Lesson 2 of 3 / Molecular genetics
DNA structure and base pairing
How can one strand tell you the bases on the other?
In this lesson: Describe the DNA double helix and nucleotide components, and apply complementary base pairing.
About 5 min
The key ideaDNA 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
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.
Step by step
- 1
Find the nucleotide parts
Identify one sugar, its phosphate group and its attached base.
- 2
Pair each base
Use A-T and C-G at the corresponding position on the other strand.
- 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.
Is this true? "The two DNA strands must have identical base sequences."
They are complementary: each A is opposite T, and each C is opposite G. The paired letters are not identical.