Bases at Higher Level: The same statement as SL, plus one thing HL asks for: whether a base is a purine or a pyrimidine. That pairing of one purine with one pyrimidine is what keeps the DNA helix the same width everywhere.
Practise this as you read
- Name the bases of DNA and RNA in full.
- Sort them: purines A and G, pyrimidines C, T and U.
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The big idea: Each nucleic acid uses four bases. Three are shared; DNA has thymine where RNA has uracil.
The points to remember
- DNA has four bases: adenine (A), cytosine (C), guanine (G) and thymine (T).
- RNA has four bases too: adenine, cytosine, guanine and uracil (U).
- Uracil takes the place of thymine: T only in DNA, U only in RNA.
- Each base is a nitrogenous base.
Remember it as: A, C and G in both: T for DNA, U for RNA.
Real example: the first genome ever read: In 1977 Fred Sanger's team read every base of a virus that infects bacteria, phiX174: 5 386 bases, written in just A, C, G and T.
When a question asks for a base by name, the full word earns the mark:
Spell the names in full: When a question asks you to name a base, write the full name: adenine, cytosine, guanine, thymine or uracil. And never put thymine in RNA or uracil in DNA.
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The big idea: The bases are the letters of the genetic code. The information is in their order along the strand.
The points to remember
- Every nucleotide carries one base, sticking out from the backbone.
- The backbone is the same all along a strand; only the bases vary.
- Bases can follow each other in any order.
- The order of the bases (the base sequence) carries the information, like the order of letters in a word.
Real example: one letter changed: In sickle cell anaemia, one base in the gene for part of haemoglobin is changed: an A becomes a T. That single change in the order alters the protein made, and the red blood cells change shape.
The big idea: The five bases come in two shapes: purines have two rings, pyrimidines have one.
The points to remember
- Purines (two rings): adenine and guanine.
- Pyrimidines (one ring): cytosine, thymine and uracil.
- In a base pair, one purine always pairs with one pyrimidine: A with T (or U), G with C.
Remember it as: Pure As Gold: purines are A and G.
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How it is asked: One mark for one base, named in full. At HL the base may be asked for by its type: purine or pyrimidine.
A table shows the ratios of the bases in DNA from six species, including the ratio of purines to pyrimidines. State one example of a purine.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
If two answers are written and one is wrong, the mark is usually lost, so give one name only.