Summary
Highlights
Many nucleotides linked together form a DNA strand, also known as a polynucleotide. A DNA strand grows only at its 3' end. DNA is a double-stranded molecule, forming a spiral structure called a double helix. The two sugar-phosphate backbones (the 'handrails' of the ladder) run parallel but in opposite directions (5' to 3' and 3' to 5'). The bases connect across the two strands via hydrogen bonds, forming the 'rungs' of the ladder.
DNA (deoxyribonucleic acid) is the carrier of our genetic information, found in every cell of our bodies. It's comparable to a spiral staircase and is made of repeating units called nucleotides. In eukaryotes, DNA is mainly located in the cell nucleus, condensed into 46 human chromosomes. In prokaryotes, it floats freely in the cytoplasm.
The basic building block of DNA is the nucleotide. Each nucleotide consists of three components: a sugar molecule (deoxyribose), a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, and cytosine). Deoxyribose is a five-carbon sugar, and its carbon atoms are numbered. The sugar connects to a base at its C1 carbon atom and to a phosphate group at its C5 carbon atom.
Specific base pairing occurs: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This is known as complementary base pairing. This complementarity means that if you know the sequence of one DNA strand, you can deduce the sequence of the other. The double-stranded DNA forms a coiled double helix, rather than a linear ladder.
In summary, DNA is a double-stranded, helical molecule composed of nucleotide units. Each nucleotide contains a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The two strands are held together by hydrogen bonds between complementary bases. In eukaryotic cells, DNA is tightly packed as chromosomes within the cell nucleus.