Describe the double-helix structure of DNA and the single stranded structure of RNA. (pages \(37-39\) )

The double-helix structure of DNA is a twisted ladder-like shape formed by two complementary strands of deoxyribonucleic acid (DNA) molecules held together by hydrogen bonds. This structure was first described by James Watson and Francis Crick in 1953.

DNA is composed of building blocks called nucleotides. Each nucleotide consists of three parts: a sugar molecule (deoxyribose), a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine, or cytosine). The sugar and phosphate components form the backbone of each DNA strand, while the nitrogenous bases face inward and pair with complementary bases on the opposite strand. The base pairing rules for DNA are as follows: Adenine pairs with Thymine (A-T), and Guanine pairs with Cytosine (G-C). These specific base pairings, known as complementary base pairs, are held together by hydrogen bonds and play a crucial role in the stability and replication of the DNA double helix.

RNA, or ribonucleic acid, is another type of nucleic acid found in cells. Unlike DNA, which is double-stranded, RNA is typically single-stranded. Its structure is similar to that of one strand of DNA, but with a few key differences.

RNA is also composed of nucleotides, but these nucleotides contain a different sugar molecule, ribose, instead of deoxyribose. Moreover, RNA has a different nitrogenous base, uracil (U), which replaces the thymine found in DNA. The base pairing rules for RNA involve Adenine pairing with Uracil (A-U) and Guanine pairing with Cytosine (G-C). RNA molecules can fold into various secondary structures such as hairpins, loops, and bulges, by forming complementary base pairs within the same strand. This structural versatility allows RNA to perform various functions within the cell, including carrying genetic information from DNA to ribosomes (messenger RNA or mRNA), forming part of the ribosomal structure and facilitating protein synthesis (ribosomal RNA or rRNA), and transferring specific amino acids to the growing polypeptide chain during protein synthesis (transfer RNA or tRNA). By following these four steps, you should now have a clear understanding of the double-helix structure of DNA and the single-stranded structure of RNA, along with their key components and features.