Topic 2 Amino Acids and Peptides

Figure 2.1: The Central Dogma

Figure 2.1 outlines the central dogma: the flow of genetic information in a cell. The central dogma occurs in two different steps: transcription and translation.

Transcription is the process of transferring one strand of DNA to RNA via the enzyme RNA polymerase. First, the DNA-dependent RNA polymerase binds to the promoter and catalyzes the formation of mRNA (i.e., messenger RNA) in the 3’ to 5’ direction.

When RNA polymerase approaches the terminal sequence, the enzyme stops transcription and the mRNA strand is released; post-transcriptional modification (e.g., splicing by spliceosomes) may also occur after this step.

Figure 2.2: The Codons and Their Genetic Code

Translation is the process of converting a mRNA sequence to a specific protein. This process requires energy (which is provided in the form of charged tRNA [i.e., transfer RNA]) molecules. Note that the mRNA strand is read in a triplet of bases called a codon; figure 2.2 displays the outcomes of all 64 possible codons.

Ribosomes - an organelle comprised of a large and a small subunit - drive translation. As the mRNA enters the ribosome, it is held in place by two tRNA molecules in the large subunit. These two tRNA molecules are also held in close proximity to one another - a peptide bond is also formed between them. As this process repeats, long polypeptide chains are formed until a stop codon is reached.