Transcript Translation - Santa Susana High School

Translation
• Translation is the process of building a protein from the
mRNA transcript. The protein is built as transfer RNA (tRNA)
bring amino acids (AA), one at a time, to the ribosomal unit
holding the mRNA.
tRNA
• 3D structure has an attachment site for
an AA on one side and the anticodon on
the other
– anticodon is the complimentary sequence
to the mRNA codon
• if the mRNA is AAA the tRNA is UUU the AA
attached is phe (phenylalanine)
– is "L" shaped with a 3' attachment site for
an AA and a loop on the other side that
carries the anticodon
• each tRNA is specific for the AA it attaches
to
• 45 different versions in the cytoplasm at all
times that correspond to the different AAs
– should be 61 for the different codons
– a few can bind to multiple codons due to a
loosely attached 3' site - wobble
• tRNA are joined to the
correct AA by the enzyme
aminoacyl-tRNA synthetase
– each synthetase fits ONLY the
tRNA and AA it is specific for
– 20 different versions in the
cytoplasm at all times
– catalyzes the attachment of
the AA to the tRNA with the
use of ATP
• the released tRNA/AA complex
is called an activated tRNA or
aminoacyl tRNA
• the activated tRNA delivers its
AA to the growing polypeptide
chain
Ribosomes
• Ribosomes facilitate the binding of the tRNA to the mRNA by
aligning the active sites and holding them in place.
• Structure
– ribosomes are made of mostly ribosomal RNA (rRNA) that is made
from DNA that resides in the nucleolus
– consists of a large unit and a small unit
• each is made of rRNA and proteins imported from the cytoplasm
• once assembled the 2 units are exported to the cytoplasm via membrane
pores in the nuclear envelope
– has 3 binding sites
• A - site binds next tRNA with AA atached
• P - site binds the tRNA whose AA is being attached to the growing chain
• E - site binds the leaving tRNA minus the AA (already attached to the growing
chain)
Initiation of translation
• Small ribosomal subunit binds with the mRNA and searches for the
start codon (AUG)
• initiation tRNA (Met) binds to the P-site of the small unit
– union of the mRNA & tRNA establishing the reading frame
• large unit attaches after the union is made completing the initiation
complex - translation begins
– proteins called initiation factors bring the units together
– energy from GTP is used in the formation of the unit
Elongation of Growing Polypeptide
– requires several proteins called elongation factors
• 3 GTP are required for each translocation steps (APE)
– mRNA moves from 5' to 3'
– AAs are added N-terminus to C-terminus with new AAs added to
the carboxyl end
– elongation ends when a stop is reached - UAG, UAA, UGA
• do not code for an AA but binds to a release factor that binds to the
adjacent A-site
• the release factor binds a water molecule to the end of the
polypeptide hydrolyzing the chain
– the ribosomal unit comes apart
– may be translated by many ribosomes at the same time ployribosomal unit
Post Translation Modifications
• proteins begin to spontaneously fold
as they are synthesized
– some will require additional proteins
(chaperonins)
• free ribosomes make proteins
destined for the cytosol
• bound ribosomes make proteins for
the membrane systems of the cell
– (Nuc Env, ER, Golgi, Lysosomes,
Vacuoles, Plasma membrane) as well
as secreted proteins (Insulin)
• ALL start as free ribosomes and bind
to the ER if a signal peptide is
present
• about 20 peptides that form the signalrecognition particle (SRP) which
attaches to the ER
• after attachment the SRP is removed
by enzymes and translation of the
membrane protein continues
• each membrane system has its own
unique SRP - much like a zip code