• Nucleotides are monomers that consist of a sugar, a phosphate group, and a nitrogen-containing
base. Ribonucleotides polymerize to form RNA. Deoxyribonucleotides polymerize to form
• DNAand RNA’s primary structure consists of a sequence of nitrogen-containing bases, which
contain information in the form of a molecular code.
• DNA’s secondary structure consists of two DNAstrands running in opposite directions, held
together by complementary base pairing, and twisted into a double helix. RNA’s secondary
structure includes short double helices and structures called hairpins.
What Is a NucleicAcid?
• Anucleic acid is a polymer of nucleotides that are each composed of a phosphate group, a
sugar, and a nitrogenous base,
• The sugar is ribose in ribonucleotides and deoxyribose in deoxyribonucleotides.
• There are two groups of nitrogenous bases: Purines (adenine,
• guanine) and Pyrimidines (cytosine, uracil, and thymine).
• U is found only in ribonucleotides, and T is found only in deoxyribonucleotides.
Could Chemical Evolution Produce Nucleotides?
• Simulations of chemical evolution have not yet produced nucleotides.
• Sugars and purines are easily made, but pyrimidines and ribose are not easily synthesized.
• Ribose problem: Ribose would have had to have been dominant on ancient Earth for nucleic
acids to form.
Nucleotides Polymerize to Form NucleicAcids
• Method: Formation of a phosphodiester bond between the phosphate group on the 5′ carbon
of one nucleotide and the –OH group on the 3′ carbon of another through a condensation
• Types of nucleotides involved:
1. Ribonucleotides, which contains the sugar ribose and forms RNA
2. Deoxyribonucleotides, which contains the sugar deoxyribose and forms DNA
• In cells, enzyme-catalyzed polymerization of nucleotides require the addition of two extra phosphate groups.
The Sugar-Phosphate Backbone Is Directional
• The sugar-phosphate backbone of a nucleic acid is directional—one end has an unlinked 5′
carbon, and the other end has an unlinked 3′ carbon.
• The nucleotide sequence is written in the 5′ ® 3′ direction. This reflects the sequence in which
nucleotides are added to a growing molecule.
What Is the Nature of DNA's Secondary Structure?
• Erwin Chargaff established two empirical rules for DNA:
1. The total number of purines and pyrimidines is the same.
2. The numbers of A’s and T’s are equal an