DNA Structure, Function, and Replication

Learning Objectives for Genomic Studies

• Requirements for Genetic Material: Understanding the fundamental criteria necessary for a molecule to function as the carrier of genetic information.

• The Central Dogma: Identifying the flow of genetic information within a biological system.

• Historical Experiments: Explaining how the work of Griffith, Avery et al., and Hershey and Chase provided evidence that DNA is the primary genetic material.

• Biochemical Structure of DNA: Describing monomeric subunits, the orientation of strands, major and minor grooves, and the specific helix type.

• Chemical Drawing: Proficiency in drawing the basic chemical structure of a nucleotide.

• Directionality and Synthesis: Understanding why DNA synthesis exclusively occurs in the $5'$ to $3'$ direction.

• Alternative DNA Structures: Listing different structural versions of DNA beyond the standard B-form.

• Topography: Explaining why the physical shape and layout of DNA are significant in molecular biology.

• DNA vs. RNA: Comparing and contrasting the biochemical structures of these nucleic acids.

• Non-DNA Genetic Material: Identifying organisms (such as certain viruses) that use genetic materials other than DNA.

Fundamental Genetic Terminology

• Genome: Defined as the complete set of genetic information within an organism.

• Chromosome: A packaged DNA structure that contains heritable genetic information.

• Gene: Considered the basic unit of heredity.

Genomic Statistics and Comparisons

• General Bacterial Genomes: Usually circular in shape, though linear forms can exist. They typically range from $0.6 \times 10^6$ to $9.4 \times 10^6$ base pairs ($bp$).

• The Human Genome: Contains approximately $4 \times 10^9$ base pairs ($4 \text{ billion bp}$).

  • Relative size: The human genome is roughly $1000$ times larger than that of $E. coli$.

  • Gene count comparison: Humans possess approximately $30,000$ genes, whereas $E. coli$ possesses about $4,000$.

• Coding Efficiency: Bacterial genes are more sequence-efficient; a typical bacterial gene consists of about $1000$ bases.

The Biochemical Structure of Nucleotides

• Components of a Nucleotide: A repeating structural unit consisting of three parts:

  1. A Phosphate Group.

  2. A Pentose Sugar (Ribose in RNA or Deoxyribose in DNA).

  3. A Nitrogenous Base.

• The Sugar Backbone:

  • Phosphate groups are attached to the $5'$ carbon of the (deoxy)ribose sugar.

  • The phosphate group of one nucleotide links to the $3'$ hydroxyl ($OH$) group of the next base.

  • The nitrogenous base is always attached at the $1'$ carbon of the sugar.

• Structural Nomenclature:

  • Nucleoside: Represents the combination of a Base + Sugar.

    • Example: Adenine + Ribose = Adenosine.

    • Example: Adenine + Deoxyribose = Deoxyadenosine.

  • Nucleotide: Represents the combination of a Base + Sugar + Phosphate(s).

    • AMP: Adenosine monophosphate (one phosphate group).

    • ADP: Adenosine diphosphate (two phosphate groups).

    • ATP: Adenosine triphosphate (three phosphate groups).

• Chemical Bonding: The phosphate groups are connected to the sugar via a phosphoester bond.

Information Storage and Base Pairing

• Information Encoding: Genetic information is stored in the specific order of the nitrogenous bases.

• DNA Bases: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Thymine ($T$).

• RNA Bases: Adenine ($A$), Guanine ($G$), Cytosine ($C$), and Uracil ($U$).

• Strand Orientation: For base-pairing to occur, strands must be antiparallel, meaning they orient in opposite directions (one $5' \rightarrow 3'$ and the other $3' \rightarrow 5'$).

• Complementary Pairing:

  • $A$ pairs with $T$ (forming $2$ hydrogen bonds).

  • $C$ pairs with $G$ (forming $3$ hydrogen bonds).

• Fidelity of Information: Both DNA strands contain the same amount of information because they are complementary. If a sequence is $5'-A-T-C-C-T-G-G-A-3'$, the complementary strand must be $3'-T-C-C-A-G-G-A-T-5'$.

Physical Properties and Topography of DNA

• Melting Curve Analysis: A standard technique to determine relative $GC$ content. Heat is applied to denature double-stranded DNA into single strands.

  • Hyperchromic Effect: Single-stranded DNA absorbs more UV light at $260 \, nm$ ($A_{260}$) than double-stranded DNA.

  • Organisms with higher $GC$ content require higher temperatures to denature because $C-G$ pairs have more hydrogen bonds ($3$ vs. $2$).

• Stabilizing Forces:

  1. Hydrogen bonding between complementary bases.

  2. Base stacking interactions.

• Surface Features: The double helix features two asymmetrical grooves:

  1. Major Groove.

  2. Minor Groove.

  • These grooves are sites where specific proteins can bind and interact with particular base sequences.

• Structural Variations: DNA can exist in three primary forms: $A-DNA$, $B-DNA$ (the standard form), and $Z-DNA$.

Genome Structure and Packaging

• Prokaryotic vs. Eukaryotic Storage:

  • Prokaryotes store DNA in a region called the nucleoid.

  • Eukaryotes utilize histone proteins for intensive packaging.

• Eukaryotic Compaction Hierarchy:

  • DNA double helix wraps around an octet of histone proteins to form nucleosomes.

  • Nucleosomes coil together to form a solenoid shape.

  • The solenoid structure is further looped back and forth for maximum density.

• Epigenetic Code: Involves modifications of DNA without changing the sequence.

  • DNA Methylation: Methyl marks added to bases typically repress gene activity.

  • Histone Modification: Molecules attach to histone "tails," altering the activity of the wrapped DNA.

RNA Structure

• Molecular Composition: Consists of a ribose sugar (with an $OH$ at the $2'$ position), a phosphate group, and nitrogenous bases ($A$, $U$, $C$, $G$).

• Secondary and Tertiary Structure: Although typically single-stranded, RNA can form short double-stranded regions through complementary base pairing ($A$ to $U$ and $C$ to $G$).

• Dynamic Folding: Interactions such as base pairing, base stacking, and connections with ions or proteins create complex 3D shapes.

  • Example: tRNAphe contains both single- and double-stranded regions that spontaneously interact to form its specific functional structure.

DNA Replication: Mechanics and Process

• Nature of Replication: An anabolic polymerization process that requires energy and monomers.

  • Triphosphate deoxyribonucleotides serve both as the building blocks and the energy source for the reaction.

• Semiconservative Model: Each new DNA molecule is composed of one original (parental) strand and one newly synthesized (daughter) strand.

• Initiation: Replication begins at a specific site called oriC (Origin of Replication). The DNA "melts" (unzips) at this location, and polymerization proceeds around the chromosome.

The Replication Machinery (Prokaryotic Model)

• Helicase: Separates (unzips) the DNA strands at the replication fork.

• Topoisomerase: Relieves the additional coiling or over-winding tension ahead of the replication fork.

• Single-Strand Binding Proteins (ssb): Bind to and stabilize the single-stranded DNA to prevent the double helix from re-forming prematurely.

• Primase: Synthesizes a short RNA primer ($5-10 \text{ nucleotides long}$) to provide a starting $3'-OH$ group.

• DNA Polymerase III: The primary enzyme that adds nucleotides to the $3'$ end of the growing nucleic acid chain. It synthesizes DNA only in the $5' \rightarrow 3'$ direction.

• DNA Polymerase I: Replaces the RNA primer sequences with DNA.

• DNA Ligase: Seals the gaps between Okazaki fragments to create a continuous DNA strand.

Leading vs. Lagging Strands

• Continuous vs. Discontinuous: Because DNA strands are antiparallel but synthesis only occurs $5' \rightarrow 3'$:

  • Leading Strand: Synthesized continuously moving toward the replication fork.

  • Lagging Strand: Synthesized discontinuously in short segments called Okazaki fragments, moving away from the fork.

• Bidirectionality: Replication generally proceeds in both directions from the origin.

DNA Methylation and Mutations

• Roles of Methylation:

  • Control of genetic expression.

  • Initiation of DNA replication.

  • Protection against viral infection.

  • DNA repair mechanisms.

• Mutations: Errors in replication that remain uncorrected. These are changes in the nucleotide sequence which can subsequently alter the protein sequence coded by the DNA.

• Types of Mutations:

  • Point Mutations: Includes Silent, Missense, and Nonsense mutations.

  • Frameshift Mutations: Caused by insertions or deletions that shift the reading frame.

  • Chromosome Mutations: Larger-scale structural changes to the chromosome.