The inherited traits of an organism, such as eye color or blood group, are ultimately controlled by which type of molecule within the cell?

Introduction / Context:
Inherited traits such as eye color, blood group, and many disease susceptibilities are passed from parents to offspring through genetic material. Understanding what kind of molecule stores and transmits this genetic information is fundamental to genetics and molecular biology. This question asks which molecule ultimately controls inherited traits in organisms.

Given Data / Assumptions:

• Inherited traits follow patterns that can be explained by genes.
• Genes are segments of a particular molecule inside cells.
• The options list DNA, enzymes, nucleotides, and RNA.
• We assume standard knowledge from classical and molecular genetics.

Concept / Approach:
DNA, or deoxyribonucleic acid, is the hereditary material in almost all living organisms. Genes are specific sequences of DNA that encode information for making proteins or functional RNA molecules. These gene products then influence cell structure and function, leading to observable traits. Enzymes are proteins that catalyze biochemical reactions, often encoded by genes, but they are not themselves the primary hereditary material. Nucleotides are the building blocks of DNA and RNA, but they are individual components, not the complete information carrying polymer. RNA plays many roles in gene expression and some viruses use RNA genomes, but in typical eukaryotic cells, DNA is the main hereditary molecule.

Step-by-Step Solution:
Step 1: Recall that Mendel discovered that traits are controlled by factors now known as genes. Step 2: Remember that genes are found on chromosomes, which are composed largely of DNA and protein, with DNA identified as the information carrying component. Step 3: Understand that DNA sequences specify the amino acid sequences of proteins and the sequences of many RNA molecules, which in turn influence the phenotype. Step 4: Evaluate enzymes. These are proteins produced according to instructions encoded in DNA and they carry out reactions, but the underlying instructions originate from DNA. Step 5: Evaluate nucleotides. They are the monomer units that form DNA and RNA, but the polymer DNA, not individual nucleotides, serves as the stable hereditary material. Step 6: Evaluate RNA. It plays roles in gene expression and some viruses carry RNA genomes, but in most organisms described in basic genetics, DNA is the primary hereditary molecule controlling inherited traits. Step 7: Conclude that DNA is the molecule that ultimately controls inherited traits.

Verification / Alternative check:
Classical experiments such as the Hershey and Chase bacteriophage experiments showed that DNA, not protein, enters bacterial cells and directs production of new viruses. The structure of DNA, elucidated by Watson and Crick, revealed a double helix capable of accurate replication and information storage. Modern techniques like DNA sequencing and genetic testing further confirm that changes in DNA sequences correlate with inherited traits and diseases. While enzymes and RNA are crucial intermediates, the stable heritable information is encoded in DNA.

Why Other Options Are Wrong:
Option B: Enzymes perform vital functions but they are themselves products encoded by genes. They do not serve as the main hereditary material.

Option C: Nucleotides are the chemical units that form DNA and RNA, but isolated nucleotides do not store complex hereditary information by themselves.

Option D: RNA carries out roles such as messenger RNA, transfer RNA, and ribosomal RNA. In some viruses, RNA is the genetic material, but in the context of standard organisms in many biology courses, DNA is the primary controller of inherited traits.

Common Pitfalls:
Students sometimes confuse the molecules that store information with the molecules that carry out functions. It is easy to see enzymes at work in metabolic pathways and imagine that they are the primary controllers of traits. However, the long term information that specifies these enzymes and other structural proteins resides in DNA. Always distinguish between the information storage role of DNA and the functional roles of the molecules encoded by DNA.

Final Answer:
The inherited traits of an organism are ultimately controlled by DNA, which stores and transmits genetic information from one generation to the next.