Explain, with a labelled diagram, how one DNA molecule gives rise to two identical molecules before a cell divides. Say what DNA is made of, including its four bases.

1. 1 · Define. DNA replication is the copying of one DNA molecule into two identical molecules before a cell divides.
2. 2 · Say why it is needed. Each daughter cell must get the same number of identical chromosomes.
3. 3 · Describe the structure. DNA is a double helix of two strands of nucleotides; each nucleotide is a base + deoxyribose sugar + phosphate.
4. 4 · Name the bases. Adenine, guanine, thymine and cytosine.
5. 5 · Write the steps.
   • Enzymes unwind the double helix and expose the two strands.
   • DNA polymerase builds a new daughter strand along each old strand.
   • Two DNA molecules form, each with one old and one new strand; they become two chromatids joined at the centromere.
6. 6 · Draw it. [Diagram: double helix opening into a Y; new strands along each old strand; two daughter double helices; labels: parent strand, new strand, DNA polymerase, bases]
7. 7 · Close in one line. So every new cell gets an exact copy of the DNA.

Scoring tip: Number the three steps and label the old and new strands in the diagram; the labels carry marks.