A Level Biology Practice Exam 2025 – All-in-One Study Guide to Excel in Your A Levels!

Complementary base pairing refers to the specific hydrogen bonding between nucleic acid bases, which is fundamental to the structure and function of DNA and RNA. In DNA, for instance, adenine pairs with thymine (using two hydrogen bonds), and cytosine pairs with guanine (using three hydrogen bonds). This precise pairing is crucial for the accurate replication of genetic information during cell division and for the transcription of DNA into RNA.

The concept of complementary base pairing is essential for maintaining the double helix structure of DNA, allowing it to carry genetic information reliably from one generation to the next. It's also key in processes such as PCR (polymerase chain reaction) and the synthesis of mRNA, where accurate base pairing ensures that the correct genetic code is transcribed.

Other options provided do not accurately describe the concept of complementary base pairing and instead refer to different aspects of molecular biology. Sugar molecules in DNA do not form base pairs; rather, they connect the nucleotides together. The replication process of RNA is associated with the synthesis of RNA strands but does not specifically address base pairing itself. Lastly, peptide chains and their connections relate to protein structure and function, which is entirely separate from nucleic acid base pairing.