How Is a DNA Replicated

This is quite clear at least in principle by now. It is based on the specific interaction between A and T, and between G and C. That is, take, for example, the double helix in Fig. 4.1. Let us label the left strand as 1 strand and the other r strand. (This is the complementary strand of I )- Suppose that you separate the two strands and the 1 strand is isolated. Then you provide a pool of components A, C, G, and T and a means to bind nucleotides (enzyme called DNA polymerase) for the 1 strand. This enzyme binds nucleotides one by one sequentially. The top bead A on the 1 strand binds a bead T (laterally through hydrogen bond), and next another bead on 1 binds laterally a bead T. Beads T and T are then connected through the phosphate group by the enzyme. Next the bead G on 1 binds a bead C, and the bead C then is connected to the previous T on the right hand by the enzyme. This is repeated then you see that an 1 strand will reproduce the complementary r strand. The reverse will also be true i.e., an r strand will reproduce the corresponding 1 strand. Thus, a double strand will have been replicated (see Fig. 4.5). 

How this is accomplished, i.e., mechanics of these chemical reactions are currently very intensely studied, is beyond the level of this book. Hence, this topic will not be pursued further here. But, the very basic reason why we are like our parents or in other words why a gene molecule (DNA) is (almost) faithfully replicated and transmitted to a progeny can be understood as in the previous paragraph. This replication mechanism of DNA, however, applies to only cell division. 

The issue of inheritance in sexual organisms like us is a little more comphcated, because we get half of the gene from mother and the other half from father. But again we are not able to elaborate on this issue here. The issue is more of biology (so-called genetics) than chemistry. The chemical principles are about the same. 

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