Cats and dogs have different numbers and types of genes and chromosomes. This causes dogs and cats to look different.
The physical traits of a species are a result of their genes. Genes are segments of DNA that code for a specific protein. Proteins are involved in the expression of traits.
DNA is a nucleic acid. In fact, DNA stands for deoxyribonucleic acid. DNA is composed of smaller building blocks called nucleotides. Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base. There are four kinds of nitrogenous bases in DNA. DNA’s nitrogenous bases are adenine, thymine, cytosine, and guanine. The order of the nucleotides determines the genes that an individual will have. Dogs and cats have different genes, thus dogs and cats make different proteins and look different from one another.
Furthermore, each species has a unique number of chromosomes. Chromosomes are composed of DNA that is wound around proteins. Chromosomes look like thread-like strands. Dogs have 78 diploid chromosomes. Cats have 38 diploid chromosomes. Again, this results in a different physical appearance of dogs and cats.
On a larger scale, we can see that these differences in genes is a result of the individual genetic adaptations each species has experienced over time.