DNA: Molecular Structure and Discovery

Historical Experiments and Key Scientists

The discovery that genes are made of DNA was a pivotal moment in biology, supported by a series of landmark experiments. Understanding these experiments provides context for the molecular basis of inheritance.

• TH Morgan: Demonstrated that genes are located on chromosomes using fruit flies (Drosophila melanogaster).

• Frederick Griffith: Showed transformation in bacteria, suggesting a "transforming principle" (later identified as DNA).

• Avery, McCarty, and MacLeod: Identified DNA as the transforming substance in Griffith's experiment.

• Chargaff: Discovered base pairing rules; the amount of adenine equals thymine, and cytosine equals guanine.

• Hershey-Chase: Used bacteriophages to confirm DNA as the genetic material, not protein.

• Watson and Crick: Proposed the double helix structure of DNA.

• Franklin and Wilkins: Provided X-ray crystallography data crucial for elucidating DNA's structure.

Example: The Hershey-Chase experiment used radioactive isotopes to label DNA and protein, showing only DNA entered bacteria during infection.

Basic Parts of DNA and Base Pairing Rules

DNA is composed of nucleotides, each containing a phosphate group, a deoxyribose sugar, and a nitrogenous base. The four bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

• Base Pairing: A pairs with T, C pairs with G.

• Chargaff's Rule: and

Example: In a DNA sample with 30% adenine, there will also be 30% thymine.

DNA Replication

Players in DNA Replication

DNA replication is a complex process involving multiple enzymes and proteins.

• DNA Polymerase: Synthesizes new DNA strands.

• Helicase: Unwinds the DNA double helix.

• Primase: Synthesizes RNA primers to initiate replication.

• Single-Strand Binding Proteins (SSB): Stabilize unwound DNA.

• Ligase: Joins Okazaki fragments on the lagging strand.

• Topoisomerase: Relieves supercoiling ahead of the replication fork.

Origins of Replication: Specific DNA sequences where replication begins. Eukaryotes have multiple origins; prokaryotes typically have one.

Mechanism and Chemistry of DNA Synthesis

DNA synthesis occurs in the 5' to 3' direction, using existing strands as templates.

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously, forming Okazaki fragments.

• Okazaki Fragments: Short DNA segments on the lagging strand, later joined by ligase.

Example: The lagging strand requires multiple primers and produces Okazaki fragments, while the leading strand needs only one primer.

Evidence for Semi-Conservative Replication

The Meselson-Stahl experiment demonstrated that DNA replication is semi-conservative: each new DNA molecule contains one old and one new strand.

• Key Evidence: Density gradient centrifugation showed intermediate density DNA after one replication cycle.

Differences Between RNA and DNA Polymerases

• DNA Polymerase: Requires a primer, synthesizes DNA, proofreads.

• RNA Polymerase: Does not require a primer, synthesizes RNA, less proofreading.

Example: RNA polymerase initiates transcription without a primer, unlike DNA polymerase in replication.

Replication of Chromosome Ends: Telomeres

Replicating the ends of linear chromosomes poses a challenge due to the inability of DNA polymerase to complete the 3' end. Telomeres are repetitive sequences that protect chromosome ends.

• Telomerase: Enzyme that extends telomeres in certain cells.

• Function: Prevents loss of genetic information during replication.

Example: Telomerase is active in stem cells and cancer cells, but not most somatic cells.

Chromosome Structure and Packaging

Nucleosomes and Histone Proteins

DNA is packaged into nucleosomes, which consist of DNA wrapped around histone proteins. This compaction is essential for fitting DNA into the nucleus.

• Nucleosome: Fundamental unit of chromatin, ~146 base pairs of DNA wrapped around a histone octamer.

• Histones: Proteins that facilitate DNA packaging.

Levels of Chromosome Structure:

• Nucleosome

• Chromatin fiber (30 nm)

• Looped domains

• Condensed chromosome (during mitosis)

Comparison: Eukaryotic chromosomes are linear and highly organized; prokaryotic chromosomes are circular and less complex.

Biotechnology Applications

PCR and DNA Cloning

Polymerase Chain Reaction (PCR) is a technique to amplify DNA. DNA cloning uses restriction enzymes to cut and insert DNA fragments into vectors.

• PCR: Uses primers, DNA polymerase, and thermal cycling to amplify specific DNA sequences.

• Restriction Enzymes: Cut DNA at specific sequences; used in cloning.

Example: PCR can detect pathogens by amplifying their DNA from patient samples.

Restriction Endonucleases and Bacterial Protection

Restriction endonucleases are enzymes that cleave DNA at specific sites. Bacteria protect their own DNA by methylating recognition sequences.

• Function: Defense against viral DNA.

• Methylation: Prevents self-DNA from being cut.

DNA Sequencing Methods

DNA sequencing determines the order of nucleotides in DNA. Three common methods are:

• Sanger Sequencing: Uses chain-terminating nucleotides.

• Pyrosequencing: Detects nucleotide incorporation by light emission.

• Nanopore Sequencing: Measures changes in electrical current as DNA passes through a pore.

Example: Nanopore sequencing allows real-time analysis of long DNA fragments.

CRISPR and Gene Drives

CRISPR is a genome editing tool derived from bacterial defense systems. It uses a guide RNA and the Cas9 enzyme to target and modify DNA.

• Gene Drive: A genetic system that increases the likelihood a particular gene will be inherited, violating Mendelian ratios.

• Application: Can spread traits rapidly through populations.

• Jennifer Doudna: Co-inventor of CRISPR-Cas9, Nobel Prize winner in 2020.

Example: CRISPR gene drives can be used to control mosquito populations by spreading genes that reduce disease transmission.

Table: Comparison of DNA Sequencing Methods

Additional info: Table entries inferred for clarity and completeness.