Final answer:
The sequence ATCG is expected to occur once every 256 base pairs in a DNA strand containing equal amounts of A, T, C, and G. This is determined by the probability of each base occurring independently, according to Chargaff's rules and the structure of DNA.
Step-by-step explanation:
Given that the DNA of an organism contains equal amounts of adenine (A), thymine (T), cytosine (C), and guanine (G), the chance of finding a specific sequence like ATCG depends on the probability of each base occurring independently. If they all occur with equal frequency, each base has a 1/4 chance of occurring at any position.
To find the sequence ATCG, we need to calculate the probability of A occurring, followed by T, then C, and G in that exact order. The probability of finding the sequence ATCG is therefore (1/4) x (1/4) x (1/4) x (1/4), which equals 1/256. Thus, we would expect to find the sequence ATCG once every 256 base pairs.
In DNA replication and cell division (mitosis), the complementarity of base pairing ensures the genetic code is accurately preserved due to the Chargaff's rules, where A pairs with T and G pairs with C. This is further backed by Chargaff's findings and the application in the Watson and Crick DNA double helix model. The genetic blueprint for each organism is based on its unique DNA sequence, resulting from these base pairings.