DNA Structure and Function
DNA's double helix relies on complementary base pairing to store genetic information with high fidelity. The specific way bases pair across the two strands is what makes accurate replication and transcription possible.
Base-Pairing Rules in DNA
A DNA molecule consists of two antiparallel polynucleotide strands wound into a double helix. Each nucleotide has three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base.
The four nitrogenous bases in DNA fall into two structural categories:
- Purines (adenine and guanine): larger, double-ring structures
- Pyrimidines (thymine and cytosine): smaller, single-ring structures
Complementary base pairing always occurs between a purine and a pyrimidine on opposite strands. This purine-pyrimidine pairing keeps the helix diameter consistent at about 2 nm. If two purines paired together, the helix would bulge; two pyrimidines would leave a gap.
The specific pairings are:
- Adenine (A) pairs with Thymine (T) through two hydrogen bonds
- Guanine (G) pairs with Cytosine (C) through three hydrogen bonds
Because G≡C pairs have three hydrogen bonds compared to A=T's two, DNA regions rich in G-C content are more thermally stable and harder to denature. This specificity is described by the Watson-Crick model of base pairing, and it's what ensures that each strand carries enough information to reconstruct the other during replication.
Features of the DNA Double Helix
The double helix has several structural features worth knowing:
- Antiparallel strands: One strand runs 5'→3' while the other runs 3'→5'. This orientation matters for replication and transcription, since polymerases only synthesize in the 5'→3' direction.
- Sugar-phosphate backbone on the outside, with nitrogenous bases facing inward toward each other.
- Major groove and minor groove: These are unequal gaps along the helix surface. Proteins like transcription factors bind in these grooves to read the base sequence without unwinding the DNA.
- Helical dimensions: The helix completes one full turn every ~10.5 base pairs, with a rise of 3.4 Å per base pair.
- Base stacking: Adjacent base pairs stack on top of each other, and the aromatic rings of the bases engage in - stacking interactions along with hydrophobic effects. These base-stacking forces actually contribute more to overall helix stability than the hydrogen bonds between base pairs.
Central Dogma of Molecular Genetics
The central dogma describes the flow of genetic information in cells: DNA → RNA → protein. Three processes carry this out.
1. Replication
The cell copies its entire DNA before dividing. Replication is semiconservative, meaning each daughter DNA molecule contains one original (parent) strand and one newly synthesized strand. This ensures genetic information passes accurately to daughter cells during mitosis and meiosis.
2. Transcription
RNA polymerase reads a DNA template strand and synthesizes a complementary RNA molecule. The primary product is messenger RNA (mRNA), which carries the genetic code from the nucleus to ribosomes. Transcription also produces other functional RNAs, including tRNA, rRNA, and regulatory RNAs like miRNA.
3. Translation
Ribosomes read the mRNA sequence in three-nucleotide units called codons, each specifying a particular amino acid. Transfer RNA (tRNA) molecules recognize codons through their anticodon loops and deliver the matching amino acid to the growing polypeptide chain. The final amino acid sequence determines how the protein folds and what it does, whether that's catalyzing reactions (enzymes), providing structural support (collagen), or transmitting signals (hormones).