How is genetic information encoded in a DNA molecule?

DNA, or deoxyribonucleic acid, is the blueprint for all living organisms. It is composed of two strands that wind around each other, forming the iconic double helix. Imagine twisting a ladder until it spirals; that's what the DNA structure looks like. Each 'rung' of this spiral staircase is made up of a pair of nucleotides.

Each nucleotide in the DNA structure includes a sugar molecule (deoxyribose), a phosphate group, and a nitrogenous base. Four types of nitrogenous bases are present in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases form pairs - A with T and C with G - creating a strict pairing rule due to their specific chemical structure. This pairing is the critical aspect of DNA's function in encoding genetic information, as the sequence of these bases holds the instructions for building and maintaining an organism.

The nucleotide sequence is the order of the nitrogenous bases along a DNA strand, and it's akin to the letters forming sentences in a book. The 'language' of DNA is comprised of only four 'letters': A, T, C, and G. These letters combine in various ways to encode genetic instructions. Each group of three bases, or a codon, specifies a particular amino acid—the building block of proteins. For instance, the codon ATG is a command for the amino acid methionine. This language, read in groups of three, dictates how amino acids should be assembled to create proteins that perform critical functions. Genetic information is effectively written in this concise genetic code within the long DNA molecules found in every cell.

Transcription is the first step in the journey from DNA to proteins. It's like copying a critical page out of an extensive archive so that it can be used. During transcription, an enzyme called RNA polymerase reads one strand of the DNA, known as the template strand, and synthesizes a complementary strand of messenger RNA (mRNA).

As the enzyme moves along the DNA, it unwinds the double helix and composes a strand of mRNA by matching each DNA base with its counterpart - except in RNA, uracil (U) replaces thymine. This mRNA strand then detaches from the DNA once the gene is fully transcribed, leaving the DNA intact and ready to be transcribed again whenever the cell needs more of the encoded protein.

In translation, the cell interprets the genetic 'sentences' composed during transcription and uses them to assemble amino acids into proteins. This takes place in the cell's cytoplasm, where ribosomes act as microscopic factories.

Transfer RNA (tRNA) molecules transport specific amino acids to the ribosome. Each tRNA has an anticodon that pairs with a corresponding codon on the mRNA strand. When a tRNA anticodon and an mRNA codon form base pairs, the tRNA deposits its amino acid, which is linked to create a protein chain. This process continues until a stop codon is reached, signaling the end of protein synthesis. The resulting chain of amino acids then folds into a three-dimensional shape to function as a completed protein.