Gene Expression ② Translation and Protein Synthesis

The information transcribed into mRNA is converted into an amino acid sequence through translation, and a protein is synthesized. Proteins carry out the actual functions of life.

Basic Knowledge

Translation takes place on the ribosome, an RNA-protein complex in the cytoplasm.
Process of translation:
① mRNA binds to a ribosome.
② tRNA (transfer RNA) carries an amino acid and uses its anticodon to bind to the complementary base sequence of an mRNA codon.
③ Amino acids are joined by peptide bonds on the ribosome.
④ The codons are read one after another, and translation ends at a stop codon (UAA・UAG・UGA).
A codon is a three-base code that specifies one amino acid. Since it consists of groups of three from four bases, there are 4³=64 possibilities. These specify 20 types of amino acids, and multiple codons may specify the same amino acid (degeneracy). The start codon is AUG (methionine).
A protein is a macromolecule formed when a chain of many amino acids (polypeptide) folds into a three-dimensional structure. Combinations of 20 amino acids produce unlimited variation, allowing proteins to perform diverse functions as enzymes, antibodies, structural materials, receptors, hormones, and more.

Further Study

The genetic code table is almost universal among all organisms. Except for a few cases, humans, E. coli, and pine trees use the same codons. This is strong evidence that all organisms evolved from a common ancestor. For example, “GCU,” “GCC,” “GCA,” and “GCG” all specify alanine (degeneracy). Thus, a one-base mutation may occur without changing the amino acid (synonymous substitution).
The chemistry of translation: Amino acids are linked on the ribosome by peptide bonds (-CO-NH-, a type of amide bond). This reaction is catalyzed by rRNA on the ribosome, which acts as a ribozyme (an RNA enzyme). The discovery that a reaction thought to be catalyzed by a protein is catalyzed by RNA supports the RNA-world hypothesis, which proposes that life originated with RNA.
The synthesized polypeptide folds into a three-dimensional structure (folding). There are four levels: primary structure (amino acid sequence), secondary structure (α-helix・β-sheet), tertiary structure (three-dimensional structure), and quaternary structure (multiple subunits). Incorrect folding causes loss of function and may sometimes form amyloid fibers, causing neurodegenerative diseases such as Alzheimer's disease and prion diseases.
The one gene-one enzyme hypothesis (1941, Beadle and Tatum, experiments with red bread mold) was an important early hypothesis in molecular biology. Today, it is known to be imprecise because of the one gene-one polypeptide concept and alternative splicing.

Points to Note

① Do not confuse the codon (on mRNA) with the anticodon (on tRNA). They bind complementarily. ② A codon consists of three bases and specifies one amino acid. 4³=64 codons specify 20 types of amino acids, with degeneracy. ③ Start codon AUG = methionine; stop codons UAA・UAG・UGA do not specify amino acids. No tRNA corresponds to a stop codon. ④ Distinguish translation (mRNA→protein), transcription (DNA→mRNA), and replication (DNA→DNA).

Practice

1. What cell structure is the site of translation?
2. What is the name of the base sequence on mRNA that specifies one amino acid?
3. What chemical bond links amino acids together?