DNA → RNA → Proteins

The central dogma of life can be defined in a fairly simple way: DNA makes RNA, which in turn makes proteins:

In transcription, your genetic code is transcribed, or written, into RNA. In translation, this RNA is then translated into proteins. Of course, the processes of transcription and translation are a little more complicated than that. Let’s review the two processes:

Transcription: DNA → RNA

Transcription is the first half of the Central Dogma. This is where DNA is translated into RNA. Transcription occurs in the nucleus of the cell—DNA cannot leave the nucleus. There are three steps in transcription: initiation, elongation, and termination (these are also the same steps as in translation; however, different things happen in the steps of the different processes).

1. Initiation: Transcription begins at a promoter: a specific region of a gene. RNA polymerase binds to the promoter. This signals the DNA to unwind. The enzyme is now ready to make mRNA
2. Elongation: Nucleotides are added to the mRNA strand
   • Remember: thymine only occurs in DNA, and uracil only occurs in RNA!
3. Termination: Transcription ends when RNA polymerase encounters a stop (termination) sequence in the gene.

RNA Polymerase

There are three types of eukaryotic RNA Polymerase. Fittingly, they’re named RNA Polymerase I, RNA Polymerase II, and RNA Polymerase III.

• RNA polymerase I is located in the nucleolus, and facilitates the transcription of ribosomal RNA (rRNA), which is then processed and assembled into ribosomes.
• RNA polymerase II is located in the nucleus and synthesizes all protein-coding nuclear pre-mRNAs.
• RNA polymerase III is also located in the nucleus. This polymerase transcribes a variety of structural RNAs that includes the 5S pre-rRNA, transfer pre-RNAs (pre-tRNAs), and small nuclear pre-RNAs.

mRNA Processing

After transcription, eukaryotic pre-mRNAs must undergo several processing steps before they can be translated.

Pre-mRNAs are first coated in RNA-stabilizing proteins; these protect the pre-mRNA from degradation while it is processed and exported out of the nucleus. The three most important steps of pre-mRNA processing are the addition of stabilizing and signaling factors at the 5′ and 3′ ends of the molecule, and the removal of intervening sequences that do not specify the appropriate amino acids. In rare cases, the mRNA transcript can be “edited” after it is transcribed.

Translation: RNA → protein

Translation occurs in the cytoplasm. There are three steps in translation: initiation, elongation, and termination (these are also the same steps as in transcription; however, different things happen in the steps of the different processes).

1. Initiation: Protein synthesis begins with the formation of an initiation complex. Translation begins with a methionine at each polypeptide chain
2. Elongation: The A site binds incoming charged aminoacyl tRNAs. The P site binds charged tRNAs carrying amino acids. Elongation proceeds with charged tRNAs entering the A site and then shifting to the P site followed by the E site with each single-codon “step” of the ribosome.
3. Termination: A nonsense codon (UAA, UAG, or UGA) is encountered. Upon aligning with the A site, these nonsense codons are recognized by release factors

Ribosomes and tRNAs

Codons

The amino acids that make up proteins are encoded by a nucleotide triplet codon: for example, the protein serine is encoded by the codons UCU, UCC, UCA, and UCG.