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DNA is a long, double‑helical polymer composed of four nucleotide bases—adenine, thymine, cytosine, and guanine—that encodes the genetic instructions for every living organism. In human cells, these strands are folded with histone proteins into nucleosomes, which further condense into the 23 pairs of chromosomes that reside in the nucleus. Genes—specific DNA segments—are transcribed into messenger RNA and ultimately translated into proteins that shape and sustain the body.
During sexual reproduction, gametes (sperm and egg) contain a single set of 23 chromosomes. When a sperm fertilizes an egg, the paternal and maternal genomes combine to form a diploid zygote with 46 chromosomes. This fusion passes a unique combination of alleles to the next generation. The sex of the offspring is determined by the X and Y chromosomes: two X chromosomes produce a female, while one X and one Y produce a male. Subsequent embryonic divisions are guided by differential gene expression, leading to the diverse array of cell types that compose the human body.
Genes dictate the synthesis of proteins—enzymes, hormones, structural proteins—that carry out all biochemical processes. Complex regulatory networks, including transcription factors and epigenetic marks, control which genes are expressed in which cells. Mutations—whether point mutations, insertions, or deletions—can disrupt protein function, resulting in congenital anomalies such as cleft palate, or in inherited disorders such as cystic fibrosis and Down syndrome.
Beyond nuclear DNA, human mitochondria harbor 37 genes encoded on a circular genome that produce essential RNAs and proteins for oxidative phosphorylation. Mutations in mitochondrial DNA can impair energy production, leading to severe metabolic disorders that may manifest in newborns. While some DNA damage triggers programmed cell death, the full mechanisms of DNA degradation during apoptosis remain an active area of research.
