DNA Structure & Replication

DNA stores and transmits genetic information, and it works the same way in every living thing. It's a double-stranded helix: two sugar–phosphate backbones running antiparallel (one $5'\to3'$, the other $3'\to5'$), with pairs of nitrogenous bases holding them together like the rungs of a twisted ladder.

Complementary base pairing

The rungs only fit one way — this is the single most important rule in the topic:

• A pairs with T — held by 2 hydrogen bonds
• G pairs with C — held by 3 hydrogen bonds

Antiparallel strands held by complementary base pairs — A–T (2 bonds), G–C (3 bonds).

Where DNA lives

• Eukaryotes: long linear chromosomes, wound around histone proteins, inside the nucleus (plus small circular DNA in mitochondria and chloroplasts).
• Prokaryotes: a single circular chromosome, generally not bound to histones, free in the cytosol.

Semi-conservative replication

Before a cell divides it must copy all of its DNA exactly. Because each strand is a template for a new partner, every daughter molecule ends up with one old strand and one new strand — hence semi-conservative.

The key players, in order:

• Helicase unwinds the helix and breaks the hydrogen bonds, exposing two template strands.
• DNA primase lays down a starting point.
• DNA polymerase adds free complementary nucleotides in the $5'\to3'$ direction.
• DNA ligase seals the new strand together.

Because base pairing is complementary, the code is conserved — each new molecule is identical to the original.