Biology · Grades 8–11
DNA structure and complementary sequences
Keep bases, strands and genes distinct.
Learn the idea
DNA
DNA contains nucleotide units, each with a sugar, phosphate and base.
Pairing
In the standard double-stranded DNA model, A pairs with T and C with G.
Direction
DNA strands are antiparallel; sequence questions must say how direction is written.
Gene
A gene is a functional DNA sequence, not one whole organism or one isolated base.
Worked examples
Which base pairs with adenine in ordinary DNA?
The canonical DNA pair is A–T.
Thymine.
Which base pairs with cytosine in ordinary DNA?
The canonical DNA pair is C–G.
Guanine.
Check your understanding
A double-stranded DNA sample has 20% adenine in the ideal model. What percentage is thymine?
The same sample has 20% A and 20% T. What percentage is G if the remainder is equally C and G?
Which describes antiparallel strands?
- Both have no direction.
- Their bases never pair.
- Their chemical directions run oppositely.
Answers and marking notes
Answers and marking notes
- 20 %
A and T occur in matching counts across the double-stranded sample.
- 30 %
100 − 40 = 60%, divided equally gives 30% G.
- Their chemical directions run oppositely.
One strand's 5′ to 3′ direction opposes the other's.
Further reading and review references
- openstax.org · reference 1
- www.genome.gov · reference 2
- www.genome.gov · reference 3
- www.genome.gov · reference 4
- www.genome.gov · reference 5
- www.genome.gov · reference 6
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.