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Chapter 27
Heterocyclic Compounds

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27-00CO
Title
RNA Catalyst
Caption
Ribbon structure of an RNA catalyst.
Notes
RNA has traditionally been thought to perform only three functions in the cell. It carries genetic information from the DNA in the nucleus to the ribosome, where it is decoded to produce a protein. Secondly, RNA is what ribosomes are made of. Finally, transfer RNA carries amino acid molecules to the ribosome, where they are covalently bonded together to form proteins. Recently, it has been found that RNA molecules can act as catalysts as well, like proteins in enzymes. The term "ribozymes" was coined to refer to RNA molecules acting like enzymes.
Keywords
RNA, catalyst, ribbon, structure
27-02
Title
Figure 27.2
Caption
Interaction between ATP magnesium ion, and arginine and lysine residues at the active site of an enzyme.
Notes
Although ATP is thermodynamically unstable, it is kinetically stable because it is highly negatively charged at ordinary pH. This charge tends to ward off nucleophiles and keep it stable until it is needed. Enzymes which use ATP generally neutralize the negative charge on this ion with ammonium ions like the protonated amine groups of the side chains of lysine or arginine, or metal ions like magnesium.
Keywords
figure, 27.2, ATP, magnesium, arginine, lysine, enzyme
27-05
Title
Figure 27.5
Caption
Base pairing in DNA.
Notes
Adenine and thymine link together forming two hydrogen bonds, and cytosine and guanine link together forming three hydrogen bonds.
Keywords
figure, 27.5, base, pairing, adenine, thymine, cytosine, guanine, DNA
27-06
Title
Figure 27.6
Caption
Structure of DNA showing the sugar–phosphate backbone on the outside (left and right) and bases on the inside.
Notes
The bases are depicted as A, T, C, and G in the structural formula, but are depicted completely in the ball-and-stick model. The two strands which are hydrogen bonded to each other are antiparallel—they run in opposite directions.
Keywords
figure, 27.6, structure, DNA
27-07
Title
Figure 27.7
Caption
The DNA double helix.
Notes
(a) Shows a side view in space-filling format. (b) Shows an end-on view in space-filling format. (c) Shows a ball-and-stick side view, which makes the planar parallel arrangement of bases apparent.
Keywords
figure, 27.7, DNA, double, helix
27-09
Title
Figure 27.9
Caption
The three helical forms of DNA.
Notes
The helix on the left is a B-helix, the one in the middle is an A-helix, and the one on the right is a Z-helix.
Keywords
figure, 27.9, DNA, helical, forms, B-helix, A-helix, Z-helix
27-13
Title
Figure 27.13
Caption
Structure of the transfer RNA molecule which carries alanine.
Notes
On the left is a schematic depiction showing sequence (primary structure) of ribonucleotides in an RNA molecule and folded secondary structure held together by hydrogen bonding between complementary ribonucleotide bases. On the right is a space-filling model.
Keywords
figure, 27.13, transfer, RNA, structure
27-15
Title
Figure 27.15
Caption
Aminoacyl-tRNA synthetase enzyme.
Notes
An aminoayl-tRNA synthetase enzyme has a binding site for a specific tRNA molecule and another binding site for a specific amino acid molecule. The enzyme attaches the amino acid to its proper tRNA. There is a different aminoacyl-tRNA synthetase enzyme for every amino acid so that every amino acid gets bound only to a tRNA molecule with the correct triplet sequence in its anticodon loop.
Keywords
figure, 27.15, aminoacyl-tRNA, synthetase
27-17
Title
Figure 27.17
Caption
Protein synthesis.
Notes
The sense strand of DNA is used to synthesize a complementary strand of pre-mRNA (transcription) in the nucleus. Next, pre-mRNA is modified by enzymes and ribozymes in various ways, producing mRNA in the nucleus. Mature mRNA is recognized by nuclear pores and exported out of the nucleus. It finds a ribosome and lays down on the ribosome to begin the process of translation. tRNA molecules carrying amino acids hydrogen bond to regions of the mRNA which have the correct sequence of three bases complementary to the anticodon loop triplets on the bottom of the tRNA molecules. Enzymes covalently bond the amino acids in adjacent positions together, removing these amino acids from their tRNA carriers. The ribosome ejects tRNA molecules which are no longer attached to amino acids and spits out the developing protein strand where waiting enzymes fold it into its correct shape.
Keywords
figure, 27.17, protein, synthesis
27-18
Title
Figure 27.18
Caption
An autoradiograph.
Notes
Autoradiography is used to determine the sequence of nucleotides in strands of DNA.
Keywords
figure, 27.18, autoradiography
27-19
Title
Figure 27.19
Caption
Space-filling model of a DNA triple helix.
Notes
Natural double-stranded DNA can be mixed with synthetic strands of DNA, and the synthetic strands can be made to base pair with complementary regions of one of the natural DNA strands. This occurs because one adenine can simultaneously hydrogen bond to two thymines, and a guanine can do the same thing with two cytosines, in a process known as Hoogsteen base pairing. When a synthetic strand of DNA binds to one of the two strands of natural DNA, a triple helix results.
Keywords
figure, 27.19, space-filling, DNA, triple, helix, Hoogsteen

Title
Table 27.1 The Names of the Bases, the Nucleosides, and the Nucleotides
Caption
Notes
Keywords

Title
Table 27.2 The Genetic Code
Caption
Notes
Keywords

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