K327 / K328 / 2027

Lesson 2 of 3 / Molecular genetics

DNA structure and base pairing

How can one strand tell you the bases on the other?

I can describe the DNA double helix and nucleotide components, and apply complementary base pairing.

  • I can describe two nucleotide strands in a double helix and nucleotide components.Syllabus

    Syllabus K327 / K328, Topic 10(b). Describe two nucleotide strands in a double helix and nucleotide components.

  • I can state and apply complementary base pairing.Syllabus

    Syllabus K327 / K328, Topic 10(c). State and apply complementary base pairing.

About 5 min

Make a guess. You are not marked.

A DNA sample is 30% adenine (A). What percentage is thymine (T)?

  1. 30%, because every A pairs with a T.
  2. 25%, because there are four bases.
  3. 70%, because the rest must be T.
Show the answer

30%, because every A pairs with a T.

In double-stranded DNA each A is opposite a T, so there are always equal amounts of A and T.

The key idea

DNA has two nucleotide strands twisted into a double helix. Each nucleotide contains sugar, phosphate and a base; A pairs with T, and C pairs with G.

The four DNA bases are adenine (A), thymine (T), cytosine (C) and guanine (G). A pair forms between bases on opposite strands.

Explore the idea

Complete the opposite strand

DNA pairing practice at position 1Each nucleotide contains a blue phosphate circle, a pale sugar square and one base. Opposite bases pair A with T and C with G. Choose the base directly opposite A.A?G?G?C?T?A?Sugar + phosphate + base = nucleotide

Choose the base directly opposite A.

A-T and C-G. Original flat ladder model of a tiny DNA section; real DNA twists into a double helix. Partner positions are aligned directly across, without chemical direction labels. This exploration is separate from the lesson's saved quick check.

Explanation

DNA is a double helix: two strands wound around one another. Each strand consists of many nucleotides joined together. Every DNA nucleotide has a sugar, a phosphate group and one of the four bases A, T, C or G.

Sugar and phosphate form the outer backbone of each strand. Bases project inwards and pair with bases on the opposite strand. Adenine pairs with thymine, and cytosine pairs with guanine. The two strands therefore have complementary sequences.

If the bases on one strand are A C G T T A, the bases directly opposite them are T G C A A T. Write each partner opposite its matching base and keep the positions aligned. This exercise compares corresponding positions; it does not require you to reverse a strand or use chemical direction labels.

A flat ladder drawing helps show the pairing, but real DNA is twisted. Likewise, a six-pair diagram shows a tiny model section, not an entire gene. The base order carries information; the repeating sugar-phosphate backbone provides the connected structure.

Base pairing lets you calculate base percentages. In double-stranded DNA every A pairs with a T and every C pairs with a G, so %A = %T and %C = %G. If 22% of the bases in a sample are G, then C is also 22%. That leaves 100 - 44 = 56% for A and T together, so A = 28% and T = 28%. The same rule tests a claim. A sample with 30% A and 18% T cannot be double-stranded DNA, because the A and T percentages would have to be equal.

To draw a labelled nucleotide, draw a circle for the phosphate group, a pentagon for the sugar and a rectangle for the base, joined in a line: phosphate to sugar, sugar to base. Label all three parts. The sugar in DNA is deoxyribose. The base is one of adenine (A), thymine (T), cytosine (C) or guanine (G). In a strand, the phosphate of one nucleotide joins to the sugar of the next, forming the sugar-phosphate backbone.

Step by step
  1. 1

    Find the nucleotide parts

    Identify one sugar, its phosphate group and its attached base.

  2. 2

    Pair each base

    Use A-T and C-G at the corresponding position on the other strand.

  3. 3

    Check the whole sequence

    Every base must have a complementary partner; do not copy the same letters unchanged.

Worked example

Complete a short DNA section

One strand has bases G A T C C A in the order shown. Give the bases directly opposite them.

One way to explain it

The complementary bases are C T A G G T, aligned opposite the given positions. G pairs with C, A with T, T with A and C with G.

Why this answer works
  • Apply the rule to one position at a time.
  • Retain the shown alignment.
  • Check that every pair is A-T or C-G.

Watch out for this

A student says: "The two DNA strands must have identical base sequences." What is wrong with this?

Show the answer

They are complementary: each A is opposite T, and each C is opposite G. The paired letters are not identical.

Check your understanding

Which bases sit directly opposite T G A C in the displayed order?

  1. T G A C
  2. A G T C
  3. A C T G
Show the answer

A C T G

Apply T-A, G-C, A-T and C-G at the four corresponding positions.

Exam-style questions

Check your understanding

A DNA sample contains 30% adenine. What percentage of its bases is cytosine?

  1. 20%
  2. 70%
  3. 40%
  4. 30%
Show the answer

20%

A pairs with T, so T = 30% and A + T = 60%. That leaves 40% for C and G, so C = 20%.

Quick recall

Card 1 of 3. Answer in your head, then check.

How can one strand tell you the bases on the other?

You can return to this lesson any time.