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

Which part of the nucleotide varies among different nucleotides?

1. Phosphate group

2. Sugar molecule

3. Base

4. Covalent bond

The correct answer is: Base

In a nucleotide, the component that varies among different nucleotides is the base. Nucleotides are the building blocks of nucleic acids such as DNA and RNA, and each nucleotide is composed of three parts: a phosphate group, a sugar molecule, and a nitrogenous base. The nitrogenous base is responsible for the genetic coding and is the variable component that distinguishes one nucleotide from another. For instance, in DNA, the bases can be adenine, thymine, cytosine, or guanine, while in RNA, uracil replaces thymine. This variation in the base is what allows nucleotides to form different sequences and thus contribute to genetic diversity and encoding of information. The phosphate group and the sugar molecule are consistent within a particular type of nucleic acid; DNA nucleotides all share the same deoxyribose sugar and phosphate group, while RNA nucleotides all consist of ribose sugar and phosphate. Covalent bonds are also fundamental components that hold the structures of nucleotides together; however, they do not vary among the nucleotides themselves.