Deoxyribonucleic acid, commonly known as DNA, serves as the fundamental blueprint containing the genetic instructions used in the development, functioning, growth, and reproduction of all known organisms. The smallest single functional unit that makes up this complex structure is called a nucleotide. Understanding the nucleotide is the first step in comprehending how the vast amount of genetic information is stored and transmitted within every living cell.
The Nucleotide: Components of the Smallest Unit
Each DNA nucleotide is a composite molecule built from three distinct chemical parts: a phosphate group, a five-carbon sugar known as deoxyribose, and a nitrogenous base. The phosphate group and the deoxyribose sugar are identical in every nucleotide across the entire DNA strand.
The deoxyribose sugar is a pentose sugar, meaning it contains five carbon atoms. It is defined by the absence of an oxygen atom at one specific position when compared to the ribose sugar found in RNA. This sugar molecule forms the central attachment point, connecting the phosphate group on one side and the nitrogenous base on the other.
The nitrogenous base is the only part that varies between the four types of DNA nucleotides. These bases are adenine (A), guanine (G), cytosine (C), and thymine (T). Adenine and guanine are classified as purines (double-ring structure), while cytosine and thymine are pyrimidines (single-ring structure). The specific presence of one of these four bases determines the identity of the individual nucleotide.
How Nucleotides Form the DNA Strand
Individual nucleotides link together in a process called polymerization to create a long, single strand of DNA. The phosphate group of one nucleotide joins with the deoxyribose sugar of the next nucleotide in the chain. This sugar-phosphate linkage is a strong covalent bond known as a phosphodiester bond.
The continuous chain of alternating sugars and phosphates forms the structural framework of the DNA strand, often referred to as the sugar-phosphate backbone. This backbone provides great stability to the genetic material.
In its natural state, DNA exists as a double helix, consisting of two nucleotide strands twisted around each other, resembling a spiral staircase. The two strands are held together by bonds formed between the nitrogenous bases extending inward from the backbones. Adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C), a rule known as complementary base pairing.
These pairings are stabilized by weak hydrogen bonds. This arrangement ensures that the two strands are complementary, meaning the sequence of bases on one strand dictates the exact sequence on the opposing strand. The antiparallel nature of the two strands, where they run in opposite directions, is a specific characteristic of the double helix structure.
The Genetic Code Stored in the Sequence
The functional information of DNA is not contained within the sugar-phosphate backbone but within the specific linear order of the nitrogenous bases. The sequence of A’s, T’s, C’s, and G’s along the strand constitutes the genetic code, which is read by cellular machinery in groups of three consecutive bases.
A sequence of three bases is called a codon. Each codon specifies either a single amino acid or a signal to stop protein synthesis. Amino acids are the molecular building blocks of proteins, which perform most of the work within a cell. The sequence of codons along a gene determines the specific order of amino acids, which dictates the type and function of the resulting protein.
There are 64 possible three-base combinations, which is more than enough to code for the 20 common amino acids used by life. This redundancy means that most amino acids are coded for by more than one codon. This feature helps protect against potential errors during DNA replication or transcription.