Deoxyribonucleic acid (DNA) carries the genetic blueprint for every living organism—from single‑cell bacteria to African elephants. It stores two sets of essential instructions: one for synthesizing proteins required by the cell, and another for faithfully replicating itself so that future cell generations inherit the same genetic code.
To keep a cell alive long enough to divide, it must produce a wide array of proteins. DNA directs this production by transcribing specific gene segments into messenger RNA (mRNA), which then travels to ribosomes where proteins are assembled.
Transcription converts DNA into mRNA, while translation builds proteins from that mRNA template.
During translation, ribosomes stitch amino acids together through peptide bonds, forming polypeptide chains. The human body relies on 20 standard amino acids, each encoded by a triplet codon in the mRNA.
Successful translation requires a coordinated interaction among mRNA, aminoacyl‑tRNA complexes, and the two ribosomal subunits, as well as other molecular players.
Nucleic acids are polymers of nucleotides. Each nucleotide comprises a five‑carbon sugar (ribose in RNA, deoxyribose in DNA), a phosphate group, and a nitrogenous base.
Four bases—adenine (A), guanine (G), cytosine (C), and either thymine (T) in DNA or uracil (U) in RNA—provide the chemical diversity that defines each nucleotide.
Beyond structural roles, nucleotides like adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are central to cellular energy metabolism.
At the molecular level, DNA uses deoxyribose, lacking a hydroxyl group at the 2’ carbon, whereas RNA uses ribose. This “deoxy” difference accounts for DNA’s greater stability.
Both nucleic acids share adenine, guanine, and cytosine, but DNA incorporates thymine while RNA incorporates uracil. Base‑pairing rules (A‑T/U, C‑G) ensure accurate genetic information transfer during transcription and translation.
DNA is typically double‑stranded and adopts a double‑helix conformation, whereas RNA is single‑stranded. The double helix allows complementary strands to be perfectly paired, whereas RNA’s single strand permits diverse secondary structures.
DNA resides mainly in the nucleus, mitochondria, and chloroplasts, while RNA is found throughout the nucleus and cytoplasm.
Three primary RNA classes perform distinct functions:
The central dogma—DNA → RNA → protein—begins with transcription. DNA unwinds, exposing single strands for RNA polymerase to synthesize a complementary mRNA sequence, substituting uracil for thymine.
For example, the DNA segment ATTCGCGGTATGTC yields the mRNA sequence UAAGCGCCAUACAG. During splicing, introns are removed, leaving only coding exons in the mature mRNA.
Translation requires:
Translation relies on a triplet codon system: 4³ = 64 possible codons map to 20 amino acids, allowing multiple codons to encode the same amino acid (degeneracy) while each codon specifies only one amino acid.
In prokaryotes, initiation starts with a specific start codon, while eukaryotes universally use AUG (methionine). Ribosomes recognize the A (aminoacyl), P (peptidyl), and E (exit) sites for tRNA binding, peptide bond formation, and translocation.
During elongation, the ribosome moves one codon at a time, shifting the growing polypeptide from the P site to the A site. Peptide bonds link consecutive amino acids, extending the chain.
Termination occurs when a stop codon (UAA, UAG, UGA) is encountered, recruiting release factors that release the completed polypeptide and dissociate the ribosome.
Post‑translational modifications—including folding, cleavage, and chemical tagging—transform the nascent polypeptide into a functional protein. Proper folding is guided by intramolecular interactions among amino acids.
Ribosomes faithfully translate the provided mRNA but cannot detect template errors. Mutations can alter single amino acids (e.g., sickle cell anemia) or introduce frameshifts and premature stop codons, leading to dysfunctional proteins.
Understanding and correcting such mutations remains a major focus of genetic medicine.
