In the history of molecular biology, which key discovery is attributed to Phoebus Levene, the Russian American biochemist who worked on nucleic acids?

Introduction / Context:
This question comes from basic history of genetics and molecular biology. Before modern DNA technology, several scientists contributed step by step to our understanding of nucleic acids. Phoebus Levene is one such important figure, and competitive exams often ask you to match his name to the specific discovery he made. Remembering who did what helps you avoid mixing up contributions of Levene, Watson and Crick, Avery, and others.

Given Data / Assumptions:

• We are asked which discovery is attributed to Phoebus Levene.
• The options mention sugars, RNA as messenger, direct observation of DNA, building the double helix model, and discovering the four bases.
• The task is to link Levene with the correct landmark in nucleic acid research.

Concept / Approach:
The core concept here is to organise the timeline of major DNA related discoveries. Levene worked in the early twentieth century. He analysed nucleic acids chemically and identified the sugar components in RNA and DNA. Later scientists such as Avery worked on DNA as genetic material, and Watson and Crick built the structural model. So the approach is to match Levene with chemical identification of ribose and deoxyribose, not with later structural or functional discoveries.

Step-by-Step Solution:
Step 1: Recall that Levene proposed the nucleotide as a phosphate sugar base unit of nucleic acids.Step 2: He identified that RNA contains ribose sugar, while DNA contains a related sugar lacking one oxygen atom, called deoxyribose.Step 3: The option that explicitly mentions identification of ribose and deoxyribose in nucleic acids matches this contribution.Step 4: Recognise that recognition of RNA as the messenger of DNA function is associated with later work on transcription and translation, not with Levene.Step 5: Extraction and observation of DNA link more closely to Friedrich Miescher and others, and the double helix model is due to Watson and Crick.Step 6: The four DNA bases were known before Levene, though he worked with them, so that option is not the unique discovery they are testing here.

Verification / Alternative check:
An alternative way to verify is to recall exam tables that summarise contributions: Miescher isolated nuclein, Levene discovered ribose and deoxyribose and described nucleotides, Avery showed DNA as transforming principle, Hershey and Chase confirmed DNA as genetic material, and Watson and Crick proposed the double helix. Only one option in the list fits Levene exactly, which confirms your choice.

Why Other Options Are Wrong:
Recognition of RNA as DNA’s messenger is wrong because that belongs to later genetic studies on mRNA and protein synthesis.First extraction and observation of DNA is incorrect as that is credited to Miescher, who worked with white blood cells and salmon sperm.Construction of the accurate DNA model is wrong because Watson and Crick, with inputs from Franklin and Wilkins, proposed that structure in 1953.Discovery of the four bases is not a unique contribution of Levene; those bases were already known, even though he studied them.

Common Pitfalls:
Students often confuse Levene’s name with Watson and Crick or believe that every major DNA discovery came from the same few people. Another pitfall is assuming that because Levene worked on nucleic acids, he must have discovered the double helix or the messenger role of RNA. To avoid such confusion, memorise a simple timeline of who did what and in which decade.

Final Answer:
The correct answer is Identification of the sugars ribose and deoxyribose in nucleic acids.