Summary
Highlights
RNA (ribonucleic acid) is similar to DNA but has three key differences: it's single-stranded, its sugar is ribose (with one more oxygen atom than deoxyribose), and instead of thymine, it contains uracil, which pairs with adenine. RNA is crucial for protein production and DNA replication.
DNA is described as a beautiful, mesmerizing, and incredibly complex molecule essential for life. Untangled DNA from a single cell can be taller than a person, and all the DNA from a human body could stretch to the sun 600 times. DNA, or deoxyribonucleic acid, stores genetic instructions, a 6-billion letter code that programs all cellular activities and defines living organisms.
Every somatic cell in humans contains 46 chromosomes, each with one large DNA molecule tightly packed in the nucleus. DNA is a nucleic acid, belonging to the fourth major group of biological molecules alongside carbohydrates, lipids, and proteins. Nucleic acids are polymers made of repeating molecular units called nucleotides, forming polynucleotides.
Each nucleotide consists of a five-carbon sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). DNA exists as a pair of molecules held together in a double helix, resembling a twisted ladder. The sugar and phosphate groups form the twin backbones, with bonds running in opposite directions, creating 5' and 3' ends.
The two long DNA chains are linked by nitrogenous bases through hydrogen bonds. Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C), forming base pairs. The order of these base pairs, known as the base sequence, is what allows DNA to create unique individuals. Human chromosome 1 contains 247 million base pairs, and all 46 chromosomes together comprise roughly 6 billion base pairs in every cell.
The understanding of DNA's structure and function involved many scientists over nearly a century. Friedrich Miescher discovered DNA (which he called nuclein) in 1869 while studying white blood cells. Rosalind Franklin, a biophysicist, used X-ray diffraction in the 1950s to confirm DNA's helical structure and that the sugar-phosphate backbone was on the outside. Her contributions were critical but largely uncredited during her lifetime, and she tragically died young from ovarian cancer, possibly due to radiation exposure.
Cells constantly divide, requiring complete copies of DNA. This process, called replication, involves unwinding the double helix and using original strands as templates. The enzyme helicase unwinds the DNA, breaking hydrogen bonds between base pairs and creating a replication fork with a 'leading strand' and a 'lagging strand'.
The leading strand is replicated continuously. DNA polymerase adds matching nucleotides to the main stem. Before DNA polymerase can start, the enzyme RNA primase lays down a short RNA primer. Once the primer is in place, DNA polymerase continuously adds new nucleotides as the DNA unwinds.
The lagging strand is more complex because DNA polymerase can only copy in the 5' to 3' direction, while the lagging strand runs 3' to 5'. RNA primase lays down multiple RNA primers, allowing DNA polymerase to synthesize short segments called Okazaki fragments (1,000-2,000 base pairs long). Another DNA polymerase replaces the RNA primers, and finally, DNA ligase joins all the fragments together.
DNA replication is remarkably accurate, with errors occurring about one in every 10 billion nucleotides. DNA polymerases can proofread, removing mismatched bases, ensuring the integrity of the genetic code. The immense complexity and precision of DNA make it an incredibly celebrated molecule.