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Translation steps

1. fMet tRNA finds AUG
2. 16s rRNA on 30s subunit finds SD sequence
3. 50s comes into contact with 30s and forms complete 70s ribosome
4. Translation begins and continues until stop codon reached


When 50s subunit combines with 30s subunit what happens with tRNA?

fMet tRNA starts off in p site of ribosome


A site

Accepts incoming charged tRNAs


P site

Peptide site; growing protein


E site

Exit site where uncharged tRNAs exit ribosome


Translation continues until

Stop codon is in the A site


What is needed to form the peptide bonds to attach 1 amino acid to another?



What do stop codons code for?

Not an amino acid so the ribosome A site sits empty, ribosome stalls and dissociates


Function of 16s rRNA gene

- shine delgarno sequence flags ribosome and it has a complimentary sequence to the conserved region in 16s rRNA
- part of the 30s subunit of ribosome
- conserved regions involved in recognizing and binding to SD sequence at the 5' end of mRNA that is coming off of transcription


Genetic exchange

- conjugation: direct cell to cell contact via sex pilus; exchanges plasmid DNA
- transformation: cells pickup naked DNA from enviro.
- transduction: viruses serve as vehicle for introduction of DNA into cell


DNA binding proteins

DNA transport protein to bring DNA into the cell
DNA is brought into the cell as ssDNA and bound to single-stranded binding proteins to protect it


Rec A

binds ssDNA and incorporates it into chromosomes of the cell


F plasmid

- plasmids containing tra genes
- contains genes for variety of functions eg. antibiotic resistance


Tra genes

code for the enzymes used in formation of sex pilus



donor; has F plasmid and tra genes



recipient; no F plasmid and no tra genes


Can conjugation occur between 2 donors?



Conjugation steps

- pilus forms
- pilus retracts slightly
- F plasmid nicked in one strand
- transfer of one strand from F+ to F-; F plasmid simultaneously replicated in F+
- synthesis of complimentary strand in recipient cell
- completion of DNA transfer and synthesis; cells separate



bacterial virus


Viral DNA codes for...

nuclease designed to destroy host chromosome and replication of itself


How is transducing virus created?

host cell packages DNA in capsid but puts piece of host chromosome in instead of viral NA


How does transducing virus DNA get inserted into chromosome?

Rec A proteins



- redirect host metabolism and resources to create more virus
- no metabolism themselves
- every known organism/cell type has viruses that infect it


Morphology/structure of virus

- 0.02-0.3 um diameter (TEM best to view)
- RNA or DNA genomes or hybrid
- Variable genome structure (dsRNA, dsDNA, dsRNA:RNA, ssRNA, etc. linear, circular, segmented, etc.)
- viral genome replication occurs independently of host cell/genome replication
- capsid



- protein coat/structure designed to protect genetic material and recognition of infection of host cell
- helical
- icosahedral
- complex


helical capsid

- individual protein subunits come together to form helix, subunits wrap around a ss genome
- pushes genome into host cell via unwinding genome throuhg cell layers
- eg. tobacco mosaic virus


icosahedral capsid

- flat surfaces and corners, highly stable structure
- spontaneously forms inside host cells
- capsid docks on host cell and experiences protein conformation change that push viral genome into host cell
- can have membrane or cell wall degrading enzymes attached to introduce holes which the genome can enter through


complex capsid

- tail fibers find appropriate receptor on host cell surface, change conformation to contract tail, syringe protein pushed through cell layers to release viral genome into cytoplasm



- some viruses have envelopes
- made out of host cell membrane
- picked up as virus buds out
- envelope is not metabolically maintained
- short half-life outside of host cell
- susceptible to degredation
- w/o envelope virus no longer infective
- masks virus to evade immune system


Some viruses carry around enzymes in capsid

- HIV - reverse transcriptase RNA --> DNA --> RNA
- other viruses: RNA --> RNA RNA-dependent RNA polymerase
- seen with genomes that otherwise cannot be replicated by host cell machinery
- can include enzymes for entry or exit from host:
- neuraminidases - enzymes that breakdown membranes
- lysozyme - enzyme that degrades cell walls


Viral replication cycle

1. recognition and attachment to host cell
2. penetration
3. replication
4. assembly and packaging
5. release of virus from host cell


recognition and attachment to host cell

- protein: protein biding between virus and host
- host cell proteins are not virus specific, carry out normal function for host
- irreversible connection



viral capsid or envelope facilitates delivery of viral genome into host cell


replication (viral)

1. early replication: viral nucleic acid alter host cell machinery; shuts down host cell processes
2. late replication: active viral genome replication and capsid synthesis, and synthesis of viral enzymes


assembly and packaging

packaging of viral NA into capsids


Release of virus from host cell

- eg. envelope viruses bud out of host cell
- eg. some viruses degrade membrane/cell wall
- eg. some viruses cause host cell lysis (host cell is too full)


Burst size

number of viruses released by an infected host cell; varies from virus to virus and host cell to host cell
- larger burst size = higher number of viruses made by host cell


latent, lysogenic, temperate viruses

- can enter state of lysogeny where viral NA is not being expressed (no early replication)
- at some point in lysogeny become activated and enter lytic state (active viral infection)
- viral genome will get copied by host cell as host cell replicates



- non-enveloped icosahedral virus
- infects CNS cells
- also infects GI cells - spread via fecal:oral route
- RNA genome virus: RNA --> RNA - RNA dependent RNA polymerase
- genome = VpG 5' ssRNA
- one of first proteins translated by host ribosome is 2A protease and it looks for 7mg 5' and cuts it off, rendering euk. mRNA unrecognizable to ribosomes
- once host cell stopped from translating own mRNA, only get viral synthesis