27.23 Pyrimidines and Purines,Pyrimidines and Purines,In order to understand the structure and properties of DNA and RNA, we need to look at their structural components. We begin with certain heterocyclic aromatic compounds called pyrimidines and purines.,Pyrimidines and Purines,Pyrimidine and purine are the names of the parent compounds of two types of nitrogen-containing heterocyclic aromatic compounds.,Pyrimidine,Purine,Important Pyrimidines,Pyrimidines that occur in DNA are cytosine and thymine. Cytosine and uracil are the pyrimidines in RNA.,Uracil,Thymine,HN,N H,NH2,O,Cytosine,Important Purines,Adenine and guanine are the principal purines of both DNA and RNA.,Adenine,Guanine,Caffeine and Theobromine,Caffeine (coffee) and theobromine (coffee and tea) are naturally occurring purines.,27.24 Nucleosides,Nucleosides,The classical structural definition is that a nucleoside is a pyrimidine or purine N-glycoside of D-ribofuranose or 2-deoxy-D-ribofuranose. Informal use has extended this definition to apply to purine or pyrimidine N-glycosides of almost any carbohydrate. The purine or pyrimidine part of a nucleoside is referred to as a purine or pyrimidine base.,Uridine and Adenosine,Uridine and adenosine are pyrimidine and purine nucleosides respectively of D-ribofuranose.,Uridine (a pyrimidine nucleoside),Adenosine (a purine nucleoside),27.25 Nucleotides,Nucleotides,Nucleotides are phosphoric acid esters of nucleosides.,Adenosine 5-Monophosphate (AMP),Adenosine 5-monophosphate (AMP) is also called 5-adenylic acid.,Adenosine 5-Monophosphate (AMP),Adenosine 5-monophosphate (AMP) is also called 5-adenylic acid.,1,2,3,4,5,Adenosine Diphosphate (ADP),Adenosine Triphosphate (ATP),ATP Stores Energy,AMP,ADP,ATP,Each step is endothermic. Energy for each step comes from carbohydrate metabolism (glycolysis). Reverse process is exothermic and is the source of biological energy. DG for hydrolysis of ATP to ADP is 35 kJ/mol,Adenosine 3-5-Cyclic Monophosphate (cAMP),Cyclic AMP is an important regulator of many biological processes.,27.26 Nucleic Acids,Nucleic Acids,Nucleic acids are polymeric nucleotides (polynucleotides). 5 Oxygen of one nucleotide is linked to the 3 oxygen of another.,Fig. 27.22,A section of a polynucleotide chain.,27.27 Structure and Replication of DNA: The Double Helix,Composition of DNA,Erwin Chargaff (Columbia Univ.) studied DNAs from various sources and analyzed the distribution of purines and pyrimidines in them. The distribution of the bases adenine (A), guanine (G), thymine (T), and cytosine (C) varied among species. But the total purines (A and G) and the total pyrimidines (T and C) were always equal. Moreover: %A = %T, and %G = %C,Composition of Human DNA,Adenine (A) 30.3% Thymine (T) 30.3% Guanine (G) 19.5% Cytosine (C) 19.9% Total purines: 49.8% Total pyrimidines: 50.1%,For example:,Purine,Pyrimidine,Base Pairing,Watson and Crick proposed that A and T were equal because of complementary hydrogen bonding.,2-deoxyribose,2-deoxyribose,A,T,Base Pairing,Likewise, the amounts of G and C were equal because of complementary hydrogen bonding.,2-deoxyribose,2-deoxyribose,G,C,The DNA Duplex,Watson and Crick proposed a double-stranded structure for DNA in which a purine or pyrimidine base in one chain is hydrogen bonded to its complement in the other.,Fig. 27.24,Two antiparallel strands of DNA are paired by hydrogen bonds between purine and pyrimidine bases.,Fig. 27.25,Helical structure of DNA. The purine and pyrimidine bases are on the inside, sugars and phosphates on the outside.,Fig. 27.26 DNA Replication,C,G,T,A,As the double helix unwinds, each strand acts as a template upon which its complement is constructed.,Fig. 27.26 DNA Replication,A,C,G,T,T,A,G,C,27.28 DNA-Directed Protein Biosynthesis,DNA and Protein Biosynthesis,According to Crick, the “central dogma“ of molecular biology is: “DNA makes RNA makes protein.“ Three kinds of RNA are involved. messenger RNA (mRNA) transfer RNA (tRNA) ribosomal RNA (rRNA) There are two main stages. transcription translation,Transcription,Transcription is the formation of a strand of mRNA using one of the DNA strands as a template. The nucleotide sequence of the mRNA is complementary to the nucleotide sequence of the DNA template. Transcription begins at the 5 end of DNA and is catalyzed by the enzyme RNA polymerase.,Transcription,As double-stranded DNA unwinds, a complementary strand of mRNA forms at the 5 end.,5,3,Transcription,A,G,G,T,C,A,C,T,G,T,C,C,A,G,T,G,A,C,A,T,G,C,T,T,T,T,T,T,C,C,C,C,C,A,A,A,A,A,G,G,A,G,G,G,5,3,Uracil is incorporated in RNA instead of thymine.,Translation,The nucleotide sequence of mRNA codes for the different amino acids found in proteins. There are three nucleotides per codon. There are 64 possible combinations of A, U, G, and C. The genetic code is redundant. Some proteins are coded for by more than one codon.,Table 27.4: mRNA Codons,Alanine Arginine Asparagine