OF THE BACTERIA
• PLASMIDS REPLICATE INDEPENDENTLY OF THE
CHROMOSOME
• EPISOMES = PLASMIDS THAT BECOME
INCORPORATED INTO THE BACTERIAL
CHROMOSOME
F PLASMID
• THE F PLASMID (F FOR FERTILITY) HAS
ABOUT 25 GENES, MOST OF WHICH ARE
INVOLVED IN THE PRODUCTION OF SEX PILI
• WHEN A RECIPIENT BACTERIUM RECIEVES
THE F PLASMID, IT TOO CAN BECOME A
DONOR CELL
R PLASMID
• R PLASMIDS PROVIDE BACTERIA WITH
RESISTANCE AGAINST ANTIBODIES
• SOME CARRY UP TO 10 GENES FOR
RESISTANCE TO ANTIBODIES
• R PLASMIDS CAN TRANSFER RESISTANCE
GENES TO BACTERIA OF DIFFERENT
SPECIES INCLUDING PATHOGENIC
STRAINS. AS A CONSEQUENCE,
RESISTANCE STRAINS OF PATHOGENS ARE
BECOMING MORE COMMON
TRANSPOSONS
• TRANSPOSONS ARE PIECES OF DNA, OR TRANSPOSIBLE
GENETIC ELEMENTS, WHICH CAN ACTUALLY MOVE
FROM ONE LOCATION TO ANOTHER IN A CELL’S
GENOME
• CONSERVATIVE TRANSPOSITION = MOVEMENT OF
PREEXISTING GENES FROM ONE GENOMIC LOCATION
TO ANOTHER; THE TRANSPOSON’S GENES ARE NOT
REPLICATED BEFORE THE MOVE, SO THE NUMBER OF
GENE COPIES IS CONSERVED
• REPLICATIVE TRANSPOSITION = MOVEMENT OF
GENE COPIES FROM THEIR ORIGINAL SITE OR
REPLICATON TO ANOTHER LOCATION; THE
TRANSPOSON’S GENES ARE INSERTED AT SOME NEW
SITE WITHOUT BEING LOST FROM THE ORIGINAL SITE
INSERTION SEQUENCES
• THE SIMPLEST TRANSPOSONS, WHICH CONTAIN
ONLY THE GENES NECESSARY FOR THE
PROCESS OF TRANSPOSITION. INSERTION
SEQUENCE DNA INCLUDES TWO ESSENTIAL
TYPES OF NUCLEOTIDE SEQUENCES:
– 1) NUCLEOTIDE SEQUENCE CODING FOR
TRANSPOSASE
– 2) INVERTED REPEATS
TRANSPOSASE = ENZYME THAT CATALYZES
INSERTION OF TRANSPOSONS INTO NEW SITES
INVERTED REPEATS (IR) = SHORT NONCODING
NUCLEOTIDE SEQUENCES OF DNA THAT ARE
REPEATED IN REVERSE ORDER ON OPPOSITE ENDS
OF A TRANSPOSON
INSERTION SEQUENCES, THE SIMPLEST TRANSPOSONS
INSERTION OF A TRANSPOSON AND CREATION OF
DIRECT REPEATS
OPERONS: THE BASIC CONCEPT
• OPERON = A REGULATED CLUSTER OF
ADJACENT STRUCTURAL GENES WITH
RELATED FUNCTIONS
• COMMON IN BACTERIA AND PHAGES
• HAS A SINGLE PROMOTER REGION, SO AN
RNA POLYMERASE WILL TRANSCRIBE ALL
STRUCTURAL GENES ON AN ALL OR NONE
BASIS
OPERONS
• THERE ARE 4 MAJOR COMPONENTS OF AN
OPERON:
• 1) A REGULATORY GENE PRODUCES A
REPRESSOR PROTEIN, A SUBSTANCE THAT
CAN PREVENT GENE EXPRESSION BY
BLOCKING THE ACTION OF RNA POLYMERASE
• 2) THE PROMOTER REGION IS A SEQUENCE OF
DNA TO WHICH THE RNA POLYMERASE
ATTACHES TO BEGIN TRANSCRIPTION
• 3) THE OPERATOR REGION CAN BLOCK THE
ACTION OF THE RNA POLYMERASE IF THE
REGION IS OCCUPIED BY A REPRESSOR
PROTEIN
• 4) THE STRUCTURAL GENES CONTAIN DNA
SEQUENCES THAT CODE FOR SEVERAL
RELATED ENZYMES THAT DIRECT THE
PRODUCTION OF SOME PARTICULAR END
PRODUCT
REGULATION OF A METABOLIC PATHWAY
Trp OPERON
• AN OPERON IN E.COLI, TRP OPERON
PRODUCES ENZYMES FOR THE SYNTHESIS
OF THE AMINO ACID TRYPTOPHAN. THE
REGULATORY GENE PRODUCES AN
INACTIVE REPRESSOR THAT DOES NOT
BIND TO THE OPERATOR. AS A RESULT,
THE RNA POLYMERASE PROCEEDS TO
TRANSCRIBE THE STRUCTURAL GENES
NECESSARY TO PRODUCE ENZYMES THAT
SYNTHESIZE TRPTOPHAN
• WHEN TRYPTOPHAN IS AVAILABLE TO E.COLI
FROM THE SURROUNDING ENVIRONMENT, THE
BACTERIUM NO LONGER NEEDS TO MAKE ITS
OWN TRYPTOPHAN. IN THIS CASE, RISING
LEVELS OF TRPTOPHAN INDUCE SOME
TRYPTOPHAN TO REACT WITH THE INACTIVE
REPRESSOR AND MAKE IT ACTIVE. HERE
TRPTOPHAN IS ACTING AS A COREPRESSOR.
THE ACTIVE REPRESSOR NOW BINDS TO THE
OPERATOR REGION, WHICH, IN TURN,
PREVENTS THE TRANSCRIPTION OF THE
STRUCTURAL GENES. SINCE THESE
STRUCTURAL GENES STOP PRODUCING
ENZYMES ONLY IN THE PRESENCE OF AN
ACTIVE REPRESSOR, THEY ARE CALLED
REPRESSIBLE ENZYMES
THE TRP OPERON
LAC OPERON
• THE LAC OPERON IN E.COLI CONTROLS THE
BREAKDOWN OF LACTOSE. THE REGULATORY
GENE IN THE LAC OPERON PRODUCES AN ACTIVE
REPRESSOR THAT BINDS TO THE OPERATOR
REGION. WHEN THE OPERATOR REGION IS
OCCUPIED BY THE REPROSSOR, RNA
POLYMERASE IS UNABLE TO TRANSCRIBE
SEVERAL STRUCTURAL GENES THAT CODE FOR
ENZYMES THAT CONTROL THE UPTAKE AND
SUBSEQUENT BREAKDOWN OF LACTOSE.
• WHEN LACTOSE IS AVAILABLE, SOME
OF THE LACTOSE (IN A CONVERTED
FORM) COMBINES WITH THE
REPRESSOR TO MAKE IT INACTIVE.
WHEN THE REPRESSOR IS
INACTIVATED, RNA POLYMERSE IS
ABLE TO TRANSCRIBE THE GENES THAT
CODE FOR THE ENZYMES THAT BREAK
DOWN LACTOSE. SINCE A SUBSTRATE
(LACTOSE) IS REQUIRED TO INDUCE
(TURN ON) THE OPERON, THE ENZYMES
THE OPERON PRODUCES ARE SAID TO
BE INDUCIBLE ENZYMES.
LAC OPERON