Reasons:

* Functional constraints: Proteins are directly involved in cellular processes, such as catalysis, transport, and signaling. Changes in protein sequence can disrupt these functions, leading to deleterious effects. Therefore, protein sequences are subjected to stronger evolutionary pressure to maintain their integrity.

* Redundancy in the genetic code: There are multiple codons that code for the same amino acid. This redundancy allows for some variation in DNA sequences without affecting the resulting protein sequence.

* Protein structure and function: The three-dimensional structure of a protein is crucial for its function. Even small changes in amino acid sequence can alter protein folding and stability, potentially disrupting its function.

* Evolutionary selection: Natural selection favors proteins with optimal function. Mutations that result in non-functional proteins are typically eliminated from the population.

Examples:

* Histones: These proteins are involved in packaging DNA and are highly conserved across species.

* Ribosomal proteins: Essential for protein synthesis and exhibit remarkable conservation.

* Cytochrome c: An electron carrier protein with a highly conserved sequence across a wide range of organisms.

Exceptions:

* Non-coding DNA: Some non-coding DNA sequences may exhibit significant conservation, particularly those involved in regulatory functions.

* Rapidly evolving proteins: Some proteins, such as those involved in immune responses, may evolve rapidly due to selective pressure from pathogens.

Conclusion:

While both DNA and protein sequences can be conserved, protein sequences are generally more conserved due to their direct role in cellular function, the redundancy of the genetic code, and the importance of protein structure and stability.