Long Summary
DNA, or deoxyribonucleic acid, is a crucial biological molecule responsible for storing the genetic information in both eukaryotic and prokaryotic cells. This genetic information guides the synthesis of proteins and enzymes necessary for cell survival and reproduction. During cell division, DNA must replicate accurately to pass genetic material to offspring cells.
DNA is made up of repeating units called nucleotides, each consisting of three components: a five-carbon deoxyribose sugar, a phosphate group, and a nitrogenous base. There are four types of nitrogenous bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and guanine are purines with a two-ring structure, while cytosine and thymine are pyrimidines with a single-ring structure.
Nucleotides link together through phosphodiester bonds formed between the phosphate group of one nucleotide and the 3rd carbon of the sugar of the adjacent nucleotide, establishing a sugar-phosphate backbone. The four nucleotide varieties are often referred to by their nitrogenous bases, which define their identity and bonding properties. DNA molecules typically exist as two complementary strands bonded together to form a double helix, which is stabilized by hydrogen bonds between specific nitrogenous base pairs; adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds.
Importantly, the two DNA strands run in opposite directions, known as anti-parallel orientation: one strand runs 5′ to 3′, while the complementary strand runs 3′ to 5′. This anti-parallel configuration ensures proper hydrogen bonding between complementary bases and prevents spontaneous unzipping of the helix. The sugar-phosphate backbone faces outward, protecting the nitrogenous bases which are inside the helix where hydrogen bonding occurs. External factors such as increased temperature can eventually disrupt these hydrogen bonds, leading to separation of the strands.
