Final answer:
Before DNA was identified as the genetic material, a complex structure was not considered a key characteristic of genetic material. Proteins, with their diverse amino acids, were initially thought to be the genetic carriers due to their apparent structural complexity compared to DNA's four nucleotides.
Step-by-step explanation:
The one characteristic that was NOT considered a key attribute of the genetic material before nucleic acids were identified is that the genetic material must have a complex structure. Historically, even though DNA does have a complex structure, early scientific thought leaned towards proteins as the biomolecules with sufficient complexity to encode genetic information. This belief was due to proteins being composed of 20 different amino acids, allowing for a high degree of variability and complexity, while DNA, composed of only four nucleotides, seemed too simple to account for the vast genetic diversity observed in living organisms.
It wasn't until the transformative experiments of Avery, McCarty, McLeod, and eventually the definitive work of Hershey and Chase, that DNA was confirmed to be the genetic material. The initial complexity perceived in proteins overshadowed the abstract concept of steep structural complexity in DNA. Afterward, it was understood that through the processes of replication and expression, DNA's arrangement of four bases could indeed encode and express the complexity necessary for life.