Ribosome Structure and Prokaryotic Translation Initiation

Comprehensive practice flashcards covering prokaryotic ribosome structure, translation speeds, subunit composition, and the detailed steps of initiation based on lecture notes.

What is the primary function of ribosomes as described in the lecture notes?

They are molecular machines that catalyse peptide bond formation directed by information in mRNA.

Which enzyme connects the correct amino acids to transfer RNAs (tRNAs) before they reach the ribosome?

The tRNA synthetases.

Which two high-resolution structural techniques have been used to change our structural knowledge of ribosomes?

Crystallography and cryo electron microscopy (cryoEM).

According to Figure 32-35, which organism was used for the X-Ray structure of the 70S ribosome?

T. thermophilus.

Why must ribosomes in polysomes operate at a slower speed than RNA polymerase?

To prevent collisions.

What is the translation speed in bacteria according to the transcript?

15 aminoacids/s

What is the transcription speed in bacteria according to the transcript?

$$50\,nucleotides/s$$

How does translation elongation speed in eukaryotes compare to bacteria?

It is slower, at only about 2 amino acids/s.

What does the term "polycistronic" mean in the context of prokaryotic messenger RNA?

It can encode multiple proteins simultaneously.

What is the approximate molecular mass of the prokaryotic ribosome?

$$2.5\,megadalton$$

How are the subunits of the prokaryotic ribosome named, and what are their specific units?

30S (small subunit) and 50S (large subunit), using Svedberg units (S).

What percentage of the prokaryotic ribosome's mass is ribosomal RNA (rRNA)?

$$66\%$$

What percentage of the prokaryotic ribosome's mass is protein?

$$40\%$$

Which ribosomal RNA (rRNA) components and how many proteins make up the 50S subunit?

5S rRNA, 23S rRNA, and 34 proteins.

Which ribosomal RNA (rRNA) component and how many proteins make up the 30S subunit?

16S ribosomal RNA and $21$ proteins.

What is the role of the 30S subunit during translation?

Crucial for correctly positioning the messenger RNA and placing the initiator tRNA.

What is the binding order for the components involved in the translation cycle before the large subunit docks?

The small subunit (30S) binds the messenger RNA and the initiator tRNA first.

What are the three defined stages of the translation process?

Initiation, Elongation, and Termination.

What happens to the polypeptide chain as it exits the ribosomal channel?

It immediately begins folding.

List the components required for the initiation of translation.

Small ribosomal subunit, mRNA, Initiation factor, fmet-tRNAfMet, GTP, and the Large subunit.

What occurs at the A site (Acceptor or Aminoacyl site) of the ribosome?

Incoming tRNAs bind to it.

What occurs at the P site (Peptidyl site) of the ribosome?

Peptide bond formation typically takes place.

What occurs at the E site (Exit site) of the ribosome?

Spent tRNAs drop out of the ribosome.

What specific mRNA sequence is responsible for accurate AUG codon placement in bacteria?

The Shine-Dalgarno sequence (or ribosomal binding site).

Which ribosomal component base pairs with the mRNA's Shine-Dalgarno sequence?

The 16S ribosomal RNA of the small subunit.

What are the chemical characteristics of the sequence pairing between the 16S rRNA and the mRNA?

The mRNA contains a G-rich sequence which pairs with a C-rich sequence on the 16S rRNA.

Who discovered the G-rich sequences while working in Linn Delgado's lab?

John Shine.

What is the N-terminal residue of prokaryotic polypeptides?

Formylated methionine (fMet).

Why does the formyl group on fMet help in the positioning of tRNA?

It mimics a peptide bond, ensuring the tRNA is correctly placed in the ribosome.

How does the human immune system treat proteins starting with formylated methionine?

It recognises it as foreign and is primed to attack.

Which enzyme is responsible for formylating the methionine on the initiator tRNA?

Transformylase.

How are translation factors (IFs) defined relative to their permanence in the ribosome?

They are temporary proteins that cycle on and off, not permanent components of the ribosome.

What is the specific function of Initiation Factor 3 (IF3)?

It binds to the small 30S ribosomal subunit to free it from the large 50S subunit after translation.

What are the two roles of Initiation Factor 1 (IF1)?

It assists IF3 binding and occludes the A site to ensure the initiator tRNA binds to the P site.

What is the role of Initiation Factor 2 (IF2)?

It is a GTP binding protein that binds the initiated tRNA and helps it dock onto the P site.

What specific complex is formed when IF-2-GTP binds the initiator tRNA?

It binds fMet-tRNAfMet to help it dock with the small ribosomal subunit.

Is a specific large subunit required to assemble with the same small subunit every cycle?

No, they are interchangeable.

What happens to the initiation factors after the large subunit docks onto the small subunit?

They leave the complex.

According to the diagram on Page 4, how many nucleotides of space are between the G-rich sequence and the AUG?

$$9\,nucleotide\,space$$

What is the sequence on the 16S rRNA that is complementary to the Shine-Dalgarno sequence as shown in the diagrams?

CCUCC