OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

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What are the two strands of DNA primarily made up of?

Amino acids joined together
Nucleotides joined in a chain
Fatty acid chains
Proteins linked together

The correct answer is: Nucleotides joined in a chain

The correct answer is that the two strands of DNA are primarily made up of nucleotides joined in a chain. DNA, or deoxyribonucleic acid, consists of two long strands that form a double helix structure. Each nucleotide in the DNA strand is composed of three components: a phosphate group, a sugar molecule (deoxyribose), and a nitrogenous base (adenine, thymine, cytosine, or guanine). These nucleotides are linked together by covalent bonds between the phosphate group of one nucleotide and the sugar of the next, creating a strong and stable backbone for the DNA structure. This nucleotide arrangement is crucial because it not only forms the genetic code but also allows the two strands to be complementary to each other through base pairing, which is essential for processes like DNA replication and transcription. Other options do not represent the building blocks of DNA accurately; amino acids are the building blocks of proteins, fatty acids are components of lipids, and proteins are made up of chains of amino acids, none of which relate to the structure or formation of DNA.