DNA Structure & Replication: The Code of Life and How It Copies Itself

Three things every CBE asks: the four bases (A, T, G, C) and how they pair, the double helix structure, and semi-conservative replication. Plus the four enzymes that make replication work.

9 min TEKS 6B,6C,6D Biology

DNA in three pieces

The CBE tests DNA in three predictable pieces:

Structure — the double helix, sugar-phosphate backbone, and base pairs.
Pairing rules — A always with T, G always with C.
Replication — how the double helix copies itself before cell division.

Structure: the double helix

DNA looks like a twisted ladder:

The sides of the ladder are made of sugar (deoxyribose) and phosphate, alternating.
The rungs are pairs of nitrogen bases connected by hydrogen bonds.
The whole ladder is twisted into a helix (hence “double helix”).

Six base-pair rungs. A always pairs with T (2 H-bonds), G always pairs with C (3 H-bonds).

The four bases — and the only two pairings

There are exactly four bases in DNA:

A denine — pairs only with T
T hymine — pairs only with A
G uanine — pairs only with C
C ytosine — pairs only with G

The base-pair rule

A pairs with T. G pairs with C. Always. Memorize “Apples in Trees, Cars in Garages.”

If one strand reads 5'-ATCG-3', the complementary strand must read 3'-TAGC-5' — every A becomes T, every G becomes C, and vice versa.

RNA: the cousin with one swap

RNA is similar to DNA with three differences:

RNA is single-stranded; DNA is double-stranded.
RNA uses ribose; DNA uses deoxyribose.
RNA replaces thymine (T) with uracil (U). So in RNA, A pairs with U.

Replication: semi-conservative

Before any cell can divide, it must copy its DNA. The copying mechanism is called semi-conservative replication — meaning each new double helix contains one old strand and one newly made strand. Half is preserved, half is new.

One old strand + one new strand = semi-conservative. The image visualizes the rule; the term is your job to remember.

The four enzymes you must know

Replication needs a coordinated team:

Helicase — unwinds and separates the two parent strands at the replication fork.
DNA polymerase — adds new nucleotides to build the new strand, reading the template 3'→5' and synthesizing 5'→3'.
DNA ligase — joins the small Okazaki fragments on the lagging strand.
Primase — lays down a short RNA primer that polymerase needs to start.

Helicase at the fork unwinds; polymerase synthesizes; ligase joins fragments. Three roles — memorize each.

Check yourself

Quick check #1

If one DNA strand reads 5'-ATCG-3', what is the complementary strand?

Quick check #2

Each daughter DNA molecule contains one strand from the parent and one newly synthesized strand. What is this called?