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Chapter 4

BIOCHEM 2B03 Chapter Notes - Chapter 4: Erwin Chargaff, Ribose, Nitrogenous Base

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Margaret Fahnestock

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Bio 1A03
Unit One: The Molecules of Life
Chapter 4: Nucleic Acids and The RNA World
Two attributes to distinguish life from non-life
o Reproduction the ability of something to make a copy of itself
o Metabolism the ability to acquire particular molecules and use them in controlled chemical reactions
that maintain conditions suitable for life and that contribute to growth
o To many biologists, the presence of a plasma membrane is also required for life, because enzymes and
reactants are contained inside a cell by a plasma membrane
4.1 What Is a Nucleic Acid?
Nucleic Acids: Polymers made up of monomers called nucleotides
3 components of a nucleotide:
o Phosphate Group
o Sugar
Organic compound with a carbonyl group and several hydroxyl groups
C#3 OH is involved in bonding with the next nucleotide
C#1 is attached to the nitrogenous base
C#5 is attached to the phosphate group
o Nitrogenous Base
Ribonucleotides - Sugar is ribose; has an OH group bonded to C#2 in the ring
o Purines: Adenine (A) and Guanine (G)
o Pyrimidines: Cytosine (C) and Uracil (U)
Deoxyribonucleotides - Sugar is deoxyribose; has an H bonded to the C#2 in the ring
o Purines: Adenine (A) and Guanine (G)
o Pyrimidines: Cytosine (C) and Thymine (T)
DNA and RNA are Polynucleotides
The ribose problem
o When formaldehyde molecules are heated in solution, they react with one another to form almost all of
the sugars that have five or six carbons
o The ease of forming these sugars creates a problem in modern experiments, the various pentose’s and
hexoses are produced in approximately equal amounts, but it seems logical to predict that ribose would
have had to be particularly abundant for RNA or DNA to form in the prebiotic soup
The origin of pyrimidine
o Origin-of-life researchers have yet to discover a plausible mechanism for the synthesis of cytosine,
uracil and thymine molecules prior to the origin of life
o Purines are readily synthesized by reactions among hydrogen cyanide (HCN) molecules
How Do Nucleotides Polymerize to Form Nucleic Acids?
Nucleic acids form when nucleotides polymerize
Phosphodiester linkage/bond: Condensation reaction between the phosphate group of one nucleotide and the
hydroxyl group of the sugar component of another nucleotide
o -C-O-P-O-C
o A phosphodiester linkage joins the C#5 on the ribose of one nucleotide to the C#3 carbon on the ribose
of the other
o Figure 4.2, page 73
Chain of phosphodiester linkages in a nucleic acid acts as a backbone and is directional
o One end has an unlinked 5’ carbon, which the other end has an unlinked 3’ carbon
o The sequence of bases found in RNA or DNA is always written in a 5’ 3’ direction
When RNA or DNA is synthesized, bases are added at the 3’ end of the growing molecule
o Figure 4.3, page 74
The sequence of nitrogenous bases forms the primary structure of the molecule
Polymerization reactions that form nucleotides are catalyzed by enzymes polymerization reactions are
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