connecting the elements The unique bonding and chemical arrangement of fatty acids determines the molecules function and metabolism Nucleic acids the amino acids of proteins carbohydrates carbohydrates and fatty acids all are naturally occurring chemical compounds Each is also capable of being broken down to yield discrete components These four basic chemical molecules are critical to life and understanding and researching their structure and function has led to many important insights in biology biology chemistry medicine and other fields 14 EXAMINING BASIC CHEMICAL MOLECULES
CHAPTER 1 Nucleic Acids A nucleic acid is a naturally occurring chemical compound that is capable of being broken down to yield phosphoric acid sugars and a mixture of organic bases purines and pyrimidines Nucleic acids are the main information-carrying molecules of the cell and by directing the process of protein synthesis they determine the inherited characteristics of every living thing The two main classes of nucleic acids are deoxyribonucleic acid DNA and ribonucleic acid RNA DNA is the master blueprint for life and constitutes the genetic material in all free-living organisms and most viruses RNA is the genetic material of certain viruses but it is also found in all living cells where it plays an important role in certain processes such as the making of proteins 15
NUCLEOTIDES Nucleic acids are polynucleotides long chainlike molecules composed of a series of nearly identical building blocks called nucleotides Each nucleotide consists of a nitrogen-containing aromatic base a stable organic compound attached to a pentose five-carbon sugar which is in turn attached to a phosphate group Each nucleic acid contains four of five possible nitrogen-containing bases adenine A guanine G cytosine C thymine T and uracil U A and G are categorized as purines and C T and U are collectively called pyrimidines All nucleic acids contain the bases A C and G T however is found only in DNA while U is found in RNA The pentose sugar in DNA 2 deoxyribose differs from the sugar in RNA ribose by the absence of a hydroxyl group OH on the 2 carbon of the sugar ring Without an attached phosphate group the sugar attached to one of the bases is known as a nucleoside The phosphate group connects successive successive sugar residues by bridging the 5 hydroxyl group on one sugar to the 3 hydroxyl group of the next sugar in the chain These nucleoside linkages are called phosphodiester bonds and are the same in RNA and DNA Nucleotides are created or synthesized from readily available precursors in the cell i e smaller molecules of the right chemical composition The 16 EXAMINING BASIC CHEMICAL MOLECULES
NUCLEIC ACIDS ribose phosphate portion of both purine and pyrimidine pyrimidine nucleotides is synthesized from the sugar glucose via a chemical process called the pentose phosphate pathway The six-atom pyrimidine ring is synthesized first and subsequently attached to the ribose phosphate The two rings in purines are synthesized while attached to the ribose phosphate during the assembly of adenine or guanine nucleosides nucleosides In both cases the end product is a nucleotide nucleotide carrying a phosphate attached to the 5 carbon on the sugar Finally a specialized enzyme called a kinase adds two phosphate groups using adenosine triphosphate ATP an energy-carrying molecule found in the cells of all living things as the phosphate phosphate donor to form ribonucleoside triphosphate the immediate precursor of RNA For DNA the hydroxyl OH group from the 2 position is removed from the ribonucleoside diphosphate to give deoxyribonucleoside deoxyribonucleoside diphosphate An additional phosphate group from ATP is then added by another kinase to form a deoxyribonucleoside triphosphate the immediate precursor of DNA During normal cell metabolism RNA is constantly constantly being made and broken down The purine and pyrimidine residues are reused by several salvage pathways to make more genetic material Purine is salvaged in the form of the corresponding nucleotide whereas pyrimidine is salvaged as the nucleoside 17
