4.22 : Nucleic Acid Structure

The pentose sugar in DNA is deoxyribose, while in RNA the pentose sugar is ribose. The difference between the sugars is the presence of the hydroxyl group on the ribose's second carbon and a hydrogen on the deoxyribose's second carbon. The phosphate residue attaches to the hydroxyl group of the 5′ carbon of one sugar and the hydroxyl group of the 3′ carbon of the sugar of the next nucleotide, which forms a 5′ to 3′ phosphodiester linkage.

DNA Structure

DNA has a double-helix structure. The sugar and phosphate lie on the outside of the helix, forming the DNA's backbone while the nitrogenous bases are stacked in the interior, like a pair of staircase steps. Hydrogen bonds bind the bases to each other. Every base pair in the double helix is separated from the next base pair by 0.34 nm. The helix's two strands run in opposite directions, meaning that the 5′ carbon end of one strand will face the 3′ carbon end of its matching strand. Only certain types of base pairing are allowed— A can pair with T, and G can pair with C.

Types and Functions of RNA

Ribonucleic acid, or RNA, is primarily involved in protein synthesis under the direction of DNA. RNA is usually single-stranded and consists of ribonucleotides linked by phosphodiester bonds.

There are four major types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and microRNA (miRNA). First, mRNA carries the message from DNA, which controls all the cellular activities in a cell. If a cell requires a specific protein, a corresponding gene is transcribed, and the messenger RNA is synthesized in the nucleus. The RNA base sequence is complementary to the DNA's coding sequence from which it has been copied. In the cytoplasm, the mRNA interacts with ribosomes and other cellular machinery.

The sequence of mRNA is read in sets of three bases known as codons. Each codon codes for a single amino acid; in this pattern, the mRNA is read by the ribosome, and the protein product is made. Ribosomal RNA (rRNA) is a major constituent of ribosomes. They ensure the proper alignment of the mRNA and the ribosomes. The ribosome's rRNA also has an enzymatic activity (peptidyl transferase) and catalyzes peptide bond formation between two aligned amino acids. Transfer RNA (tRNA) is one of the smallest of the four types of RNA, usually 70–90 nucleotides long. It carries the correct amino acid to the protein synthesis site. The base pairing between the tRNA and mRNA allows the correct amino acid to insert itself into the polypeptide chain. MicroRNAs are the smallest RNA molecules, and their role is to regulate gene expression by interfering with the expression of certain mRNA messages.

Even though the RNA is single-stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function.

This text is adapted from Openstax, Biology 2e, Chapter 3.5: Nucleic Acids