25.1 DNA, Chromosomes, and Genes • When a cell is not dividing, its nucleus is occupied by chromatin, DNA (deoxyribonucleic acid), twisted around organizing proteins known as histones. • During cell division, chromatin organizes itself into chromosomes. • Each chromosome contains a different DNA molecule; the DNA is duplicated so each new cell receives a complete copy. © 2013 Pearson Education, Inc.

25.1 DNA, Chromosomes, and Genes • Each DNA molecule is composed of genes— individual segments of the DNA molecule containing the instructions that direct the synthesis of a polypeptide. • Some genes code for functional RNA molecules. • Organisms differ widely in their numbers of chromosomes. A horse has 64 chromosomes (32 pairs), a cat has 38 chromosomes (19 pairs), a mosquito has 6 chromosomes (3 pairs), and a corn plant has 20 chromosomes (10 pairs). A human has 46 chromosomes (23 pairs). © 2013 Pearson Education, Inc.

25.2 Composition of Nucleic Acids • Nucleic acids are polymers known as polynucleotides. • Each nucleotide has three parts, a fivemembered cyclic monosaccharide, a nitrogenous base, and a phosphate group.

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25.2 Composition of Nucleic Acids • Nucleic acids are polymers known as polynucleotides. • There are two types of nucleic acids, DNA and RNA (ribonucleic acid). • Each nucleotide has three parts: a five-membered cyclic monosaccharide, a nitrogenous base, and a phosphate group.

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25.2 Composition of Nucleic Acids The Sugars • In RNA, the sugar is D-ribose, as indicated by the name ribonucleic acid. • In DNA, the sugar is 2-deoxyribose, giving deoxyribonucleic acid.

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25.2 Composition of Nucleic Acids The Bases • The five nitrogenous bases found in DNA and RNA are all derived from two parent compounds, purine and pyrimidine.

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25.2 Composition of Nucleic Acids The Bases • In addition to differing in the sugars they contain, RNA and DNA differ in their bases: – Thymine is present only in DNA molecules (with rare exceptions).

– Uracil is present only in RNA molecules. – Adenine, guanine, and cytosine are present in both DNA and RNA. © 2013 Pearson Education, Inc.

25.2 Composition of Nucleic Acids Sugar + Base = Nucleoside • The sugar and base are connected by a b-N-glycosidic bond to the anomeric carbon of the sugar. • Nucleoside names are the nitrogenous base name modified by the suffix -osine for the purine bases and the suffix -idine for the pyrimidine bases. • The prefix deoxy- is added for those that contain deoxyribose. • To distinguish atoms in the sugar ring from atoms in the base ring (or rings), numbers without primes are used for atoms in the base, and numbers with primes for atoms in the sugar. © 2013 Pearson Education, Inc.

25.2 Composition of Nucleic Acids Nucleoside + Phosphate = Nucleotide • Nucleotides are the building blocks of nucleic acids; they are the monomers of DNA and RNA polymers. • A nucleotide is a 5’-monophosphate ester of a nucleoside:

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25.2 Composition of Nucleic Acids Nucleoside + Phosphate = Nucleotide

• Nucleotides are named by adding 5’-monophosphate at the end of the nucleoside name. • Nucleotides containing ribose are ribonucleotides. • Those that contain 2-deoxy-D-ribose are deoxyribonucleotides, designated by leading their abbreviations with a lower case “d”. • Phosphate groups can be added to any of the nucleotides to form diphosphate or triphosphate esters. These esters are named with the nucleoside name plus diphosphate or triphosphate. © 2013 Pearson Education, Inc.

25.2 Composition of Nucleic Acids

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25.2 Composition of Nucleic Acids Summary—Nucleoside, Nucleotide, and Nucleic Acid Composition • Nucleoside – A sugar and a base

• Nucleotide – A sugar, a base, and a phosphate group

• DNA (deoxyribonucleic acid) – A polymer of deoxyribonucleotides – The sugar is 2-deoxy-D-ribose – The bases are adenine, guanine, cytosine, and thymine

• RNA (ribonucleic acid) – A polymer of ribonucleotides – The sugar is D-ribose – The bases are adenine, guanine, cytosine, and uracil © 2013 Pearson Education, Inc.

25.3 The Structure of Nucleic Acid Chains • The nucleotides in DNA and RNA are connected by phosphate diester linkages between the —OH group on C3’ of the sugar ring of one nucleotide and the phosphate group on 5’ of the next nucleotide.

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25.3 The Structure of Nucleic Acid Chains • The structure and function of a nucleic acid depend on the sequence in which the nucleotides are connected. • The sequence of nucleotides in a nucleic acid chain is read by starting at the 5’ end and identifying the bases in the order of occurrence.

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25.4 Base-Pairing in DNA: The Watson-Crick Model

• Analysis of the nitrogenous bases in DNA samples from many different species revealed that the amounts of adenine and thymine were always equal, and the amounts of cytosine and guanine were always equal (A=T and G=C). • The proportions of each (A/T:G/C) vary from one species to another. • This is Chargoff’s Rule, and it suggests that the bases occur in discrete pairs. © 2013 Pearson Education, Inc.

25.4 Base-Pairing in DNA: The Watson-Crick Model • In 1953, James Watson and Francis Crick proposed a structure for DNA that accounts for the pairing of bases and the storage and transfer of genetic information. • According to the Watson–Crick model, a DNA molecule consists of two polynucleotide strands coiled around each other in a helical, screw-like fashion. • The sugar–phosphate backbone is on the outside of the right-handed double helix, and the heterocyclic bases are on the inside, so that a base on one strand points directly toward a base on the second strand.

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25.4 Base-Pairing in DNA: The Watson-Crick Model

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25.4 Base-Pairing in DNA: The Watson-Crick Model • The two strands of the DNA double helix are said to be antiparallel. • The stacking of hydrophobic bases in the interior and the alignment of the hydrophilic groups on the exterior provide stability to the structure. • Each pair of bases in the center of the double helix is connected by hydrogen bonding. Adenine and thymine (A-T) form two hydrogen bonds, and cytosine and guanine (C-G) form three hydrogen bonds.

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25.4 Base-Pairing in DNA: The Watson-Crick Model • The pairing of the bases is complementary. • Wherever a thymine occurs in one strand, an adenine falls opposite it in the other strand. • Wherever a cytosine occurs in one strand, a guanine falls opposite it on the other strand. • A and T and C and G occur in equal amounts.

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25.5 Nucleic Acids and Heredity • Duplication, transfer, and expression of genetic information occur as the result of three fundamental processes: replication, transcription, and translation. – Replication is the process by which a replica, or identical copy, of DNA is made when a cell divides, so that each daughter cell has the same DNA. – Transcription is the process by which DNA is read and copied. The products of transcription are ribonucleic acids, which carry the instructions stored by DNA to the sites of protein synthesis. – Translation is the process by which the messages carried by RNA are used to build proteins.

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