Nucleic Acids and the RNA World: Structure, Function, and Evolution chapter 4

Chapter 4: Nucleic Acids and an RNA World

Introduction

Nucleic acids are essential biomolecules that store and transmit the genetic information necessary for life. This chapter explores the structure and function of nucleic acids, the differences between DNA and RNA, and the hypothesis that life may have originated in an "RNA world."

• Chemical evolution led to the production of molecules capable of self-replication.

• Deoxyribonucleic acid (DNA) stores genetic information and is replicated using proteins.

• RNA world hypothesis proposes a time in evolution when RNA both stored genetic information and catalyzed its own replication.

• Once self-replicating molecules evolved, biological evolution began.

What is a Nucleic Acid?

Structure of Nucleic Acids

Nucleic acids are polymers made up of nucleotide monomers. Each nucleotide consists of three components:

• Phosphate group

• Five-carbon sugar (either ribose or deoxyribose)

• Nitrogenous base (a nitrogen-containing base)

The phosphate group and nitrogenous base are bonded to the sugar molecule.

Types of Nucleotides

• Ribonucleotides are the monomers of RNA:

  • Contain ribose as their sugar

  • Have an –OH group bonded to the 2′ carbon

• Deoxyribonucleotides are the monomers of DNA:

  • Contain deoxyribose (lacking oxygen at the 2′ carbon)

  • Have an H instead of –OH at the 2′ carbon

• Both sugars have an –OH group bonded to the 3′ carbon.

Nitrogenous Bases

• Purines (two-ring structure):

  • Adenine (A)

  • Guanine (G)

• Pyrimidines (one-ring structure):

  • Cytosine (C)

  • Uracil (U) – found only in RNA

  • Thymine (T) – found only in DNA

Mnemonic: "CUT the Py" (Cytosine, Uracil, Thymine are Pyrimidines)

Origin and Polymerization of Nucleotides

Chemical Evolution of Nucleotides

• Amino acids could have been synthesized on early Earth, but the prebiotic synthesis of nucleotides is more complex.

• Some experiments show that nitrogenous bases and sugars can be synthesized under certain conditions.

• Recent work suggests that deep-sea hydrothermal vents and reactive minerals may have played a role in concentrating ribose.

Polymerization of Nucleic Acids

• Nucleic acids polymerize via condensation reactions, forming phosphodiester linkages between the 5′ phosphate of one nucleotide and the 3′ hydroxyl of another.

• Phosphodiester linkages create a sugar-phosphate backbone that is directional (5′ to 3′).

• The sequence of nitrogenous bases encodes genetic information.

Example: The primary structure of DNA can be written as 5′-ATTAGC-3′.

Energy for Polymerization

• Polymerization requires energy, which is provided by nucleoside triphosphates (e.g., ATP).

• Energy is released when activated nucleotides polymerize, making the reaction spontaneous.

DNA Structure and Function

Secondary Structure of DNA

• DNA is polymerized through phosphodiester linkages, forming a sugar-phosphate backbone.

• Chargaff's rules: Number of purines equals number of pyrimidines (A = T, G = C).

• X-ray crystallography revealed that DNA is helical.

Double Helix and Base Pairing

• James Watson and Francis Crick determined that DNA consists of two antiparallel strands held together by hydrogen bonds between complementary bases.

• Base pairing: A pairs with T, G pairs with C (Watson-Crick pairing).

• The double helix has a sugar-phosphate backbone on the outside and nitrogenous bases on the inside.

• DNA has major and minor grooves, which are important for protein binding.

Tertiary Structure of DNA

• DNA can form more compact three-dimensional structures in cells.

• Supercoiling occurs when DNA is wound too tightly or loosely.

• DNA wraps around histone proteins to form nucleosomes.

DNA as an Information-Containing Molecule

• DNA stores the information required for growth and reproduction.

• The sequence of bases functions like letters in an alphabet, encoding genetic instructions.

DNA Replication

1. Two strands are separated by breaking hydrogen bonds.

2. Free deoxyribonucleotides form hydrogen bonds with complementary bases on the template strand.

3. Phosphodiester linkages form to create a new complementary strand.

Complementary base pairing allows each strand to be copied exactly, producing two identical daughter molecules.

Stability of the DNA Double Helix

• The double helix is stabilized by phosphodiester linkages, hydrogen bonds, and hydrophobic interactions.

• DNA's stability makes it an effective information-storage molecule.

• There is no evidence that the first life forms consisted of DNA alone.

RNA Structure and Function

Primary and Secondary Structure of RNA

• RNA contains ribose and uracil (instead of thymine).

• The 2′-OH group on ribose makes RNA more reactive and less stable than DNA.

• RNA's secondary structure results from complementary base pairing (A with U, G with C) within the same strand, forming hairpin structures.

Tertiary Structure of RNA

• RNA molecules can fold into complex three-dimensional shapes.

• RNA is more diverse in size, shape, and reactivity than DNA.

Comparison of DNA and RNA Structure

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Primary Bases	A, T, G, C	A, U, G, C
Strands	Double-stranded (usually)	Single-stranded (usually)
Stability	More stable	Less stable
Function	Information storage	Information transfer, catalysis

RNA's Versatility

• RNA can fold into complex shapes, allowing it to perform many tasks.

• Messenger RNA (mRNA) transmits information from DNA to protein synthesis machinery.

• Some RNAs regulate gene expression or catalyze reactions (ribozymes).

RNA as a Catalytic Molecule

• Ribozymes are RNA molecules that catalyze chemical reactions.

• Their three-dimensional structure is vital to their catalytic activity, similar to protein enzymes.

• Some ribozymes can catalyze the formation of phosphodiester bonds, suggesting that RNA could have replicated itself in early evolution.

The RNA World Hypothesis

Origin of Life and the Role of RNA

• The theory of chemical evolution suggests that life began as a naked self-replicator, a molecule capable of copying itself without a membrane.

• Such a molecule would need to provide a template for copying and catalyze its own replication.

• Most researchers propose that the first life-form was made of RNA, as it can both store information and catalyze reactions.

Experimental Evidence for the RNA World

• Laboratory studies have generated ribozymes capable of catalyzing RNA replication (e.g., adding nucleotides to an RNA strand).

• Some ribozymes can synthesize RNA nucleotides or add specific bases, supporting the plausibility of an RNA world.

Transition from RNA to Modern Biology

• Modern ribozymes are essential for protein production; without them, proteins could not be made.

• This suggests that RNA likely preceded proteins in evolution.

• The evolution of protein enzymes marked the end of the RNA world and the beginning of modern biology.

• Three characteristics of life were established: information processing, replication of hereditary information, and evolution by random changes in nucleic acids.