Chapter 4 – Nucleic Acids and the RNA WorldI. What Is a Nucleic Acid? C, H, O, N, Pa. What is a nucleic acid made of?i. A nucleic acid is a polymer that is made up of monomers called nucleotidesii. One nucleotide consists of a phosphate group, a sugar, and a nitrogenous baseiii. The sugar (a pentose) can either be ribose (in RNA) or deoxyribose (in DNA)iv. The nitrogenous bases:- There are two classes of nitrogenous bases: purines and pyrimidinesa. Adenine (A) and guanine (G) are purinesb. Cytosine (C), uracil (U), and thymine (T) are pyrimidines- Ribonucleotides can have the bases A, G, C, and U- Deoxyribonucleotides can have the bases A, G, C, and TNucleosides contain only the sugar and the Nitrogenous base, and NO phosphate group.b. How do nucleotides polymerize to form nucleic acids?(i) Nucleic acids form when nucleotides polymerize  a condensation reaction that forms a 3’ 5’ phosphodiester linkage, linking the (phosphate group of) 5’ carbon of one nucleotide to ( –OH of) 3’ carbon of the sugar of the next nucleotide  sugar-phosphate backbone-The sequence of nitrogenous bases forms the primary structure of the RNA or DNA molecule(ii) DNA and RNA strands are directional1. One end has an unlinked 5’ carbon with a free phosphate group2. The other end as an unlinked 3’ carbon with a free –OH group, where new nucleotides can be added3. The sequences of bases of a strand of DNA or RNA is conventionally written from the 5’ end to the 3’ end(iii) Polymerization is an endergonic process that requires energyII. DNA Structure and Functiona. DNA’s primary structure  Base sequencei. DNA has a sugar-phosphate backbone with four types of nitrogenous basesii. DNA’s primary structure is the sequence of nitrogenous basesa. What is the nature of DNA’s secondary structure?i. The discovery of DNA’s secondary structure by Watson and Crick (1953) was oneof the greatest scientific breakthroughs of the twentieth centuryii. Early data provided clues:1. Chemists already knew that DNA must have a sugar-phosphate backbone2. Chargaff found that nucleotides seem to be “paired” in some way:-The total numbers of purines and pyrimidines are equal in any given nucleic acid molecule-The amount of A is always equal to the amount of T, and C is always equal to Giii. Watson and Crick put the pieces together:1-Antiparallel strands: by building models, Watson and Crick deduced that the two DNA strands must run in opposite directions (antiparallel) with the nitrogenous bases in the middle-Complementary base pairing: to maintain the correct distance between the two strands, a purine must always pair with a pyrimidine.  hydrogen bonds form between A and T (2) and between G and C (3) 5’ ATTGCCAT write the complementary strand? 5’3’?-The double helix: the hydrogen bonds cause the backbone to twist, forming a double helix with a major groove and a minor grooveb. DNA functions as an information-containing moleculei. In all cells studied, DNA carries the information required for the organism’s growth and reproductionii. This information is contained in the sequence of nucleotides (base sequence)iii. The double helix allows the information to be copied, by using one strand as a template for the synthesis of a complementary strandc. Is DNA a catalytic molecule? NOi. DNA is highly stable because it exists as a double-stranded structure with few exposed functional groupsii. DNA is so stable, and so simple in structure, that it cannot catalyze any reactionsIII. RNA Structure and Functiona. Structurally, RNA differs from DNAi. RNA’s primary structure1. Like DNA, RNA has a sugar-phosphate backbone with phosphodiester linkages between nucleotides, with a sequence of four nitrogenous bases2. Unlike DNA, RNA contains ribose instead of deoxyribose. This makes RNA more reactive and less stable than DNA3. Unlike DNA, RNA contains uracil instead of thymine. Uracil pairs with adenineii. RNA’s secondary structure- Unlike DNA, a single RNA strand tends to fold and form complementary base pairs with itself, resulting in structures such as hairpin structure (stems & loops)iii. RNA’s tertiary and quaternary structures- Unlike DNA, RNA molecules can have tertiary structure and quaternary structure, resulting in very different shapes and chemical propertiesb. RNA’s structure makes it an extraordinarily versatile molecule 1. RNA is not as good at information storage as DNA or as good at catalysis as protein, but it can do “a little of everything”2. RNA has recently been found to perform many diverse and important functions in cellsc. RNA is an information-containing molecule(i) RNA’s sequence of base pairs can carry information, just as in DNA(ii) Unlike DNA, the template RNA molecule is a single strand rather than a double strand(iii) RNA can function as a catalytic molecule  RNA catalysts are called ribozymesd. 3 major types of RNA  mRNA, tRNA and rRNA , all 3 involved in protein synthesis2DNA and RNA - Structure: See summary table 4.1FLOW OF BIOLOGICAL INFORMATION: DNA ® RNA ® Polypeptide (protein) ® STRUCTURE /