BIOL 2500 - Topic 2 (part 10)

3 functions of ribosomes

1.) Bind mRNA and identify the start codon for translation

2.) Facilitate complementary base-pairing between mRNA codons and tRNA anticodons

3.) Catalyze peptide bond formation

Prokaryotic vs. eukaryotic ribosomes

They primarily differ based on the number of their ribosomal proteins and the sequence of the rRNA molecules, but they share structural homology

Structural homology of ribosomes throughout the 3 domains

Large ribosomal subunit and small ribosomal subunit, both of which are measured in Svedberg units

Svedberg unit

It is not directly proportional to weight and instead focuses more on size, such that the higher the S, the larger the molecule

tRNA binding sites of the ribosome

1.) A site (aminoacyl-tRNA binding site)

2.) P site (peptidyl site)

3.) E site (exit site)

A site

Site where incoming charged tRNA binds to

P site

It is where the peptide bond between the amino acids form and where the amino acid disconnects from the tRNA

E site

Site where deacylated (i.e. uncharged) tRNA are released from the ribosome

Extra centers of the ribosomes

1.) Decoding center

2.) Peptidyl-transferase center

Decoding center

Domain in the ribosome that ensures that only the tRNA with the right anticodon enters the A site

Peptidyl-transferase center

Part of the large ribosomal subunit that forms the peptide bonds

Initiator codon

The first amino acid is always methionine, with the initiation codon AUG

Initiator tRNAs

Both bacteria and eukaryotes have 2 kinds of tRNAs that code for methionine, one that holds the normal methionine amino acid and one that specifically holds the starting methionine

Methionine tRNAs in bacteria

tRNA^Met

tRNA^fMet

Methionine tRNAs in eukaryotes

tRNA^Met

tRNA_i^Met

Identifying the initiator codon

The initiator codon can be found in the 5’-UTR region of the mRNA, but how it is found differs between bacteria and eukaryotes

Identifying the initiator codon in bacteria

The ribosome binds to the RBS, which positions the small subunit to be in the correct spot to align the start codon into the P-site

Identifying the initiator codon in eukaryotes

The ribosome looks for the Kozak sequence, which is a sequence that contains the AUG start codon