Maths for A Level Biology - updated edition

1st generation: When a molecule replicates, the two heavy strands separate and each synthesises a new daughter strand. If only nucleotides with light DNA are available, the complementary daughter strand will be light. So each molecule contains a heavy and a light strand and so is intermediate in density between completely heavy and completely light. Two intermediate molecules each with two strands, one heavy and one light. Intermediate DNA 15 N 14 N 15 N 14 N 2nd generation: When the ϐirst generation molecules replicate, the two strands separate and each synthesises a new complementary strand with nucleotides containing light DNA. So now, there are four molecules, two of which contain the original heavy parental strands and two which are entirely light. They are in equal proportions so the bands are of equal height. 15 N 14 N Intermediate DNA Light DNA 14 N 15 N 14 N 14 N 14 N 14 N In equal proportions Four molecules, two of which are intermediate and two of which are light. 3rd generation: When these four molecules replicate, their strands separate and each synthesises a new light complementary strand. Two of the resulting eight molecules will each contain one of the original parental, heavy strands and one light strand, so they will be intermediate in density. Two molecules out of eight represents one quarter. So three quarters of the molecules will be light. The intermediate band is therefore drawn to be one third of the height of the light band. Pointer The band thickness represents the proportion of molecules at that density. 2 Processed numbers 41