The Molecular Basis of Heredity
In 1953, James Watson and Francis Crick described the structure of DNA — one of the most significant discoveries in the history of science. Their double helix model explained, for the first time, how genetic information is stored and copied. Decades of subsequent research have revealed the chemical details of how DNA and its close relative RNA carry out the central functions of inheritance and gene expression.
Nucleotides: The Building Blocks
Both DNA and RNA are polynucleotides — long chains of monomers called nucleotides. Each nucleotide consists of three components:
The difference between deoxyribose and ribose is a single oxygen atom: ribose has an –OH group at the 2' carbon, while deoxyribose has only –H. This seemingly minor difference has major consequences for the stability and function of each molecule.
The Four Bases
DNA contains four nitrogenous bases: - Adenine (A) and Guanine (G) — purines (two-ring structures) - Thymine (T) and Cytosine (C) — pyrimidines (one-ring structures)
RNA also contains adenine, guanine, and cytosine, but Uracil (U) replaces thymine. Uracil lacks the methyl group found on thymine's ring.
The Structure of DNA
The Phosphodiester Backbone
Nucleotides link together through phosphodiester bonds: the phosphate group of one nucleotide connects to the 3' carbon of the previous nucleotide's sugar. This creates a continuous backbone of alternating sugar and phosphate groups, with the bases projecting inward.
Each DNA strand has directionality: one end has a free phosphate on the 5' carbon (5' end) and the other has a free hydroxyl on the 3' carbon (3' end). When two strands pair, they run in opposite directions — they are antiparallel.
The Double Helix
Two DNA strands coil around a common axis in Watson and Crick's famous double helix. The sugar-phosphate backbones form the outer "rails," while the base pairs form the "rungs." The helix makes one complete turn approximately every 10 base pairs (3.4 nm), with adjacent base pairs separated by 0.34 nm.
The two strands are held together by hydrogen bonds between complementary base pairs: - Adenine pairs with Thymine via 2 hydrogen bonds: A=T - Guanine pairs with Cytosine via 3 hydrogen bonds: G≡C
This complementary base pairing (Chargaff's rules: [A]=[T], [G]=[C]) is fundamental to DNA replication and transcription. Because each strand's sequence dictates the other's, each strand serves as a template for copying.
The G≡C pair's extra hydrogen bond makes DNA with high GC content more stable and resistant to thermal denaturation — relevant in organisms like thermophilic bacteria that live in hot springs.
DNA Supercoiling and Packaging
The human genome contains approximately 3.2 billion base pairs, totaling about 2 meters of DNA if stretched out. This must fit into a cell nucleus only 6 micrometers across. The solution involves multiple levels of coiling and compaction:
- DNA wraps around histone proteins to form nucleosomes, like beads on a string
- Nucleosomes coil into 30 nm chromatin fibers
- Further looping and scaffolding produce the visible chromosomes during cell division
Topoisomerases are enzymes that manage DNA's topological state, preventing tangles and relieving torsional stress during replication and transcription.
The Structure of RNA
RNA is typically single-stranded, but single RNA strands frequently fold back on themselves to form internal base pairs and secondary structures such as: - Hairpin loops: stem-loop structures where a strand folds and pairs with itself - Bulges and internal loops: regions of unpaired bases within otherwise paired stems
These structures are functionally critical. The active sites of ribozymes, for example, depend on precise RNA tertiary structure.
Types of RNA
| Type | Abbreviation | Function |
|---|---|---|
| Messenger RNA | mRNA | Carries coding sequence from DNA to ribosome |
| Transfer RNA | tRNA | Transports amino acids; decodes codons |
| Ribosomal RNA | rRNA | Structural and catalytic component of ribosomes |
| Small nuclear RNA | snRNA | Splicing of pre-mRNA introns |
| MicroRNA | miRNA | Post-transcriptional gene regulation |
| Long non-coding RNA | lncRNA | Chromatin remodeling, gene regulation |
Transfer RNA (tRNA)
tRNA molecules are small (70–90 nucleotides) and fold into a characteristic cloverleaf secondary structure that adopts an L-shaped tertiary structure. Each tRNA: - Has an anticodon loop: three nucleotides that base-pair with a codon on mRNA - Carries a specific amino acid at its 3' CCA terminus, attached by aminoacyl-tRNA synthetase enzymes
Ribosomal RNA (rRNA)
rRNA is the most abundant RNA in cells and forms the core of ribosomes — the molecular machines that synthesize proteins. The large ribosomal subunit's rRNA acts as a ribozyme, catalyzing peptide bond formation during translation. This was a landmark discovery confirming that RNA, not protein, is the catalytic component of the ribosome.
The Central Dogma
The flow of genetic information follows the central dogma of molecular biology:
DNA → RNA → Protein
- Replication: DNA is copied to DNA by DNA polymerase before cell division
- Transcription: DNA is transcribed to mRNA by RNA polymerase
- Translation: mRNA is translated to protein at ribosomes
Reverse transcription — RNA back to DNA — also occurs, notably in retroviruses like HIV, which carry reverse transcriptase.
DNA Replication
During replication, the double helix is unwound by helicase and each strand serves as a template. DNA polymerase reads the template 3'→5' and synthesizes the new strand 5'→3', adding complementary nucleotides according to base-pairing rules.
Because DNA polymerase can only extend existing strands, primase first synthesizes short RNA primers. DNA polymerase I later replaces these primers with DNA, and DNA ligase seals the gaps.
Replication is semi-conservative: each daughter molecule consists of one original strand and one newly synthesized strand.
Transcription and the Genetic Code
During transcription, RNA polymerase binds to a promoter sequence on DNA, unwinds the helix, and synthesizes a complementary mRNA strand. The mRNA is then processed (capped, polyadenylated, and spliced to remove introns) before export from the nucleus.
The mRNA sequence is read in codons — triplets of nucleotides. The genetic code maps 64 codons to 20 amino acids plus three stop signals. The code is: - Degenerate: multiple codons can specify the same amino acid - Nearly universal: the same code is used across virtually all life on Earth
Mutations: Changes in DNA Sequence
A mutation is a permanent change in DNA sequence. Mutations can arise from: - Replication errors: misincorporation of bases (rate ~1 in 10⁹ per base pair with proofreading) - Chemical damage: alkylation, oxidation, UV-induced thymine dimers - Radiation: X-rays and gamma rays can cause double-strand breaks
Most mutations are corrected by DNA repair mechanisms. Those that persist may have no effect (silent mutations), alter protein function (missense mutations), or truncate the protein (nonsense mutations). Mutations in cancer driver genes or inherited disease genes can have profound health consequences.
Understanding the chemistry of DNA and RNA — from hydrogen bonds to the genetic code — underpins all of modern molecular biology, medicine, and biotechnology.