A piece of DNA contains 1200 bases. 240 of them are C. How many are A?
- C pairs with G, so G = 240. C + G = 240 + 240 = 480.
- A + T = 1200 − 480 = 720.
- A pairs with T, so A = T = 720 ÷ 2 = 360.
Answer: 360 of the bases are A.
DNA
Biology · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
DNA is the molecule that carries genetic information. It is made of two strands coiled together into a double helix. Each strand is a chain of nucleotides, and each nucleotide contains one base: A, T, C or G. Bonds between pairs of bases hold the two strands together. The bases always pair the same way: A with T, and C with G.
A gene is a length of DNA that codes for a protein. The sequence of bases in the gene sets the sequence of amino acids in the protein. A different amino acid sequence gives a protein with a different shape, and so a different job.
DNA controls a cell by controlling which proteins, including enzymes, the cell makes.
Rules to remember
- DNA: two strands, each a chain of nucleotides, coiled into a double helix.
- Base pairing: A pairs with T; C pairs with G.
- So in any DNA molecule: amount of A = amount of T, and amount of C = amount of G.
- A + T + C + G = 100% of the bases.
- Gene: a length of DNA that codes for a protein.
- Base sequence of the gene → sequence of amino acids → shape of the protein → its function.
Common mistake
In percentage questions students pair the wrong bases or forget to halve. Subtract the A + T total from 100%, then divide what is left equally between C and G.
Worked example
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