Class 10 Slides

mRNA

template for protein synthesis

tRNA

delivers amino acid to the ribosome to be added to the growing peptide

Ribosome

ribonucleoprotein complex where protein synthesis occurs--made of rRNA and proteins

Elongation factors

proteins that facilitate translation

Aminoacyl tRNA synthetases

add the amino acid to tRNAs

Initiation factors

control when/where translation begins, facilitate ribosome assembly on the mRNA

Nucleic acids

are polymers with directionality based on the ribose orientation. By convention, they are written from 5’ to 3’ (unless labeled otherwise)

Base pairing is achieved ONLY when

the 2 strands are in an anti-parallel orientation

RNA can base-pair with

another RNA, a DNA, or itself if the sequences are reverse-complements of each other

tRNAs overview

• Synthesized by RNA Pol III (not Pol II)

• Encoded by tRNA genes in the DNA

• Processed (cleaved, modified, spliced, extended)

• Fold into a 3D shape

• Attached to an amino acid by aaRS enzymes

• Base pairs with mRNA within the ribosome, bringing a new amino acid to the growing peptide

• Anticodon pairs with codon

tRNA processing

tRNAs are:

• cleaved

• extended

• spliced

• modified

  • covalent modifications to nucleotides

Each tRNA will have specific modifications based on its nucleotide sequence

Aminoacylation catalyzed by

aminoacyl tRNA synthetase (aaRS)

How does aaRS work?

Activation site vs. Editing site

Each aaRS enzyme has an active site that binds a subset of amino acids and

an editing site that removes incorrectly-added ones.

accuracy in aminoacylation

Multiple contacts between tRNA and aaRS ensure accuracy in aminoacylation

An example of editing

Ala-tRNA Synthetase

Too large to fit into the active site of the Ala-tRNA Synthetase

• Active site will exclude larger aa but may allow Glycine (smaller and also non-polar) to be added by mistake

• Editing site will be small enough to fit Glycine, but will exclude Alanine

Only Alanine will be added to the tRNA!

another example: Val-tRNA Synthetase

Too large to fit into the active site of the Val-tRNA Synthetase

• Threonine is a very close structural match, enters the active site frequently and can be added to the tRNAVal

• Editing site is polar so that the Threonine side chain (with the -OH

group) can stably bind there, while excluding non-polar Valine Only Valine will be added to the tRNA!

Aminoacyl-tRNA meets mRNA inside the ribosome

Ribosomal RNAs and proteins stabilize the interaction codon- anticodon

The codon and anticodon base pair with each other. The third position of the codon can form unusual base pairs with the first base of the anticodon (“wobble” position)