DNA, or Deoxyribonucleic Acid, is the hereditary material found in all living organisms. It has a distinctive double helix structure, consisting of two sugar-phosphate backbones that spiral around each other. The structure contains four nucleotide bases: Adenine, Thymine, Guanine, and Cytosine, represented by the letters A, T, G, and C. These bases follow specific pairing rules - Adenine always pairs with Thymine, and Guanine always pairs with Cytosine. This complementary base pairing is crucial for DNA's stability and its ability to store genetic information accurately.
The Watson-Crick base pairing rules are fundamental to DNA structure and function. Adenine always pairs with Thymine through two hydrogen bonds, while Guanine pairs with Cytosine through three hydrogen bonds. These specific pairings occur between purines and pyrimidines, ensuring uniform width of the DNA double helix. The hydrogen bonds, shown as dashed lines, provide the chemical basis for base pair stability. The stronger triple bond between G and C makes GC-rich regions more stable than AT-rich regions. These pairing rules are essential for DNA replication accuracy and maintaining genetic information integrity across generations.
DNA replication is a precise process that creates two identical copies of the original DNA molecule. First, the enzyme helicase unwinds the double helix, breaking the hydrogen bonds between base pairs and creating replication forks. DNA polymerase then adds complementary nucleotides to each strand. The leading strand is synthesized continuously in the 5 prime to 3 prime direction, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments. This semi-conservative replication ensures that each new DNA molecule contains one original strand and one newly synthesized strand, maintaining genetic fidelity across cell divisions.
DNA stores genetic information through the organization of base sequences into genes. The genetic code uses triplet codons, where every three consecutive bases specify a particular amino acid. With four different bases, there are 64 possible codons, but only 20 standard amino acids, creating redundancy in the genetic code. Genes are organized on chromosomes and contain promoter regions, coding sequences, and regulatory elements. The linear sequence of bases in DNA directly determines the sequence of amino acids in proteins. Mutations, such as base substitutions, insertions, or deletions, can alter this sequence and potentially change protein function, demonstrating the critical importance of DNA sequence accuracy in maintaining cellular function.
The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to protein. During transcription, RNA polymerase reads the DNA template strand and synthesizes a complementary mRNA molecule, replacing thymine with uracil. The mRNA then travels to ribosomes where translation occurs. Transfer RNA molecules bring specific amino acids corresponding to each codon on the mRNA. The ribosome reads the mRNA codons sequentially and assembles amino acids into a protein chain. This process demonstrates how the linear sequence of bases in DNA ultimately determines protein structure and function, enabling cells to express their genetic information and carry out essential biological processes.