Nucleic Acids: Structure and Function

• Nucleic acids are macromolecules present in all cells and viruses, storing and expressing genetic information.
• Deoxyribonucleic acid (DNA) stores the information needed to make proteins.
• Ribonucleic acid (RNA) exists in various forms and participates in protein synthesis.
• Nucleic acids are biopolymers of nucleotides.

Nucleotide Structure

• Nucleotides consist of a phosphate group, a nitrogen base, and a pentose molecule.
• The phosphate group includes a phosphorus atom bonded to negatively charged oxygen atoms.
• Pentose molecules in DNA and RNA are deoxyribose and ribose, both with five carbon atoms and one oxygen atom.
• Ribose has hydroxyl groups on the second and third carbon atoms, while deoxyribose has a hydroxyl group on the third and a hydrogen atom on the second.
• Nitrogen bases donate electrons, forming ring structures: single-ring pyrimidines and double-ring purines.
• Pyrimidines: thymine, uracil, and cytosine.
• Purines: adenine and guanine.
• Size differences between purines and pyrimidines affect their pairings in DNA strands.

Bonds in Nucleic Acids

• Glycosidic and ester bonds hold the pentose sugar, phosphorus, and nitrogen molecules together.
• Glycosidic bonds form between the first carbon of the pentose sugar and the 9th nitrogen atom of a nitrogenous base.
• Ester bonds form between the 5th carbon of the pentose sugar and the phosphate group.
• Polynucleotide chains form when the 5th carbon binds to the 3rd carbon of the next ribose sugar molecule, creating a phosphate-sugar backbone.
• RNA contains ribose, while DNA contains deoxyribose.
• Two antiparallel polynucleotide strands in DNA are linked by hydrogen bonds: two between adenine and thymine, and three between guanine and cytosine (complementary base pairing).

DNA Functions

• DNA stores and codes genetic information.
• DNA structure allows genes to be inherited.
• Adenine pairs only with thymine, and guanine pairs only with cytosine.
• During cell duplication, the DNA sequence is copied, ensuring exact copies are passed to other generations.
• DNA contains instructions for making proteins.

RNA Functions

• RNA is significant in protein synthesis and controls the expression of DNA-stored information.
• RNA carries genetic information in some viruses.
• RNA functions include creating new cells, acting as a messenger between DNA and ribosomes, translating DNA into proteins, and selecting appropriate amino acids.
• Transfer RNA (tRNA), messenger RNA (mRNA), and ribosomal RNA (rRNA) carry out nucleic acid functions.
• Adenosine triphosphate (ATP) comprises a nitrogenous base (adenosine), a ribose sugar, and three phosphate groups.
• ATP generates energy for cellular processes.

ATP

• High-energy bonds hold the three phosphate groups in ATP together, providing energy.
• Living cells use ATP for various cellular functions.
• Energy is released when the last phosphate group is released from ATP, forming adenosine diphosphate (ADP).
• Removing two phosphate groups from ATP forms adenosine monophosphate (AMP).
• ATP is synthesized in mitochondria through a recycling process that recharges and adds phosphate groups back to the molecule.
• ATP is involved in transporting proteins and lipids via exocytosis and endocytosis.
• ATP helps maintain the cell's structure by building cytoskeletal properties.

Discovery of Nucleic Acids

• Friedreich Miescher isolated DNA (nuclein) from white blood cells and pus in 1869.
• Altmann named the molecule nucleic acid.
• Albrecht Kossel isolated cytosine, guanine, uracil, and thymine between 1885 and 1901.

Nucleic Acid Function in the Body

• DNA stores genetic information, while RNA transfers it.
• DNA functions like a computer hard drive, while RNA functions like a flash drive.
• RNA synthesizes proteins using DNA-coded information.
• mRNA migrates from the nucleus to the cytoplasm.
• tRNA helps synthesize proteins from amino acids.
• Ribosomal RNA forms ribosomes, which participate in protein synthesis.
• Single-stranded RNA molecules form three-dimensional structures with weak hydrogen bonds.
• The three-dimensional structure of RNA determines unique cell functions, including enzyme degradation.

Structure

• Nucleic acids are polynucleotides made of nucleotides.
• Nucleotides consist of a nitrogen base attached to a pentose sugar molecule, which is attached to a phosphate molecule.
• Each nucleic acid contains the nitrogen bases adenine, thymine, guanine, and cytosine.
• Adenine and guanine are purines, while cytosine, thymine, and uracil are pyrimidines.
• All nucleic acids contain adenine, guanine, and cytosine, while thymine is unique to DNA and uracil is unique to RNA.
• The pentose sugar in DNA lacks a hydroxyl group at its second carbon.
• A sugar attached to a nitrogen base without a phosphate group is called a nucleoside.

Importance

• Nucleic acids form a chemical basis for genetic trait transfer.
• Nucleotides function as energy sources (ATP), signaling mediators, secondary messengers, and allosteric enzyme effectors.