The two organisms that would most likely have the least similar nucleotide sequences in a given gene are:

* A bacterium and a human.

Here's why:

* Evolutionary Distance: Bacteria and humans are very distantly related on the evolutionary tree. They diverged from a common ancestor billions of years ago. This long evolutionary history has allowed for significant genetic differences to accumulate.

* Genetic Complexity: Humans have a much more complex genome than bacteria. They have more genes and their genes are often larger and more complex. This difference in genetic complexity adds to the likelihood of significant differences in nucleotide sequences.

While other options like a cat and a dog might show some differences, the evolutionary distance and genetic complexity between a bacterium and a human are much greater, leading to significantly more dissimilar nucleotide sequences.