BIOL303 Translation

Proteins

Polymers composed of amino acids linked together via peptide bonds to form polypeptide chains

Amino Acid Structure

Contains a central carbon, carboxyl group, amino group, and an R group or side chain

Common Amino Acids

There are 20 common amino acids

Primary Structure

The sequence of amino acids in a polypeptide chain

Secondary Structure

Folds and twists due to neighboring amino acid interaction; forms α-helices and β-sheets

Tertiary Structure

The 3D shape of a polypeptide chain

Quaternary Structure

Two or more polypeptide chains associating with one another

Domain

A group of amino acids that form a discrete functional unit

Codon

A set of 3 nucleotides that encode a single amino acid

Codon Breakdown

64 possible codons (4^3 total): 61 sense codons encoding amino acids and 3 stop codons

Degenerate Code

The genetic code contains more codons than are needed to specify the 20 amino acids

Synonymous Codons

Different codons that specify the same amino acid

tRNA Function

Serves as an adaptor molecule that brings amino acids to the ribosome

Isoaccepting tRNAs

Different tRNAs (30–50 in most cells) that accept the same amino acid

Wobble Hypothesis

Flexibility in hydrogen bonding at the 3rd nucleotide position of a codon, allowing an anticodon to pair with different codons

Reading Frame

Three potential ways a sequence of nucleotides can be read in sets of three

Nonoverlapping Code

Property stating that genetic codons do not overlap

Initiation Codon

AUG; establishes the correct reading frame and codes for methionine

Stop Codons

UAA, UAG, and UGA; do not have a tRNA that pairs

Universality

The genetic code is mostly, but not completely, universal across organisms

3' CCA Sequence

Conserved sequence at the 3' end of all tRNAs where amino acids attach to adenine

Aminoacyl-tRNA Synthetases

Enzymes that attach amino acids to their corresponding tRNAs

tRNA Charging

The ATP-dependent reaction that attaches an amino acid to its tRNA

IF-3

Binds the small ribosomal subunit in bacteria and prevents the large subunit from binding during initiation

IF-1

Ensures appropriate initiator tRNA binding, prevents premature subunit joining, and stabilizes IF-2 in bacteria

IF-2

Complexes with GTP to attach the initiator tRNA (tRNAifMet) to the start codon in bacteria

30S Initiation Complex

Bacterial complex consisting of the small ribosomal subunit, mRNA, initiator tRNA, GTP, and initiation factors

70S Initiation Complex

Functional complex formed when IF-3 dissociates and the large subunit joins

Cap-Binding Complex (CBC)

Binds the 5' cap in eukaryotes, aids mRNA nuclear export, and promotes the pioneering round of translation

eIF-4E

Replaces CBC after the initial round of translation to promote continued translation in eukaryotes

Poly(A) Tail Function

Interacts with 5' cap-binding proteins to enhance small ribosomal subunit binding

Ribosomal tRNA Sites

Aminoacyl (A) site, Peptidyl (P) site, and Exit (E) site

tRNA Entry Rules

Initiator tRNA binds directly to the P site; all other tRNAs enter at the A site

EF-Tu

Joins GTP and a charged tRNA to enter the A site, then hydrolyzes GTP and releases EF-Tu-GDP

EF-Ts

Regenerates active EF-Tu-GTP from EF-Tu-GDP

Peptidyl Transferase

rRNA in the large subunit catalyzes peptide bond formation between amino acids in the P and A sites

P Site Release

Formation of the peptide bond releases the amino acid in the P site from its tRNA

Translocation

Ribosome movement down mRNA requiring elongation factor G (EF-G) and GTP

E Site Shift

Ribosome movement shifts the P-site tRNA to the E site, where it exits into the cytoplasm

Termination Trigger

Protein synthesis ends when the ribosome reaches a stop codon in the A site

Release Factors

Proteins that bind to the ribosome and stimulate release of the polypeptide chain, tRNA, and mRNA

Bacterial Release Factors

RF-1, RF-2, and RF-3

Eukaryotic Release Factors

eRF-1 and eRF-2

Polyribosomes

An mRNA molecule translated by multiple ribosomes at once

Expressome

Complex formed by RNA polymerase and the first ribosome in prokaryotes, linking transcription and translation

mRNA Surveillance

Mechanisms that detect and deal with errors in mRNAs during translation

Nonsense-Mediated Decay

Eukaryotic process that eliminates mRNA containing a premature stop codon

tmRNA

Bacterial transfer-messenger RNA (with tRNA and mRNA parts) that frees stalled ribosomes

Nonstop mRNA Decay

Eukaryotic mechanism for resolving ribosomes stalled on mRNAs lacking a stop codon

No-Go Decay (NGD)

Eukaryotic mechanism that removes ribosomes stalled by secondary structures, chemical damage, premature stop codons, or defects