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Brown eyes, blue eyes, dark hair, light hair, towering height, or a short stature – the visible traits that set us apart are encoded by genes, specific stretches of DNA that dictate how our bodies develop and function.
DNA’s four chemical bases – adenine (A), thymine (T), guanine (G), and cytosine (C) – pair in a precise pattern to form a double‑helix ladder. The order of these bases in each gene determines the proteins the cell produces, and the slight variations (alleles) between individuals account for the majority of human diversity.
Every cell’s nucleus houses the human genome, a molecular blueprint encoded in deoxyribonucleic acid (DNA). DNA is a sugar‑phosphate backbone threaded with four nitrogenous bases that pair specifically: A with T, and G with C. This complementary base‑pairing gives DNA its iconic double‑helix structure and allows it to store vast amounts of genetic information in a compact, stable format.
A gene is a defined segment of DNA that carries the instructions for building a protein. Humans inherit one copy of each gene from each parent, resulting in two alleles per gene. Remarkably, over 99 % of the roughly 20,000–25,000 genes in the human genome are identical across all people. The remaining <1 % – the allelic variants – introduce subtle differences that shape individual traits. Gene sizes vary widely, from a few hundred bases to more than two million bases for the largest human genes.
The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into messenger RNA (mRNA), which is then translated into proteins. During transcription, the DNA double helix unwinds, and RNA polymerase synthesizes a complementary mRNA strand. The mRNA exits the nucleus and is read by ribosomes in the cytoplasm. Each three‑base sequence (codon) on the mRNA specifies one amino acid, and the ribosome links these amino acids into polypeptide chains. After folding, the polypeptide becomes a functional protein that performs critical roles in the body’s structure, function, and regulation.
