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Chapter 24 Nucleotides, Nucleic Acids, and Heredity The Molecules of Heredity • Each cell has thousands of different proteins. • How do cells know which proteins to synthesize out of 100000s possible amino acid sequences? • From the end of the 19th century, biologists suspected that the transmission of hereditary information took place in the nucleus, more specifically in structures called chromosomes. • The hereditary information was thought to reside in genes within the chromosomes. • Chemical analysis of nuclei showed chromosomes are made up largely of proteins called histones and nucleic acids. The Molecules of Heredity • By the 1940s, it became clear that deoxyribonucleic acids (DNA) carry the hereditary information. • Other work in the 1940s demonstrated that each gene controls the manufacture of one protein. work protein • Thus the expression of a gene in terms of an enzyme protein led to the study of protein synthesis and its control. Structure of DNA and RNA (based on Nucleic Acids) • Two Kinds in cells: • ribonucleic acids (RNA) • deoxyribonucleic acids (DNA) • RNA & DNA: polymers built from monomers (nucleotides) • A nucleotide is composed of: • 1. a base, 2. a monosaccharide, 3. a phosphate, (e.g. AMP) (DNA and some RNA) (DNA onl y) (in RNA only) 1. Purine/Pyrimidine Bases 7 6 1 2 5 N N H2 N 8 N 3 N9 4 H Puri ne O 4 N N N HN 5 3 2 N Adenine (A) (DNA and RNA) N NO H N N H PyriGuani mi dine ne (G) Cytosine (C) (DNA and RNA) (DNA and some RNA) 1 2 5 N N H2 7 6 N 8 N 3 N9 4 H Puri ne Base Pairing: DNA: A-T;C-G RNA: A-U;C-G N 6 N H 2 N1 N H N H2 note: N N N N H Adenine (A (DNA and RN Nucleosides (base and sugar) • compound that consists of D-ribose or 2-deoxy-D-ribose bonded to a purine or pyrimidine base by a -N-glycosidic bond. uracil HN -D -ribos ide 1 O 5' HOCH 2 H N O H 4' 3' O H 2' HO OH Urid ine 1' H a -N -glycosid ic bon d anomeric carb on Nucleotides • a nucleoside w/ molecule of phosphoric acid esterified with an -OH of the monosaccharide, most commonly either the 3’ or the 5’-OH. NH2 N O 5' N O-P-O-CH2 O N H H 1' O H 3' H HO OH Aden os in e 5'-monophosp hate (5'-A MP) N ATP- a nucleotide In Summary Nucleoside = Base + Sugar Nucleotide = Base + Sugar + Phosphoric acid Nucleic acid = chain of nucleotides Structure of DNA and RNA Primary Structure •sequence is read from the 5’ end to the 3’ end start • -bases arranged in various patterns (like A.A.s for Proteins) GENE ((protein)) finish DNA - 2° Structure • the ordered arrangement of nucleic acid strands. • the double helix model of DNA 2° structure was proposed by James Watson and Francis Crick in 1953. Using Chargaff rules: (A-T; C-G) -X-ray (Franklin, Wilkins) Watson, Crick and Wilkins (Nobel Prize 1962) (R. Franklin, 1920-1958) • Double helix: 2° structure of DNA in which two polynucleotide strands are coiled around each other in a screw-like fashion. The DNA Double Helix -Polynucleotide chains run anti-parallel -Bases (hydrophobic) avoid water & stabilize d. helix w/ H-bonds (below) Base Pairing Higher Structure of DNA • DNA is coiled around proteins called histones. • Histones are rich in the basic amino acids Lys and Arg, whose side chains have a positive charge. • The negatively-charged DNA molecules and positivelycharged histones attract each other and form units called nucleosomes. _ + _ • Nucleosome: a core of eight histone molecules around which the DNA helix is wrapped. Chromosomes Nucleosomes are further condensed into chromatin. Chromatin fibers are organized into loops, and the loops into the bands that provide the superstructure of chromosomes.