The flow of information from a gene to it product is from DNA to RNA to protein. The properties of different protein products of genes are determined by the sequence of amino acids of the polypeptide chain and by the way in which the chain is folded. Each gene is usually responsible for the synthesis of a single polypeptide.
Gene expression begins with the enzymatic synthesis of an RNA molecule that is a copy of one strand of the DNA segment corresponding to the gene. This process is called transcription and is carried out by the enzyme RNA polymerase. This enzyme joins ribonucleoside triphosphates by the same chemical reaction used in DNA synthesis.
RNA polymerase differs from DNA polymerase in that a primer is not needed to initiate synthesis. Transcription is initiated when RNA polymerase binds to a promoter sequence. Each promoter consists of several subregions, of which two are the polymerase binding site and the polymerization start site. Polymerization continues until a termination site is reached. The product of transcription is an RNA molecule. In prokaryotes, this molecule is used directly as messenger RNA (mRNA) in polypeptide synthesis. In eukaryotes, the RNA is processed: Noncoding sequences called introns are removed, the exons are spliced together, and the termini are modified by formation of a 5 cap and usually by addition of a poly-A tail at the 3 end.
After mRNA is formed, polypeptide chains are synthesized by translation of the mRNA molecule. Translation is the successive reading of the base sequence of an mRNA molecule in groups of 3 bases called codons. There are 64 codons; 61 correspond to the 20 amino acids, of which 1 (AUG) is a start codon. The remaining 3 codons (UAA, UAG, and UGA) are stop codons. The code is highly redundant: Many amino acids have several codons. The codons in mRNA are recognized by tRNA molecules, which contain a 3-base sequence complementary to a codon and called an anticodon. When used in polypeptide synthesis, each tRNA molecule possesses a terminally bound amino acid (aminoacylated, or charged, tRNA). The correct amino acid is attached to each tRNA species by specific enzymes called aminoacyl-tRNA synthetases.
Polypeptides are synthesized on particles called ribosomes. Synthesis begins with the formation of a 30S ribosomal subunit + charged tRNA + mRNA complex, which recruits a 50S subunit to complete the mature 70S ribosome. (In eukaryotes, the 40S and 60S subunits come together to from the 80S ribosome.) Next, charged tRNA molecules are successively brought to the A site on the 50S ribosome by elongation factor EF-Tu (EF-1 αin eukaryotes). These are hydrogen-bonded to the mRNA in the 30S subunit by a codon-anticodon interaction, and the 50S subunit is shifted to the pretranslocation state. As each charged tRNA is brought aboard, its amino acid is attached by a peptide bond to the growing polypeptide chain. Translocation of the 30S ribosome one codon farther along the tRNA is the function of elongation factor EF-G (EF-2 in eukaryotes), converting the ribosome to the post translation state and shifting the uncharged tRNA to the E site and the polypetidyl tRNA (the one carrying the incomplete polypeptide) to the P site. The elongation process continues until a stop codon in the mRNA is reached. No tRNAs for the stop codons exist. Instead, specific release factors cleave the complete polypeptide from the last polypetidly tRNA and free the ribosome components for reuse in translation.
Several ribosomes can translate an mRNA molecule simultaneously, forming a polysome. In prokaryotes, translation often begins before synthesis of mRNA is completed; in eukaryotes, this does not occur because mRNA is made in the nucleus, whereas the ribosomes are located in the cytoplasm. Prokaryotic mRNA molecules are often polycistronic, encoding several different polypeptides. Translation proceeds sequentially along the mRNA molecule from the start codon nearest the ribosome-binding site, terminating at stop codons and reinitiating at the next start codon. This is not possible in eukaryotes, because only the AUG site nearest the 5 terminus of the mRNA can be used to initiate polypeptide synthesis; thus eukaryotic mRNA is monocistronic.
The author is Departmental Coordinator of Department of Plant Breeding and Genetics.