Eukaryotic Translation: Ribosomes, tRNA, and Protein Synthesis

Vocabulary-style flashcards covering the structural components of proteins, ribosomes, tRNA function, and the fidelity of aminoacyl-tRNA charging.

Proteins

Polymers constructed out of $20$ different amino acids, which serve as the monomers.

Peptide bonds

Covalent amide bonds that link individual amino acids together in linear chains through a dehydration reaction.

N-terminus

The amino end of a polypeptide chain.

C-terminus

The carboxyl end of a polypeptide chain.

Targeting signal

A ‘postal code’ sequence in a protein that directs it to the correct cellular organelle.

Nuclear Localization Signal (NLS)

A short amino acid sequence rich in positively charged residues (lysine and arginine) that directs proteins to the nucleus.

Importin

Proteins that recognize and bind the NLS to transport proteins through the nuclear pore complex.

Peptide

A short chain of amino acids linked by peptide bonds containing fewer than $20$ to $30$ residues.

Polypeptide

Longer chains of amino acids, often $200$ to $500$ residues long.

Titin

The longest described muscle protein containing more than $35,000$ amino acid residues, providing elasticity and stabilizing myosin.

AUG

The start codon specifying the amino acid methionine (Met) where synthesis of all polypeptide chains begins.

Stop codons

The three codons (UAA, UGA, and UAG) that terminate translation and do not specify an amino acid.

Ribosomes

The molecular factories for protein synthesis that provide the environment for interaction between mRNA and aminoacyl-tRNAs.

Free ribosomes

Ribosomes located in the cytosol that synthesize proteins intended for use inside the cell, such as enzymes and hormones.

ER-bound ribosomes

Ribosomes attached to the outside of the endoplasmic reticulum that synthesize proteins for export, use in lysosomes, or internal cell use.

80S Ribosome

The eukaryotic ribosome consisting of a large ($60S$) subunit and a small ($40S$) subunit.

60S Subunit

The large ribosomal subunit consisting of $28S$ rRNA, $5.8S$ rRNA, $5S$ rRNA, and more than $50$ ribosomal proteins.

40S Subunit

The small ribosomal subunit consisting of $18S$ rRNA and over $30$ ribosomal proteins.

Ribozyme

A catalytic RNA molecule, such as the $28S$ rRNA, which is responsible for peptide bond formation.

P-site (Peptidyl site)

The ribosomal active site that holds the aminoacyl-tRNA and the growing chain of amino acids.

A-site (Acceptor site)

The ribosomal active site that receives the tRNA with the next amino acid to be added to the chain.

E-site (Exit site)

The ribosomal active site that releases the used uncharged tRNA back into the cytoplasm.

Transfer RNA (tRNA)

The smallest major species of RNA ($74$ to $95$ nucleotides) showing a cloverleaf secondary structure that transfers amino acids to the ribosome.

Modified bases

Unique nucleotides in tRNA formed from the modification of regular nucleotides, such as inosinic acid or pseudouridylic acid.

Acceptor stem

The part of the tRNA where the amino acid is attached; it contains the conserved sequence ACC.

Anticodon

A three-nucleotide sequence in tRNA that base-pairs complementarily with a codon in mRNA.

Wobble Hypothesis

Crick's proposal that non-canonical base pairing occurs at the third base of a codon and the first base of an anticodon.

Aminoacyl-tRNA synthetase (aaRS)

The enzyme that catalyzes the attachment of a specific amino acid to its corresponding tRNA in a two-step reaction using ATP.

tRNA charging

The process of adding an amino acid to the acceptor arm of a tRNA molecule, resulting in an aminoacyl-tRNA.

Editing pocket

A deep cleft in some aminoacyl-tRNA synthetases used to proofread and remove incorrectly charged amino acids to ensure high fidelity.