Point 2 of 28 — Lesson 2: DNA Structure, Replication and DNA Repair EnzymesQuestion bank: 25 questions (4 with pictures)
02 The data a model had to fit
DNA is made of nucleotides. Each nucleotide has three parts:
a five-carbon sugar, deoxyribose;
aphosphate group, covalently bonded to carbon 5′ of the sugar;
one of four nitrogenous bases, covalently attached to carbon 1′ of the sugar: the single-ringpyrimidines – thymine (T) and cytosine (C)or the double-ringpurines – adenine (A) and guanine (G).
How nucleotides link. The phosphate on the 5′ carbon of one nucleotide joins to the 3′ carbon of the sugar of the neighbouring nucleotide (a phosphodiester link). The chain of alternating sugar and phosphate is the sugar–phosphate backbone. The backbone isnot symmetrical: it has direction (polarity), a free3′ hydroxyl (–OH)at one end and a free5′ phosphate at the other. The bases project sideways from the backbone.
Chargaff's rules (base ratios). In every DNA molecule, the number of adenine = thymineand the number ofguanine = cytosine(A = T, G = C).
Franklin's X-ray diffraction pictures (1952). Rosalind Franklin sent X-rays through crystals of highly purified DNA. A regular molecule scatters X-rays in a pattern of spots whose analysis shows its shape. Her photographs showed that DNA is twisted into a helix, thebases are perpendicular to the length of the fibre, thesugar–phosphate backbone is on the outside with the bases inside, and the diameter of the helix implied more than one strand.
📝 Question bank for this point
25 questions linked to this point only, 4 of them with pictures.
