DNA Structure

HSC Biology | Free Study Notes

In this lesson

• what nucleotides are

• how base pairing works

• why DNA is described as a double helix

• what the sugar-phosphate backbone is

• why DNA structure matters in heredity

What is DNA?

DNA is the molecule that stores genetic information.

This genetic information:

• carries instructions for inherited characteristics

• is passed from one generation to the next

• provides the code used in protein synthesis

Nucleotides

DNA is made of repeating subunits called nucleotides.

What a nucleotide contains

Each DNA nucleotide has three parts:

• a phosphate

• a deoxyribose sugar

• a nitrogenous base

A 2022 HSC question specifically tested the identification of deoxyribose sugar, phosphate, and the correct base in a DNA section. 

The four bases in DNA

The four nitrogenous bases in DNA are:

• adenine (A)

• thymine (T)

• cytosine (C)

• guanine (G)

Base pairing

The bases in DNA pair in a specific way.

Complementary base pairing

• adenine pairs with thymine

• cytosine pairs with guanine

This is called complementary base pairing.

Why base pairing matters

Base pairing is important because it:

• keeps the DNA structure consistent

• allows DNA to be copied accurately during replication

• helps preserve genetic information

Double helix

DNA is described as a double helix.

What this means

A double helix is:

• made of two strands

• twisted around each other like a spiral ladder

Key idea

The two strands are held together by the pairing of the nitrogenous bases in the middle.

So when students describe DNA as a double helix, they should picture:

• two long strands

• twisted together

• with paired bases between them

Sugar-phosphate backbone

The outer part of each DNA strand is called the sugar-phosphate backbone.

What it is made of

The backbone is formed by alternating:

• deoxyribose sugar

• phosphate

Where the bases are found

The nitrogenous bases project inward from the backbone and pair with bases on the opposite strand.

Why the backbone matters

The sugar-phosphate backbone:

• forms the sides of the DNA molecule

• supports the overall structure

• helps organise the bases into a stable arrangement

Putting the structure together

A simple way to describe DNA structure is:

• DNA is made of nucleotides

• each nucleotide contains phosphate, deoxyribose sugar, and a base

• the bases pair A with T and C with G

• the molecule has two strands

• the strands twist into a double helix

• the outer edges are the sugar-phosphate backbone

Why DNA structure matters

DNA structure is important because it helps explain how genetic information is:

• stored

• copied

• passed on

Link to replication

Because bases pair specifically, each strand can act as a template during DNA replication.

Link to heredity

Because DNA carries inherited information, its structure is central to understanding how reproduction ensures continuity of species.

DNA compared with RNA

Students sometimes confuse DNA with RNA.

Main differences

DNA:

• contains deoxyribose

• contains thymine

• is usually double-stranded

RNA:

• contains ribose

• contains uracil instead of thymine

• is usually single-stranded

This is useful because HSC-style questions often test the difference between DNA components and RNA components.

Worked example

Exam-style question

Describe the structure of DNA.

Worked answer

DNA is made of nucleotides, and each nucleotide contains a phosphate, a deoxyribose sugar and a nitrogenous base. The bases pair specifically, with adenine pairing with thymine and cytosine pairing with guanine. DNA has two strands twisted into a double helix, with a sugar-phosphate backbone on the outside.

Why this works

This answer:

• includes all the key structural features

• uses correct terminology

• links the parts together clearly

Common mistakes

• Saying DNA contains ribose instead of deoxyribose.

• Mixing up thymine and uracil.

• Forgetting that DNA is made of nucleotides.

• Saying adenine pairs with cytosine.

• Describing the double helix without mentioning the two strands.

Quick quiz

1. What are the three parts of a DNA nucleotide?

2. Which bases pair together in DNA?

3. What does double helix mean?

4. What forms the sugar-phosphate backbone?

5. Why is complementary base pairing important?

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