DNA Structure and Replication

Learning Objectives and Requirements for Genetic Material

  • Essential Skills to Master:

    • Discuss the specific requirements necessary for a substance to serve as genetic material.

    • State the Central Dogma of molecular biology.

    • Explain the historical significance of experiments conducted by Griffith, Avery et al., and Hershey and Chase in proving DNA is the genetic material.

    • Describe the biochemical structure of DNA: identify monomeric subunits, 5' to 3' orientation, major and minor grooves, and the specific helix types.

    • Draw the basic chemical structure of a nucleotide including all functional groups.

    • Explain the directionality of DNA and the biochemical reasons why synthesis occurs specifically in the 5' to 3' direction.

    • List alternative DNA structures beyond the standard B-form.

    • Explain the relevance of DNA topography to the field of molecular biology.

    • Compare and contrast the biochemical structures of DNA and RNA.

    • Identify organisms (such as certain viruses) whose genetic material is NOT DNA.

Fundamental Genetic Terminology

  • Genome: The complete set of genetic information within a cell or organism.

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

  • Gene: The fundamental unit of heredity.

DNA as the Repository of Cellular Information

  • Genomic Scope:

    • The genome represents the complete sequence of DNA in a cell.

    • Bacterial Genomes:

    • Structure: Usually circular, though linear genomes can occur.

    • Efficiency: A typical bacterial gene consists of approximately 10001000 bases.

Human Genomes:

  • Comparison to Bacteria: The human genome is roughly 1000×1000 \times as large as the E.coliE. coli genome.

  • Gene Count: Humans have approximately 3000030000 genes, whereas E.coliE. coli has approximately 40004000 genes (approximately 8×8 \times more in humans).

  • Information Density: E.coliE. coli genes utilize significantly less DNA sequence relative to the total genome size compared to humans.

Structure of Nucleic Acids

  • The Nucleotide (The Monomeric Subunit):

    • A nucleotide consists of three essential components:

      1. Nitrogenous Base: Attached to the 1' carbon of the sugar.

      2. Pentose Sugar: (Deoxy)ribose.

      3. Phosphate Group: Three phosphate groups are typically attached to the 5' carbon in a free nucleotide.

  • Sugars and Linkages:

    • The sugar is a ribose (RNA) or deoxyribose (DNA).

    • Phosphate groups on the 5' carbon of one nucleotide link to the 3' OH (hydroxyl group) of the next base to form the backbone.

  • Distinction between Nucleosides and Nucleotides:

    • Nucleoside: Base + Sugar.

      • Example: Adenine+ribose=Adenosine\text{Adenine} + \text{ribose} = \text{Adenosine}.

      • Example: Adenine+deoxyribose=Deoxyadenosine\text{Adenine} + \text{deoxyribose} = \text{Deoxyadenosine}.

    • Nucleotide: Base + Sugar + Phosphate(s).

      • AMP: Adenosine monophosphate (1 phosphate).

      • ADP: Adenosine diphosphate (2 phosphates).

      • ATP: Adenosine triphosphate (3 phosphates).

  • Nitrogenous Bases:

    • The information in nucleic acids is stored in the specific order of the bases.

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

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

The DNA Double Helix

  • Physical Orientation:

    • Strands must be antiparallel (oriented in opposite directions: 5' to 3' and 3' to 5').

    • The strands wind around each other to form a helical structure.

  • Stabilizing Forces:

    • Hydrogen Bonding: Occurs between complementary bases.

      • AA and TT pair via 2 hydrogen bonds.

      • CC and GG pair via 3 hydrogen bonds.

    • Base Stacking: Interactions between the flat surfaces of the bases also stabilize the double helix.

  • Structural Features:

    • Grooves: The helix features two asymmetrical grooves on the outside known as the Major groove and the Minor groove.

    • Protein Interaction: Certain proteins bind within these grooves to interact with specific sequences of bases.

  • Alternative Helix Types:

    • B-DNA: The standard, most common biological form.

    • A-DNA: A theoretical or dehydrated form.

    • Z-DNA: A left-handed helix variant.

Information Content and GC Content Analysis

  • Complementarity: Both strands of DNA contain the same amount of information because one strand is exactly complementary to the other (e.g., a 5' to 3' sequence of A-T-C-C-T-G-G-A corresponds to a 3' to 5' sequence of T-C-C-A-G-G-A-T).

  • Melting Curves and GC Content (Critical Thinking):

    • Determination of relative GC content involves applying heat to denature (melt) double-stranded DNA into single strands.

    • Single-stranded DNA absorbs more UV light at A260A260 (absorbance at 260 nm260\,nm).

    • Organisms with higher GC content require higher temperatures to denature because C-G pairs have three hydrogen bonds compared to the two bonds in A-T pairs.

Genome Packaging and Epigenetics

  • Prokaryotic Packaging:

    • DNA is found in the Nucleoid region.

    • Bacterial DNA is usually circular and associated with proteins that help it fit inside the cell.

  • Eukaryotic Chromosome Compaction:

    • DNA double helix wraps around an octet of Histone proteins to form Nucleosomes.

    • Nucleosomes coil together to form a Solenoid shape.

    • The solenoid is further looped back and forth for higher-level compaction.

  • Epigenetic Code:

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

    • Histone Modification: Molecules attach to histone "tails" to alter the activity of the DNA wrapped around them.

RNA Structure and Function

  • Biochemical Characteristics:

    • Consists of a sugar-phosphate backbone and nitrogenous bases (A, U, C, G).

    • Contains Ribose sugar (identifiable by the 2' OH group).

    • Single-stranded, but contains short double-stranded regions due to complementary base-pairing (AA to UU and CC to GG).

  • Tertiary Structure:

    • RNA molecules can fold into complex 3-dimensional shapes.

    • Tertiary structure is influenced by base-pairing, base stacking, and interactions with ions, small molecules, and large proteins.

    • Example: tRNAphetRNA^{phe} (phenylalanine transfer RNA) contains both single- and double-stranded regions that interact spontaneously to produce a functional 3-D structure.

DNA Replication

  • General Principles:

    • Replication is an anabolic polymerization process.

    • Requires monomers (triphosphate deoxyribonucleotides) and energy; the triphosphates themselves provide the energy for the reaction.

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

  • Initiation:

    • In prokaryotes, replication begins at a specific site called oriC (Origin of Replication).

    • DNA melts (strands separate) at the origin, and polymerization proceeds in both directions (bidirectional).

Prokaryotic Replication Machinery (Replisome)

  • Enzymes and Proteins:

    • Helicase: Unzips the DNA strands by breaking hydrogen bonds, forming the replication fork.

    • Topoisomerase: Relieves the additional coiling (supercoiling) ahead of the replication fork.

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

    • Primase: Synthesizes a short RNA primer (55-1010 nucleotides long), providing the 3' OH required for DNA polymerase.

    • DNA Polymerase III: The primary enzyme that adds nucleotides to the 3' end of the growing daughter strand.

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

    • DNA Ligase: Seals gaps between Okazaki fragments, creating a continuous sugar-phosphate backbone.

  • Symmetry of Synthesis:

    • DNA polymerase adds nucleotides only to the hydroxyl group at the 3' end (5′→3′5' \rightarrow 3' synthesis).

    • Leading Strand: Synthesized continuously toward the replication fork.

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

Additional Aspects of DNA Maintenance

  • DNA Methylation Roles:

    • Control of genetic expression.

    • Initiation of DNA replication.

    • Protection against viral infection.

    • Repair of DNA.

  • Mutations (Replication Errors):

    • Uncorrected errors during replication lead to mutations (changes in nucleotide sequence).

    • Mutations can alter the protein sequence encoded by the DNA.

    • Types of Mutations:

      1. Point Mutations:

        • Silent: No change in protein sequence.

        • Missense: One amino acid changed.

        • Nonsense: Introduction of a premature stop codon.

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

      3. Chromosome Mutations: Large-scale changes to chromosome structure.