BIOL2020 - Chapter 27 - Translation

What is tRNA made of?

Nucleic acids and amino acids

What sequence is always on the amino acid arm of a tRNA?

“Pu C C A”

What is the distinctive part of the D arm of a tRNA?

2-3 D residues

What is the secondary structure of tRNA called?

The “Cloverleaf”

How are Amino acids linked to tRNAs?

• With a aminoacyl-tRNA enzyme

• Requires an ATP

What are Ribosomes composed of?

RNA and protein.

What are the 2 subunits of a eukaryotic ribosome called?

• 60S (large)

• 40S (small)

What are the 2 prokaryotic ribosomal subunits made of?

• 30S made of 1 rRNA(16S)

• 50S made of 2 rRNA (5S and 23S)

What are ribosomes measured by?

Sedimentation coefficient.

Which ribosome is bigger; Eukaryotic or Prokaryotic?

Eukaryotic.

What direction does translation occur in?

N to C terminus.

What are the “jobs” of the ribosomal subunits?

• Small: recognizes codons

• Large: forms peptide bond

What are the requirements of Initiation of translation?

• ribosomal subunits

• mRNA

• initiation factors +GTP

• tRNA in activated form

What do Initiation factor 1 and 3 (IF1 and IF3) do?

Keeps subunits apart and recruits mRNA

What does Initiation factor 2 + GTP do?

recruits the first tRNA.

What unique feature does the first tRNA to be added to a peptide chain contain in prokaryotes?

An N - formyl methionine.

What happens in the first step of elongation of translation?

• First codon is read

• First tRNA enters the P site

What is the EF-Tu/EF-Ts cycle?

The cycle of elongation factor Tu bringing a tRNA to the ribosome, using a GTP → GDP, and having it phosphorylated by elongation factor Ts back to GTP

What is the only step of translation that a ribosome can perform without a cofactor?

Peptide bond formation.

What happens during the second step of elongation of translation?

Peptide bond formation

What happens during the third step of elongation of translation?

• Empty tRNA moves from P site to E site

• tRNA with peptide chain moves from A site to P site

• requires EF-G

How are EF-G’s and tRNAs bonded to EF-Tu’s similar?

Shape.

How does termination of translation occur?

• release factor binds when stop codon is read

• add water instead of an amino acid

• protein is released and subunits dissociate

What is a polysome?

mRNA with multiple ribosomes translating it

What does post translational modifications affect?

• Activity

• Folding

• Stability

• Subcellular location

What are the most common posttranslational modifications?

• Proteolytic cleavage

• Amino acid modification

• Attachment of carbohydrates

• Attachment of prosthetic groups

What posttranslational modifications are necessary to target a protein to the ER?

• recognition + cleavage of a peptide signal sequence by a signal recognition particle

What is ubiquitin attachment and what is its purpose?

• post translational modification

• targets protein for degradation by a proteosome

How does ubiquitin target proteins for degradation?

• activated by E1 activating enzyme via ATP hydrolysis

• binds Ub C-terminus to a cysteine on E1

• transferred to an E2 conjugating enzyme and Ub is target to a lysine on target protein by a E3 ligase

• cycle repeats

• protein is digested when proteosome uses poly-ubiquitin chain to feed protein into its core