Meaning: The quote "If poly A is added to poly U, to form a double or triple helix, the combination is inactive" by Francis Crick, a renowned molecular biologist and co-discoverer of the structure of DNA, refers to a fundamental principle in molecular biology and genetics. This principle is known as the complementarity of nucleic acids, which plays a crucial role in the functioning of DNA and RNA.

Poly A and poly U refer to sequences of nucleotides that are found in RNA molecules. In RNA, the nucleotides are adenine (A), uracil (U), guanine (G), and cytosine (C). Poly A is a sequence consisting entirely of adenine nucleotides, while poly U is a sequence consisting entirely of uracil nucleotides.

When Crick refers to the addition of poly A to poly U to form a double or triple helix, he is describing the process of hybridization, in which complementary nucleic acid strands come together to form a stable structure. In this case, the combination of poly A and poly U does not result in the formation of an active double or triple helix. This has important implications for the functioning of RNA and the processes of transcription and translation in molecular biology.

The concept of complementarity in nucleic acids is central to understanding the structure and function of DNA and RNA. In DNA, the nucleotides adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This complementary base pairing is essential for the stability and accurate replication of the DNA molecule.

In RNA, the rules of complementarity are slightly different. Adenine (A) in RNA pairs with uracil (U) instead of thymine (T). This means that in RNA, the complementary base pairing is A-U and G-C. The ability of nucleic acid strands to form stable double helices through complementary base pairing is crucial for the storage and transmission of genetic information.

The specific combination of poly A and poly U being described in the quote is known to result in an inactive structure. This observation underscores the importance of precise base pairing and the specific sequences of nucleotides in determining the functionality of nucleic acid molecules.

Furthermore, the quote highlights the intricate nature of molecular interactions and the delicate balance required for biological processes to occur. The inability of poly A and poly U to form an active double or triple helix suggests that the specific sequences and arrangements of nucleotides are essential for the functioning of RNA in the cell.

In conclusion, Francis Crick's quote "If poly A is added to poly U, to form a double or triple helix, the combination is inactive" reflects the foundational concept of complementarity in nucleic acids and its significance in molecular biology. The precise pairing of nucleotides is essential for the stability and functionality of DNA and RNA, and understanding these principles is crucial for unraveling the complexities of genetic information and cellular processes.