The Structure of DNA · DNALab

Photo 51

In 1952 Rosalind Franklin shone X-rays through a drawn-out fibre of DNA and recorded the famous Photo 51: an “X”-shaped cross with a strong spot at the top and bottom.

The left half of the film on the right is the diffraction pattern calculated from the helix you set up; the right half is the measured pattern. Adjust the pitch, base pairs per turn, radius and number of strands until the halves match — like Watson and Crick, you will read the shape of DNA from the photograph.

The X shows that DNA is a helix; the strong spots at top and bottom come from the 0.34 nm spacing between neighbouring base pairs; the angle of the X gives the pitch and diameter.

Build a DNA Model

The building block of DNA is the deoxynucleotide: one phosphate, one deoxyribose and one nitrogenous base. Strand I on the left is already built; first assemble a deoxynucleotide, then complete the other strand.

Bases on the two strands are joined by hydrogen bonds following complementary base pairing: A with T, G with C. Try a wrong pair — purine with purine is too wide, pyrimidine with pyrimidine too narrow.

Finally decide which way strand II runs, then twist the “ladder” into a right-handed double helix.

The DNA Double Helix

DNA consists of two antiparallel chains of deoxynucleotides coiled into a double helix. Deoxyribose and phosphate alternate on the outside, forming the backbone; the bases are on the inside, where the bases of the two strands are joined into base pairs by hydrogen bonds.

The model is generated from an X-ray crystal structure (PDB 1BNA). Click a base pair to see its hydrogen bonds: 2 between A and T, 3 between G and C.

Rewrite the sequence on the right and the readouts check Chargaff's rules live: A = T, G = C, purines = pyrimidines.

In-class testing

Look back over the three topics and answer the multiple-choice questions below.