About The Selfish Gene Book - How Replicators Molecules forms duplicates?
This is how I think about it.  Here is the original large molecule, with each of its 'parts' represented by a letter.  There are a few different kinds of part:

C-A-A-C-T-A-T-A-C-G-A-C


As it drifts around in a 'soup' of parts, new parts stick to it one by one, like for like.  Here it is after two parts stick to it:

C-A-A-C-T-A-T-A-C-G-A-C
  A         T


Here it is after a few more:

C-A-A-C-T-A-T-A-C-G-A-C
C-A   C   A-T   C   A


Eventually, all the parts have got a similar part stuck to them:

C-A-A-C-T-A-T-A-C-G-A-C
C-A-A-C-T-A-T-A-C-G-A-C


Now, if the two halves split, the molecule has replicated itself.