translation Flashcards
why is it important that ribosomes consist of 2 RNP structures
-Ribosomes are large, abundant RNP particles (always structurally similar) consisting of two unequal subunits. Ribosome structure is highly conserved
-decoding site= mRNA recognised here
-peptidyltransferase centre= where peptide bond synthesis
-large subunit and small subunit- protein synthesis accurate interface between the 2 subunits
-Codon/anticodon binding occurs on the small subunit. Peptide bond formation occurs on the large subunit.
-Three nonoverlapping tRNA binding sites (A, P and E) are found at the subunit interface.
-The PTC is RNA-rich. Peptide bond formation is an RNA-catalysed reaction
what’s ribosomes synthesis in eukaryotes like
-Ribosome synthesis is one of the most energetically expensive cellular processes. It is tightly coupled to cell growth and requires hundreds of different proteins and RNAs.
-rRNA transcription and early rRNA processing steps occur in nuclear structures called nucleoli. Later processing and assembly steps occur in the nucleoplasm and cytoplasm.
-Functionally active ribosomes are generated after export from the nucleus
-ribosomes synthesised in nucleolus
what happens in the translation elongation cycle
-Two tRNAs are bound to the ribosome at any given time during translation elongation. -The tRNAs are bound in adjacent A and P, or P and E sites- ribosome switches between 2 different structures which is called translocatocation
-Translation elongation involves reiterative cycles of
=aminoacyl-tRNA binding
=peptide bond formation
=translocation of the ribosome along the mRNA.
-Upon binding of aminoacyl-tRNA to the A site, tRNA is released from the E site
-now the next codon on mRNA is ready to be recognised by the tRNA
what role does GTPase play in translation elongation
-GTP= nucleotide triphosphate
-aminoacyl-tRNA is brought to the ribosome by the elongation factor EF1A(protein that recognises charged tRNA molecules and hydrolyses ATP when bound) (EF-Tu in prokaryotes).
-Translocation requires another elongation factor 2, EF2 (EFG in prokaryotes- GTP->GDP).
-EF1A and EF2 are GTPases.
-2 GTP molecules are hydrolysed per elongation cycle.
how does A specific Initiator tRNA bind to the start codon
-initiated codon always recognised by tRNA with methane (AUG)
-EF1A brings tRNAs to the A site. How is tRNA brought to the start codon?
-Cells contain two distinct methionyl-tRNAs.
-Elongator methionyl-tRNAs bind to internal AUG codons and are associated with EF1A (EF-Tu in prokaryotes). A distinct initiator methionyl-tRNA recognises the AUG start codon and is associated with eukaryotic initiation factor 2 (eIF2, IF2 in prokaryotes).
-eIF2, like EF1A, is a GTPase
how is the start codon positioned in the P site in regards to the 16s/mRNA base-pairing in prokaryotes
-The Shine-Dalgarno (SD) sequence within prokaryotic mRNAs is recognised by base-pairing with nucleotides at the 3’ end of the 16S rRNA.
what happens with Translation initiation in eukaryotes
-The initiator tRNAMet (bound to eIF2), together with the small subunit, is assembled at the 5’ end of the mRNA through interaction with the cap-binding complex (CBC).
-The preinitation complex scans along the mRNA, using the helicase activity of CBC, until it finds an AUG codon within an appropriate sequence context: the Kozak sequence.
-After selection of the start codon, the large subunit is recruited.
what happens with translation termination
-Stop codons are recognised by protein termination (or release) factors, not tRNAs.
-Two release factors act together to cause termination.
-Initial binding of eukaryotic release factor 1 (eRF1, RF1 or RF2 in prokaryotes) triggers peptide hydrolysis.
-Subsequently, eRF3 (RF3 in prokaryotes) allows release of RF1/RF2 from the ribosome. RF3 is a GTPase
-GTP hydrolysing enzymes needed in translation
