Translation

What is translation?

The directed synthesis of polypeptides based on the nucleotides sequence of mRNA.

Where does translation occur?

In the ribosome— the site where mRNA is translated into protein

In which direction are proteins synthesized?

from the N-terminal (amino acid) to the C-terminal (carboxyl end)

What is coupling in bacteria and archaea?

Transcription and translation occur simultaneously since there’s no nucleus in prokaryotes

What is a polyribosome (polysome)?

A complex of one mRNA with several ribosomes translating it at once.

What structure does tRNA have?

A tertiary structure due to interal base pairing

What sequence is found at the 3’ end of all tRNAs?

The CCA sequence, which is where amino acids are attached (acceptor stem)

What is the anticodon?

A sequence of three nucleotides in tRNA that base pairs with a complementary codon in mRNA during protein synthesis.

What enzyme attaches an amino acid to its tRNA?

aminoacyl-tRNA synthetase

How many synthetases exist?

at least 20, each specific for one amino acid

What is the structure of a bacterial ribosome?

70s total= 30s (small) + 50s ( large) subunits

What are the functional domains of a ribosome?

Translational domain (for translation) and exit domain (for tRNA release)

What is the role of 16s rRNA?

Helps bind mRNA’s shine-dalgarno sequence to the 30s subunit

What initiator tRNA is used in bacteria?

N-formylmethionine-tRNA ( fMet-tRNA)

What sequence aligns the start codon with the ribosome?

The shine-dalgarno sequence of mRNA binds the 16s rRNA

What are the three main steps of elongation?

Aminoacyl-tRNA binding, transpeptidation, and translocation of the ribosome.

What occurs in the transpeptidation reaction?

The 23a rRNA acts as a ribozyme and the amino group of A-site amino acid reacts with the carboxyl group of the P-site amino acid.

What happens during translocation?

Ribosomes move along mRNA one codon at a time using elongation factors (EFs) and GTP

What elongation factors are used in bacteria?

EF-TU: brings aminoacyl-tRNA to the A site

EF-G: moves the ribosome to the next codon.

What does the P-site do?

Holds the growing peptide chain

What does the A-site do?

Accepts the incoming aminoacyl-tRNA

What does the E-site do?

Releases the empty tRNA

What triggers translation termination?

A stop codon (UAA, UAG, UGA) entering the A site

What recognizes a stop codon?

Release factors that promote termination of translation.

How many release factors exist?

Three in prokaryotes and one in eukaryotes

What energy source is needed for termination?

GTP hydrolysis

Why is coupling efficient in prokaryotes?

Ribosomes cna begin translocation before transcription is finished, speeding up protein production.

What is protein splicing?

Removal of certain polypeptide portions (inteins), leaving functional regions (exteins)

What do molecular chaperones do?

Help fold proteins correctly and protect them from heat damage

What is translocation?

movement of proteins from the cytoplasm to the plasma membrane or periplasm

How does streptomycin inhibit translation?

Inhibits 70S ribosome function

How does tetracycline inhibit translation?

Binds to the 30S subunit, blocking tRNA binding

How do chloramphenicol and erythromycin act?

Bind to 23S rRNA of the 50s subunit, blocking peptide bond formation.

How does rifamycin B inhibit transcription?

Binds bacterial RNA polymerase, blocking the initiation of transcription.

How does actinomycin D affect transcription?

Binds to DNA nonspecifically, preventing RNA synthesis