Notes From Reading
CHAPTER 4: N UCLEICA CIDS AND THE RNAW ORLD (PGS.71-85)
- Nucleotides are monomers that consist of a sugar, a phosphate group, and a nitrogen-
containing base. Ribonucleotides polymerize to form RNA. Deoxyribonucleotides polymerize
to form DNA.
- DNA and RNA’s primary structure consists of a sequence of nitrogen-containing bases, which
contain information in the form of a molecular code.
- DNA’s secondary structure consists of two DNA strands running in opposite directions, held
together by complementary base pairing, and twisted into a double helix. RNA’s secondary
structure includes short double helices and structures called hairpins.
4.1 What is a Nucleic Acid?
- A nucleic acid is a polymer of nucleotides that are each composed of a phosphate group, a
sugar, and a nitrogenous base,
- The sugar is ribose in ribonucleotides and deoxyribose in deoxyribonucleotides.
- There are two groups of nitrogenous bases: Purines (adenine,
- guanine) and Pyrimidines (cytosine, uracil, and thymine).
- U is found only in ribonucleotides, and T is found only in deoxyribonucleotides.
Could Chemical Evolution Produce Nucleotides?
- Simulations of chemical evolution have not yet produced nucleotides.
- Sugars and purines are easily made, but pyrimidines and ribose are not easily synthesized.
- Ribose problem: Ribose would have had to have been dominant on ancient Earth for nucleic
acids to form.
Nucleotides Polymerize to Form Nucleic Acids
- Method: Formation of a phosphodiester bond between the phosphate group on the 5′
carbon of one nucleotide and the –OH group on the 3′ carbon of another through a
- Types of nucleotides involved:
(1) Ribonucleotides, which contains the sugar ribose and forms RNA
(2) Deoxyribonucleotides, which contains the sugar deoxyribose and forms DNA
- In cells, enzyme-catalyzed polymerization of nucleotides require the addition of two extra
The Sugar-Phosphate Backbone is Direction
- The sugar-phosphate backbone of a nucleic acid is directional—one end has an unlinked 5′
carbon, and the other end has an unlinked 3′ carbon.
- The nucleotide sequence is written in the 5′ 3′ direction. This reflects the sequence in
which nucleotides are added to a growing molecule. Notes From Reading
CHAPTER 4: NUCLEIC ACIDS AND THE RNAW ORLD (PGS .71-85)
4.2 DNA Structure and Function
What is the Nature of DNA’s Secondary Structure?
- Erwin Chargaff established two empirical rules for DNA:
(1) The total number of purines and pyrimidines is the same.