There are pretty obvious differences between plants and animals, but – at the chemical level – the cells of all plants and all animals contain DNA in the same shape – the famous “double helix” that looks like a twisted ladder. What’s more, all DNA molecules – in both plants and animals – are made from the same four chemical building blocks – called nucleotides.
What is different is how these four nucleotides in DNA are arranged. It’s their sequence that determines which proteins will be made. The way the nucleotides are arranged, and the information they encode, decides whether the organism will produce scales or leaves – legs or a stalk.
Research shows that plants and animals may produce some proteins in common. One prominent example is known as Cytochrome C. But because the DNA copying process is imperfect, mistakes accumulate over time, making Cytochrome C slightly different in different creatures. The gene regions that specify the amino acid sequence in human Cytochrome C are more similar to those in another mammal like a rabbit, and less similar to a more evolutionarily distant creature, like a sunflower.
The schematic of classifying animals and plants in kingdoms is facing competition. More recently an alternative system has arisen, based on evolutionary and molecular information. Cytochrome c is perhaps the canonical or paradigmatic molecule in this approach.
Every species has a characteristic number of chromosomes, called the chromosome number. Animals have more chromosomes; plants have fewer.
