Ask questions and construct an explanation about how the structure of DNA stores genetic information and is copied accurately by replication (Louisiana Student Standards for Science, High School Biology, HS-LS3-1).

What this topic is asking

Louisiana's LS3 standards (HS-LS3-1) ask you to explain how DNA stores the instructions for an organism's traits. For LEAP 2025 Biology you should know the structure of DNA (the double helix of nucleotides), the base-pairing rule, how the order of bases carries information, and how replication copies DNA accurately before a cell divides. The test often gives one DNA strand and asks for the complementary strand, applying the pairing rule.

The structure of DNA

The sugars and phosphates form the two backbones (the sides of the twisted ladder), and the bases stick inward and pair up to form the rungs. The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). The order of these bases along a strand is what stores the information.

The base-pairing rule

This complementary pairing is the single most useful fact about DNA on the test, because it lets you write a partner strand and because it explains both how DNA is copied and how the information is read out.

How the base sequence stores information

The sequence (order) of bases along a DNA strand is the genetic code. A length of DNA that codes for one protein (or trait) is a gene. Different orders of A, T, C, and G spell different instructions, just as different orders of letters spell different words. Because the base sequence determines which proteins a cell makes, and proteins determine traits, the DNA sequence ultimately determines the organism's characteristics, the LS3 cause-and-effect chain from gene to trait.

DNA replication

Before a cell divides, it must copy its DNA so each new cell gets a complete set. This copying is replication:

• The double helix unwinds and the two strands separate.
• Each strand acts as a template: free nucleotides pair with the exposed bases by the pairing rule (A with T, C with G).
• The result is two identical DNA molecules, each with one old strand and one new strand.

Because each base pairs with only one partner, the template specifies exactly which base goes opposite it, so replication is accurate. This accuracy is why the daughter cells of mitosis are genetically identical.

Try this

Q1. State the base-pairing rule in DNA. [1]

• Cue. Adenine pairs with thymine (A-T), and cytosine pairs with guanine (C-G).

Q2. Explain why DNA replication produces two identical molecules. [2]

• Cue. Each separated strand acts as a template, and because each base pairs with only one partner, the new strand is specified exactly, so each copy is identical to the original.