Molecular Biology of Protein Synthesis: Translation, Structure, and Genetic Code

Translation

The process of protein synthesis from an mRNA template; converts genetic information into a sequence of amino acids.

Amino acids

The building blocks of proteins; there are 20 standard amino acids.

Protein characteristics

High molecular weight, contains nitrogen, and has a complex shape and composition.

Primary structure

The linear sequence of amino acids in a protein.

Secondary structure

Folding or twisting of a polypeptide chain due to hydrogen bonding (e.g., α-helix, β-pleated sheet).

Tertiary structure

Three-dimensional conformation of a single polypeptide chain determined by R-group interactions.

Quaternary structure

Complex formed by multiple polypeptide chains in multi-subunit proteins.

Protein function rule

Protein shape determines its function.

Central dogma

DNA → RNA → Protein; transcription followed by translation.

Triplet code

Each codon of three nucleotide bases codes for one amino acid.

Continuous code

Codons are read sequentially without skipping or overlapping bases.

Universal code

The genetic code is nearly the same across all organisms.

Degenerate code

More than one codon can code for the same amino acid.

Start codon

AUG; codes for methionine and signals the start of translation.

Stop codons

UAA, UAG, and UGA; signal the end of translation (nonsense codons).

Frameshift mutation

Mutation caused by addition or deletion of bases not in multiples of three, altering the reading frame.

Ribosome

Organelle made of large and small subunits (protein + rRNA); site of translation.

tRNA (transfer RNA)

Transfers specific amino acids to the ribosome during translation; each has an anticodon complementary to an mRNA codon.

Aminoacylation (charging)

Process of attaching the correct amino acid to tRNA by aminoacyl-tRNA synthetase.

Aminoacyl-tRNA synthetase

Enzyme that charges tRNA with its corresponding amino acid; one for each of the 20 amino acids.

Wobble hypothesis

Flexibility in base pairing between the third base of the codon and anticodon, allowing some codons to code for the same amino acid.

Initiation of translation

Stage where the small ribosomal subunit binds mRNA and the initiator tRNA binds to the AUG start codon, followed by large subunit joining.

Elongation

Stage where the ribosome moves along mRNA, peptide bonds form, and the polypeptide chain grows.

Termination

Stage when a stop codon is reached, release factors bind, and the completed polypeptide is released.

Polysome (polyribosome)

Complex of multiple ribosomes simultaneously translating a single mRNA molecule.

Protein sorting

Process directing proteins to their correct cellular locations (e.g., ER, nucleus, mitochondria, lysosome).

Signal sequence

Short amino acid sequence that directs a protein to its destination within the cell.

Signal recognition particle (SRP)

Protein-RNA complex that binds signal sequences and guides ribosomes to the ER membrane.

Crick, Barnett, Brenner & Watts-Tobin experiment

Showed that the genetic code is read in triplets by observing that three base additions restored the reading frame.

Nirenberg & Khorana experiment

Used synthetic mRNAs in cell-free systems to determine which codons corresponded to specific amino acids.

Nirenberg & Leder experiment

Used ribosome binding assays to determine exact codon-amino acid pairings.

tRNA genes

Found in multiple copies; transcribed by RNA Polymerase III in eukaryotes and by one RNA polymerase in bacteria.

Precursor tRNA

Initial transcript processed to mature tRNA with CCA sequence at the 3′ end.

rRNA genes (rDNA)

Genes encoding rRNA; organized in transcription units containing 16S/18S, 5.8S, and 28S rRNA sequences.

rRNA transcription

Performed by RNA Polymerase I in eukaryotes; yields precursor rRNA containing spacer sequences.

Spacer sequences

Non-coding regions between rRNA sequences removed during processing.

Ribosomal assembly

Ribosomal proteins associate with rRNA during processing to form functional ribosomal subunits.

Summary of translation flow

DNA → mRNA → Ribosome → tRNA (with amino acid) → Polypeptide → Folded protein → Sorted to destination.