cell and molecular lecture 6: translation

central dogma

- DNA -> RNA -> protein

- transcription copies genetic information from DNA into RNA

- translation decodes the mRNA nucleotide sequence to produce a specific amino acid sequence

mRNA

- messenger RNA

- the message

- contains genetic code

- carries the nucleotide sequence that specifies the amino acid sequence of a protein

tRNA

- transfer RNA

- the adaptor

- matches an mRNA codon with its corresponding amino acid

rRNA

- ribosomal RNA

- the machinery

- forms the structural and catalytic core of the ribosome

codon

- a sequence of 3 nucleotides in mRNA that species an amino acid or a stop signal

- ex: 5'-AUG-3' (specifies methionine, also serves as the start codon)

the genetic code is nonoverlapping

- each nucleotide is read as part of only one codon at a time

the genetic code is degenerate

- most amino acids are specified by more than one codon

- different codons, same amino acids

the genetic code is universal

- with a few exceptions, organisms use the same genetic code

- the same codon will equal the same amino acid for each organism

tRNA features

- tRNA acts as an adaptor

- anticodon: recognizes a codon in mRNA and acceptor stem: carries the corresponding amino acid

tRNA charging

- aminoacyl-tRNA synthetases recognize a specific amino acid and its corresponding tRNA

- the amino acid is attached to the 3' acceptor stem of the tRNA

- charging links the correct amino acid to its tRNA

the wobble position

- one tRNA can recognize multiple codons

- wobble occurs at the 3' base of the mRNA codon and the 5' base of the tRNA anticodon

- flexible base pairing at the wobble position allows one tRNA to recognize more than one codon

translation: initiation

- assemble the preinitiation complex: 40S ribosomal subunit + initiator tRNA (Met) + GTP-bound initiation factors

- recruit the complex to the mRNA: initiation factors recognize the 5' cap and help recruit the preinitiaiton complex

- scan for the start codon: the complex scans 5' -> 3' until the initiator tRNA recognizes an AUG start codon

translation: initiation continued

- recognize the start codon: the initiator tRNA base-pairs with AUG

- hydrolyze GTP: start codon recognition triggers GTP hydrolysis and release of initiation factors

- assemble the complete ribosome: the 60S subunit joins the 40S subunit, forming the 80S ribosome

translation: elongation

- select the next tRNA: a charged tRNA is delivered to the A site by a GTP-bound elongation factor, correct codon + anticodon pairing = GTP hydrolysis

- form the peptide bond: the growing polypeptide is transferred from the tRNA in the P site to the amino acid on the tRNA in the A site

- translocate: another GTPase moves the ribosome one codon along the mRNA

- tRNAs shift A -> P -> E

translation: termination

- recognize the stop codon: a stop codon enters the A site, there is no corresponding tRNA

- recruit a release factor: a release factor binds the stop codon in the A site

- release the polypeptide: the bond linking the polypeptide to the tRNA is hydrolyzed, releasing the complete protein

- disassemble the ribosome: GTP hydrolysis helps drive termination and recycling of the translation machinery