At the molecular level, genes consist of segments of which type of chemical building blocks?

Introduction / Context:
Genes are the units of heredity that carry information from one generation to the next. They are responsible for controlling traits and directing the synthesis of proteins. To understand genetics, it is essential to know the chemical nature of genes at the molecular level. This question asks which type of chemical building blocks make up genes.

Given Data / Assumptions:

Genes store hereditary information. They are part of long molecules such as DNA (and in some viruses, RNA). Options include carbohydrates, proteins, fats, nucleotides and inorganic salts. We assume standard molecular biology concepts.

Concept / Approach:
In most organisms, genes are specific sequences of DNA (deoxyribonucleic acid). DNA is a nucleic acid built from repeating units called nucleotides. Each nucleotide consists of a sugar, a phosphate group and a nitrogenous base. The order of these nucleotides encodes genetic information. In some viruses, genes may be carried on RNA, which is also a nucleic acid composed of nucleotides. Therefore, at the fundamental level, genes are made of nucleotides arranged in a specific sequence, not of carbohydrates, proteins or fats, although the information they encode ultimately guides protein synthesis.

Step-by-Step Solution:
Step 1: Recall that DNA and RNA are nucleic acids that store and transmit genetic information. Step 2: Understand that a gene is defined as a specific segment of DNA (or RNA in some viruses) that encodes a functional product. Step 3: Remember that nucleic acids are polymers composed of repeating nucleotide monomers. Step 4: Recognise that carbohydrates, proteins and fats are other classes of biomolecules with different roles (energy storage, structure, enzymes) but are not the primary hereditary material. Step 5: Conclude that genes consist of sequences of nucleotides forming DNA or RNA strands.

Verification / Alternative check:
Classic experiments in molecular biology, such as those by Avery, Hershey and Chase, demonstrated that DNA is the substance of genes in bacteria and phages. Structural studies by Watson and Crick revealed that DNA is a double helix of nucleotide chains. Modern sequencing technologies read genetic information by determining the order of nucleotides. In all these contexts, genes are consistently treated as nucleotide sequences, which confirms that nucleotides are the building blocks of genes.

Why Other Options Are Wrong:
Carbohydrates are mainly involved in energy storage and structural roles, such as starch in plants and glycogen in animals; they do not store the primary genetic code. Proteins carry out structural and enzymatic functions and are produced according to genetic instructions, but they are not themselves the standard genetic material in cells. Fats (lipids) make up membranes and serve as energy stores but do not directly encode genetic information. Simple inorganic salts help maintain ionic balance and support various physiological processes but are not information bearing polymers. Therefore, only nucleotides forming DNA or RNA correctly describe the chemical basis of genes.

Common Pitfalls:
Some students associate genes strongly with proteins because genes ultimately determine protein structure. This can lead to the mistaken idea that genes are made of proteins. Another common confusion is between chromosomes, which contain both DNA and associated proteins, and genes, which are specific DNA segments within those chromosomes. To avoid errors, remember that genes are stretches of nucleic acid sequence, and nucleotides are the building blocks that make up those sequences.

Final Answer:
Genes are composed of segments of nucleotides forming DNA or RNA, which encode hereditary information.