Describe the two-stranded DNA double helix
Identify the two coiled strands and paired bases that hold them together.
Extended only (Supplement); Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Recall the distinction between an element, a molecule and a cell structure.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many strands are in the required DNA model?
What you will learn
- Identify the two coiled strands and paired bases that hold them together.
- Explain the conclusion from stated evidence and recognise its limits.
Build a structural description
A DNA molecule has two strands coiled together to form a double helix. Each strand contains chemicals called bases. Bonds between paired bases link the two strands together. A twisted ladder is a useful schematic: two sides represent strands and paired bases form the linking rungs.
The ladder drawing is not a microscopic photograph or a scale model. A cell nucleus contains DNA; the nucleus itself is not a strand or a base. DNA can also occur in bacterial cells without a nucleus.
Include all required features
A complete description identifies two strands, coiling, bases in each strand, and bonds connecting pairs. Saying only “a spiral” misses the two-strand and pairing structure.
The order of bases can vary along a strand. Coiling changes the overall shape of the double-stranded molecule; it does not change the complementary pairing rule taught next.
Pause and explain
What joins the two strands in this description?
Worked example
A student describes DNA as one spiral strand. Improve the description.
Show the worked solution
- Identify the missing second strand.
- State that both strands coil together.
- Add bases and bonds between paired bases.
Answer Two strands coil into a double helix and are held together by bonds between paired bases.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the overall shape of DNA.
Give me a hint
Include two and coiling.
Compare my reasoning
- Two strands coil together.
A two-stranded double helix.
Look for these in your work
- Include the named molecules or components in your answer.
- Explain the relationship using the stated building blocks or paired-base evidence.
What chemical components occur along each strand?
Give me a hint
Name the units that pair.
Compare my reasoning
- Each strand contains bases.
Bases.
Look for these in your work
- Include the named molecules or components in your answer.
- Explain the relationship using the stated building blocks or paired-base evidence.
Why is “one spiral” incomplete?
Give me a hint
Recover both strand and pairing information.
Compare my reasoning
- DNA has two coiled strands.
- Paired bases bond across them.
It omits the second strand and the paired-base bonds.
Look for these in your work
- Include the named molecules or components in your answer.
- Explain the relationship using the stated building blocks or paired-base evidence.
Common mistakes
- DNA strands are not joined by cell walls.
- The frame is not molecular structure.
What can you explain now?
What joins the two strands in this description?
Compare with the explanation
Bonds between paired bases.
Pairs link bases on opposing strands.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Describe DNA with strands, coiling, bases and bonds in one clear explanation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.