DNA structure and replication
DNA is the molecule of heredity. Its structure, discovered in 1953, immediately suggested how it copies itself.
The building blocks
DNA is a polymer of nucleotides. Each nucleotide has a sugar (deoxyribose), a phosphate group, and one of four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Sugars and phosphates form the backbone; the bases carry the information.
The double helix
James Watson and Francis Crick, using X-ray images made by Rosalind Franklin, showed that DNA is a double helix: two strands twisted around each other, held together by hydrogen bonds between bases. A always pairs with T, and G with C. The strands run in opposite directions (antiparallel).
Base pairing rules
Because pairing is complementary, one strand predicts the other: if one strand reads ATGC, its partner reads TACG. In any double-stranded DNA, the amount of A equals T and G equals C. So if 30% of the bases are A, 30% are T and the remaining 40% split into 20% G and 20% C.
Chromosomes and genomes
In eukaryotes, DNA wraps around proteins called histones and coils into chromosomes. The human genome has about 3 billion base pairs and roughly 20,000 protein-coding genes, which make up only about 1-2% of the DNA. Much of the rest regulates genes or has unknown function.
Replication
Before a cell divides, DNA copies itself. The enzyme helicase unzips the strands; DNA polymerase adds complementary nucleotides to each exposed strand. Each new molecule has one old and one new strand, a pattern called semiconservative replication, proven by the Meselson-Stahl experiment in 1958.
Leading and lagging strands
DNA polymerase can only add nucleotides in one direction. So one strand (leading) is copied continuously, while the other (lagging) is copied in short pieces called Okazaki fragments, later joined by the enzyme DNA ligase. Replication starts at many points along each eukaryotic chromosome at once.
Proofreading and repair
DNA polymerase checks its own work and fixes most mistakes, giving an error rate of about one in a billion bases after repair. Other enzymes fix damage from ultraviolet light and chemicals. Failures in repair genes, such as BRCA1 and BRCA2, raise cancer risk.
Worked example
A DNA strand reads 5'-AGCTTA-3'. Its complementary strand, written in the same left-to-right alignment, reads 3'-TCGAAT-5'. If a sample is 18% guanine, it is also 18% cytosine, leaving 64% split equally: 32% adenine and 32% thymine.
Key takeaways
DNA is a double helix of nucleotides; A pairs with T and G with C, on antiparallel strands. The human genome has about 3 billion base pairs and 20,000 genes. Replication is semiconservative: helicase unzips, polymerase builds, ligase joins Okazaki fragments. Proofreading and repair keep errors very rare.
Flashcard
Sugar + phosphate + base
Check your understanding
If 20% of the bases in a DNA sample are cytosine, what percentage is adenine?