Biochemistry of the Genome: Structure and Function of Genetic Material

Chapter 10: Biochemistry of the Genome

Introduction

This chapter explores the molecular structure, organization, and function of genetic material in cells and viruses. It covers the chemical composition of DNA and RNA, the packaging of genomes, and the relationship between genotype and phenotype.

Genetics and the Genome

Definitions and Concepts

• Genetics: The scientific study of heredity and the variation of inherited characteristics.

• Genome: The complete set of genetic material (DNA, or RNA in some viruses) present in a cell or organism.

• Chromosome: A discrete cellular structure composed of a DNA molecule and its associated proteins, which carries genetic information.

• Gene: A polynucleotide sequence that codes for a functional product, such as a protein or RNA molecule. It is the basic unit of heredity.

Genetic material can be found in chromosomes and, in some cases, in non-chromosomal sites such as mitochondria, chloroplasts, and plasmids. In viruses, the genome may be composed of either DNA or RNA.

Organization of Chromosomes

• Bacterial Chromosomes: Typically a single, circular DNA molecule located in the cytoplasm.

• Eukaryotic Chromosomes: Linear DNA molecules located in the nucleus, often associated with histone proteins.

• Viral Genomes: May be DNA or RNA, and can be single- or double-stranded.

Genotype and Phenotype

Definitions

• Genotype: The full collection of genes (genetic makeup) that a cell or organism contains. Not all genes are expressed at all times.

• Phenotype: The observable traits or characteristics of an organism, resulting from the expression of the genotype.

• Constitutive Genes: Genes that are always expressed.

• Example: The gene for an enzyme (genotype) is present in the DNA, but the enzyme itself (phenotype) is only produced when the gene is expressed.

Chemical Structure of Nucleic Acids

Nucleotides: The Building Blocks

• Nucleotide: The basic unit of DNA and RNA, consisting of three components:

  • A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

  • A phosphate group

  • A nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA)

• Sugar-Phosphate Backbone: The alternating chain of sugar and phosphate to which the DNA and RNA nitrogenous bases are attached.

Nitrogenous Bases

• Pyrimidines: Single-ring structures; cytosine, thymine (in DNA), and uracil (in RNA).

• Purines: Double-ring structures; adenine and guanine.

Base Pairing and Stability

• Base Pairing: In DNA, adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds.

• Stability: G-C pairs are more stable than A-T pairs due to the extra hydrogen bond.

Directionality of Nucleic Acids

• The 5' end of a nucleic acid strand has a free phosphate group attached to the 5' carbon of the sugar.

• The 3' end has a free hydroxyl group attached to the 3' carbon of the sugar.

• DNA strands are antiparallel, meaning they run in opposite directions (5' to 3' and 3' to 5').

Bonds in Nucleic Acids

• Phosphodiester Bond: The covalent bond that links the 3' carbon of one sugar to the 5' phosphate of the next nucleotide in the backbone.

• Hydrogen Bonds: Non-covalent bonds that hold complementary nitrogenous bases together between the two DNA strands.

DNA Structure and Packaging

Double Helix Structure

• DNA is most stable in its double-stranded, helical form.

• Base pairing facilitates the formation of the double helix.

• Denaturation (separation of strands) can occur with heat or chemical treatment.

Supercoiling and Chromatin Organization

• Prokaryotic DNA: Supercoiled by the action of DNA gyrase, compacting the chromosome into a tight bundle.

• Eukaryotic DNA: Wrapped around histone proteins to form nucleosomes, further coiled into chromatin fibers and condensed into chromosomes.

• Additional info: Chemical tags (such as methylation) on histones can affect gene expression.

Comparison of DNA and RNA

Structural Differences

• DNA contains deoxyribose sugar; RNA contains ribose sugar.

• DNA uses thymine (T); RNA uses uracil (U) as a base.

• DNA is typically double-stranded; RNA is usually single-stranded but can form secondary structures.

Stability

• DNA is more stable than RNA due to the absence of the 2' hydroxyl group in deoxyribose, which makes RNA more susceptible to hydrolysis.

Types of RNA and Their Functions

Main Types of RNA

• Messenger RNA (mRNA): Carries genetic information from DNA to the ribosome for protein synthesis.

• Transfer RNA (tRNA): Brings amino acids to the ribosome during translation.

• Ribosomal RNA (rRNA): Forms the core of the ribosome and catalyzes protein synthesis.

• Other RNAs: Includes small nuclear RNA (snRNA), microRNA (miRNA), small interfering RNA (siRNA), and ribozymes, which are involved in gene regulation and RNA processing.

Stability of RNA Types

• tRNA and rRNA are more stable than mRNA due to their extensive secondary structures and association with proteins, which protect them from degradation.

Summary Table: DNA vs. RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strandedness	Double-stranded	Single-stranded (usually)
Stability	More stable	Less stable
Main Function	Genetic information storage	Protein synthesis, regulation, catalysis

Key Equations

• Phosphodiester Bond Formation:

$\text{Nucleotide}_1\text{-}3'\text{OH} + \text{Nucleotide}_2\text{-}5'\text{P} \rightarrow \text{Nucleotide}_1\text{-}3'\text{O}-\text{P}-\text{Nucleotide}_2$

• Base Pairing:

$\text{A} = \text{T}$ (2 hydrogen bonds) $\text{G} \equiv \text{C}$ (3 hydrogen bonds)

Conclusion

Understanding the structure and function of the genome is fundamental to microbiology. The chemical properties of DNA and RNA, their organization within cells, and the mechanisms of gene expression are central to the study of heredity and cellular function.